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Q&A Spotlight

How to Install a Foundation Drain

Does it really make a difference whether the perforated drain pipe goes on top of the footing or beside the footing?

Most detail drawings show that a footing drain should be located next to the footing. But some builders prefer placing the drain pipe on top of the footing. Which way is correct?
Image Credit: GBA

On its face, the location of a foundation perimeter drain seems like the simplest of details. The perforated drain line is run around the foundation next to the bottom of the footing.

At least that’s what many construction drawings show. But in some parts of the country, the drain is placed on top of the footing rather than next to it, and this discrepancy is at the root of Steven Knapp’s dilemma.

In a question posted at Green Building Advisor’s Q&A forum, Knapp writes this approach is not typical in his area, and that his waterproofing contractor is refusing to go along with it.

“Several years ago he switched to placing the drain (a rectangular pipe) on top of the footer and thinks this is the better method,” Knapp writes. “I’m annoyed and confused since I was advised by another credible expert that placing the pipe on top of the footer would greatly increase my chances of springing a leak.”

Knapp’s builder is leaning in favor of the “on footer” method, but he’s willing to do whatever Knapp thinks best.

“So what is industry best practice?” he asks. “Putting the pipe next to the footer makes intuitive sense to me, but I know that what’s intuitive isn’t always correct. I just don’t want a basement that leaks.”

That’s the topic for this Q&A Spotlight.

The drawings are just plain wrong

Yes, writes James Morgan, we’ve all seen foundation drains drawn that way many times but the drawings are wrong.

“I know that’s the standard drawn detail and I’ve seen it a thousand times, but I think it’s a bad one, and there are several important [performance] reasons that no one ever builds it that way,…

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  1. user-2310254 | | #1

    Great summary
    FYI I decided to use two drains ( one next to the footer and one on top of the footer). It seemed like the best course of action since I could see logic in the positions argued by both sides.

  2. user-1140531 | | #2

    A Few More Thoughts

    It is nice to see your blog on this topic.

    Just to clarify, I would not backfill with sand. If I said that, it was a mistake. I would backfill with “fill sand”, which is sandy or fine granular material of varying particle size, and including some content of dispersed clay particles. This blend of material is ideal because it is compactable. It is also free draining, so if it gets wet, it will be relatively unable to hold moisture. The inability to hold moisture is a benefit in minimizing frost heave during the winter. The material that is the complete opposite in this performance is pure clay, sometimes called fat clay. This is the worst possible material for backfilling, but is nevertheless used if it is the native soil of the site. Such clay eagerly holds moisture and is most highly susceptible to frost expansion. It also the most expandable and contractible with the gain and loss of moisture aside from the issue of freezing. It is also more difficult to compact because the proper moisture content for compaction is more difficult to achieve with clay.

    As mentioned, I would pitch the perimeter drain. While it true that any amount of head pressure will push the water out of a flat tile, a flat tile will likely have minor sags that will pond water. It will not only stand in the tile, but also it will stand in the bottom of the entire drain bed. The drain tile is intended to drain free water incursion, but if you go to all that work, why not go a little further and avoid the concentrated dampness arising from puddles in the drain bed?

    The only problem with pitching the tile is that it might leave the tile too high in one area or too low in another. This can be controlled by minimizing the pitch and the length of run. However, the length of run depends on the size of the foundation. Another way to minimize the elevation change resulting from pitch is to start at the highest point, pitch the tile down grade in both opposing directions, and exit the perimeter at a low point most distant from the high point. I would not set the bottom of the drain bed lower than the bottom of the footing due the possibility of compromising the support of the bearing soil, as Steve Baczek says.

    I would set the bottom of the tile on the bottom of the drain bed rather than elevated above the bottom of the drain bed, as shown in details here. I consider the drain bed itself to simply be a functional extension of the drain tile. Both the tile and the drain bed floor are pitched. Without any pitch; and with the tile bottom sitting a couple inches above the floor of drain bed; and with the holes in the tile set a 4 and 8 o’clock; you would have a fairly large reservoir to hold water. If this were set up as a continuous active drain system, you could have maybe 500-1000 gallons of water stored continuously in that reservoir as a permanent wet moat surrounding the footing.

    I have developed a whole system of practice for building what I call a “high performance basement.” Altogether, it goes far beyond typical practice which is always seeking to minimize cost. As I mentioned in some of the Q&A discussion, I would not expect the drain tile system to ever see a drop of water. It would only be a backup measure. All of the water incursion control would be dealt with at the surface with some special measures of grading, film, and rock bedding. The backfilling that I advocate is also essential for this surface water control detailing. I would never set up a system that routinely has water draining down the face of the foundation wall and into the drain tile, as was mentioned in the Q&A discussion.

    Water issues come from two independent sources:

    1) Surface water from rain and snow.
    2) Subterranean water moving horizontally or vertically due to hydrostatic pressure.

    My overall method is to deal only with item #1. All sites have that issue. Only certain sites have issues with item #2. I can see a need for backfilling entirely with drain bed rock to combat the problems associated with item #2 such as horizontal subterranean flow. In that scenario, the tile will be required to function as an active, fulltime drain. However, rather than go down the road of problems with item #2, I would not build on a site that has those issues. Often this can be determined simply by looking at the site. It can certainly be determined by testing the site.

    If I had to build on a site with subterranean water flow, or occasional high water table, I would dewater the entire foundation site to 10-30 feet out all around the foundation. This would amount to broadening the drain perimeter and installing an active drain tile of significant diameter, with cleanouts and inspection access to monitor performance. I would not attempt to build a foundation like a boat to keep the water out. It would be more like building an island of dry land for the house.

  3. Expert Member

    High Water Table
    Ron, rather than rely on such elaborate measures, wouldn't it make more sense not to design a house with either a basement or crawlspace when sub-surface water was such an issue?

  4. user-1140531 | | #4

    Subterranean water

    As I mentioned, I would not want to build on a site with sub-surface water issues. But if I had to, I would dewater the site by the measures I mentioned. I would not want to build on such a site with the expectaion that a typical drain tile installation would take care of the problem. Building without a basement would be another option. But even then, high water and saturated ground might complicate the pier footings.

    I would certainly not want the footing tile to be a link in an active water transport system. The main problem with that is that things can change as the tile handles water over time. Silt can enter the drain rock field, reducing its permeability and ultimately plugging the holes in the tile. It can enter the tile and plug its linear flow. And any of these restrictions will cause water backup in the critical vicinity of the foundation where it can cause problems to the interior. Furthermore, these problems are hard to diagnose and repair. Preventing the overworking and life shortening of the footing drain is one reason that people advise against running the roof gutter water into the footing drain system.

    And even if you put in a good enough footing tile system to handle the water, having the water so close to the foundation will raise the possibility of water incursion. Damp-proofing measures will not necessarily stop the pentration of flowing water.

  5. user-460363 | | #5

    How about BELOW the footing?
    It strikes me as optimal to keep the water away from both the footings and the slab. Like in the drawing.

    I realize it's not common practice. But maybe it should be.

  6. Billy | | #6

    Different Approach
    I've thought a lot about this. It appears that a footer drain beside the footer is best, assuming there is plenty of gravel extending above the footer and all of it is wrapped in heavy landscape fabric. But the problem is you are disturbing soil right next to the footer, and it seems foolish to risk the structural integrity of the footer for waterproofing purposes, when there are alternative ways to achieve waterproofing that is perfectly adequate.

    The Fastfoot membrane appears good but I've never used it. Instead, the top of the footer should be waterproofed and a peel and stick membrane used for waterproofing on the outside of the foundation wall, together with the recommended sealant at the wall/footer joint. A dimpled drainage mat can be placed over the waterproofing to protect the membrane and allow water to fall to the footer drain placed on top of the footer.

    The second part of the equation is to put foamboard underneath the slab and on top of the interior of the footer, as shown in the attached diagram from Building Science. Why don't any of these GBA drawings in this article show underslab insulation???? Come on guys....

    Installing foamboard under the slab prevents wicking between the footer and slab and gravel and slab, and you can/should have a polyethylene barrier as well. MOST IMPORTANTLY it also raises the level of the slab with respect to the exterior footer drain so the water level should never reach the height of the bottom of the slab if you use 2" or 4" of foam under the slab.

    If you are super cautious you can also add a French drain in the gravel under the slab, and drain it to an interior sump or outside to daylight only if the house is built on a slope where you can get to daylight.

    I think it would be foolish to potentially risk undermining the structural integrity of the footer just to drop your footer drain by a few inches when you can achieve adequate waterproofing AND UNDERSLAB INSULATION using a different approach.



  7. ecdunn | | #7

    Upside down pipe
    Here, in Flagstaff, we always set it on the footer. Don't want to disturb anymore earth than we have to. The BIGGEST mistake I have seen with drain pipe is setting it with the holes facing up. That is a big mistake. The holes go DOWN!

  8. GBA Editor
    Martin Holladay | | #8

    Regional differences
    I'm always interested to learn about regional differences in construction practices. In one section of the country, contractors say, "That's difficult. That will cost more. We never do it that way." Elsewhere, the difficult job is completely routine.

    Ed Dunn reports that in Flagstaff, Arizona, the drain pipe is always installed on top of the footing. William Geary writes that the problem with GBA's approach is that "you are disturbing soil right next to the footer, and it seems foolish to risk the structural integrity of the footer for waterproofing purposes."

    Meanwhile, in New England, excavation contractors routinely dig holes in the ground for basement foundations. These holes have level bottoms. They are large enough to accommodate the necessary concrete footings, plus a generous amount of room to allow the concrete contractors to set their forms. This means that there is plenty of room on the outside of the footings to install the drain pipe beside the footing. Needless to say, the footing is not undermined, nor is the soil beside the footing "disturbed" -- because the hole in the ground has a level bottom, courtesy of the excavation contractor, who doesn't charge extra to dig what we consider to be a normal basement excavation.

  9. user-460363 | | #9

    4" of stone and a 4" drain pipe installed level?
    From what Pat Huelman says, it seems a granular layer under the footings could work. If there was no pitch to the 4" drain pipe, and the pipe was placed at (or very near) the bottom of the 4" granular layer, then there would be no risk of undermining by water washing out the soil under the footing (on it's way to the "deeper" drain pipe). Correct?

    It would then be a matter of ensuring you use the right granular material, and that it's uniform.

    In the previous Q&A, Ron Keagle mentioned something that sounded scary: that repeated soil saturation could somehow pump the soil material into the granular layer. And then you definitely have a problem. Differential settling at worst. And clogging of the granular layer at the best.

    What if you laid down some kind of sheet product on top of the excavation before any granular layer was added? Then a uniform granular layer, then the footings? Would that mitigate the risk of the underlying soil mixing into the granular layer?

  10. user-1140531 | | #10

    I am familiar with the
    I am familiar with the footing technique that Martin mentioned. That is where you dig the basement to the elevation matching the bottom of the footing, and extend the excavation a foot or two beyond the outside line of the footing. Then form the footing and pour it, and then strip off the forms. It is important to not excavate lower than the bottom of the footing. But footing bearing surface will probably get a little chewed up here and there, so a plate compactor is run over it before pouring the concrete.

    I don’t believe the soil alongside of the footing plays a role in support, so I see nothing wrong with placing the bottom of the drain bed at the same elevation as the bottom of the footing. But you should not excavate below the bottom of the footing elevation alongside of the footing.

    I don’t understand the reasoning for placing the drain tile on top of the footing. Are the footings in this case poured into a trench without forms? If that were the case, I could see how one would not want to go back in and dig out the material along the outside of the footing in order to place the drain bed there. That would have to be dug out by hand.

    I agree that the holes in the drain tile should not face upward. However, I would not place them on the bottom either because I would set the tile directly on the bottom of the drain bed. That would plug the holes if they were on the bottom. So I would place the holes at 4 or 8 o’clock position.

  11. user-871583 | | #11

    Keep her below the slab
    Martin - thanks for your response. You took some of the words right out of my mouth.

    In my region (Vancouver BC Canada) pretty much 90%+ of homes are built with basements now (that is were the income suite goes) and the home would be worth a lot less if it did not have a basement. So stating a home would not be built in areas with ground water is totally impracticable. A large percentage of basement homes do have ground water issues. It is often made more complicated by the fact that a lot of our municipal storm sewers are not deep enough to service a basement so we also have to pump the water up and out.

    Ground water is a very real concern for us and should be evacuated by the perimeter drain so that it can never build up and touch the bottom of the slab (pretty much the only source of water that a perimeter drain should be addressing is the ground water. We have directed our roof storm run off to a separate drain pipes for many years now). So it belongs beside the footing surrounded in a granular layer. I tell my clients that the middle to top of the pipe should be below the slab bottom surface and for a new build, always ensure a healthy granular layer below the slab (never sand or anything with fines). Fortunately in my area, a capillary break is becoming a lot more common (dimple board) along the surface of the foundation to allow the whole foundation to be protected from hydraulic pressure.

    Like Martin's region, our excavations are generally flat and there is almost always a 'work' margin around the footings to allow access to the workmen (3 - 4 ft). So there is no impediment to placing the drain beside the footing where it belongs.

    Also like AJBuilding, I have only periodically seen the back-fill compressed and since the forms are typically removed within 2 days of pouring, the danger of blowing out your wall is very high if you compact this soon after poor. Of course, this creates a problem where the sidewalk that is eventually poured beside the structure ends up back-sloped to the house once the ground below has settled, so I would like to see more compacted back-fill done after the foundation has had a chance to setup longer.

    Finally per Zenon's posting, rising damp is a very real issue and can create a lot of damage in a finished basement. There is a pretty simple way to prevent it, encapsulate your footing in a waterproof bag. I will be using the Fastfoot ( my upcoming build. Not only does it prevent rising damp, it also is easier to form than conventional forming and saves material and labour.

  12. user-460363 | | #12

    Is the Fast-foot drawing becoming code in Vancouver?
    Sean: the fast-foot/FabForm people were saying that this idea of a granular layer under the footings & draining water before it even hits the footings, is becoming code in the Vancouver area. At least that's what I thought they said. Is that true? Do you know of foundations that are entirely perched up on a draining, granular layer?

    It seems like the ideal way to avert water problems right from the start. Waterproof bagged footings help too.

  13. GBA Editor
    Martin Holladay | | #13

    Response to Zenon Tymosko
    Plenty of builders pour their footings over a layer of crushed stone. Check out the photos showing one such project here: Owner-Builders Get a Tight Shell With SIPs.


  14. user-871583 | | #14

    Footings over gravel
    The BC Building code allows for footings to be placed on "compacted granular fill" and I plan to place my footings over 4" of gravel (as long as I can keep my engineer happy). But I would say that it still is not that common for houses built to sell (spec) in my region. I see a lot of footings right on ground. In these cases, the footing is interrupted by a drain pipe at various locations to allow the area under the slab to drain to the exterior perimeter system.

    It is also interesting to note that the BCBC REQUIRES the drainage pipe to "have the top of the tile or pipe below the bottom of the floor slab or the ground cover of the crawl space." so that puts an end to that argument.

  15. MikeSaintLouis | | #15

    Footing formed against earth
    The common (though not universal) way of pouring a footing in the Saint Louis area is to trench and pour the sides as well as the bottom of the footing against undisturbed soil. If there is to be a basement, the 'flat bottom whole house' excavation is to 8" below the floor slab, then the footing trenched below that level, with the top of the footing pour level with the bottom of the basement excavation. Then place 4" of granular fill inside the basement and then your 4" slab. Outside the foundation you have 8" from the top of the footing to the level of the floor slab (4" to the level of the bottom of the slab), which is enough room to place a perimeter drain. The drain is covered in rock, which is then wrapped in filter fabric prior to backfill in order to postpone the day when fines plug everything up.

  16. user-460363 | | #16

    Would you worry about soil entering the granular layer?
    Sean, Martin, or anyone? Would you put down something like Delta MS-Underslab down first - on the soil? Would that become your only barrier again soil moisture? In other words, you wouldn't put an additional layer of a sheet product under the slab itself. Or I suppose you could, just to be extra cautious (and extra $).

    If you use XPS under the slab, it would be it's own moisture barrier. But if you used Roxul "RockBoard", it would not be its own moisture barrier. I assume.

  17. GBA Editor
    Martin Holladay | | #17

    Response to Zenon Tymosko
    Here is the order of the layers, from the bottom to the top: crushed stone, rigid foam insulation, polyethylene vapor barrier, concrete slab.

  18. user-871583 | | #18

    Totally agree with Martin's layers.

    Poly is a very effective VB and cheaper than any other product. It ALWAYS belongs against the underside of the slab and never under the insulation or under any type of fill. By including insulation you also help protect the poly from puncture. If in a ground gas region, then the poly also has to be sealed as an AB which takes a bit more effort to ensure no punctures during construction.

  19. Billy | | #19

    Foam Board on Top of Footer
    I agree with Martin's layers, but no one has responded to putting the foam board on top of the footer, where I believe it belongs so the slab is isolated and insulated from the ground. This also raises the height of the slab bottom which results in the footer drain being below the bottom of the slab, whether it is placed beside the footer or on top of the footer. GBA should consider correcting their drawing to show the layers that Martin recites.

    And yes, the drain holes face down...


  20. GBA Editor
    Martin Holladay | | #20

    Response to William Geary
    There have been several discussions on GBA about whether it is acceptable to install a layer of rigid foam between the top of a concrete footing and a concrete foundation wall. One such discussion occurred here: Construction Begins — and We Encounter a Few Snafus. The relevant section begins with these words: "Our architect, Chris Briley, cleverly added a 2-inch thick layer of high-density XPS installed at the bottom of the first course of block. The XPS creates a thermal break between the footer and the foundation. And this became the first of several rapid fire foundation snafus." You may also want to read the comments.

    Concerning "correcting" the GBA details: we have readers in all climate zones. A continuous horizontal layer of rigid foam is not required or recommended under slabs in Florida or other warm climates. I'll admit, however, that the insulation R-values in many of our details could be higher. It would be great if we could offer a greater range of details; some day we will. We add to our detail library regularly, although probably not as regularly as some readers would like.

  21. Billy | | #21

    Still Didn't Address the Question

    I appreciate your answer, but after reading it and re-reading it, and reading the article you linked to, I realized it was a non-answer.

    First, your GBA drawings don't show foam under the slab. This is pretty inexcusable for a Green Builder Advisor and it is contrary to your stated design preference. This is pretty basic, isn't it?

    Second, you should first take a look at the drawing I posted before you decide to comment on it. Your comment and the article you refer to put the foam UNDER THE STRUCTURAL FOUNDATION WALL.

    The drawing I posted does not do that in any way. In my drawing the foundation wall rests directly on the footer without any foam between them and only the slab rests on the foam board.

    I'll assume this was a mistake on your part but my first reaction was that this was a bit flippant of you to say "There have been several discussions on GBA about whether it is acceptable to install a layer of rigid foam between the top of a concrete footing and a concrete foundation wall..." when this was not at all what I proposed and this can be seen plainly in the drawing posted in my Comment 6 above.

    Third, I disagree that a slab in Florida would not benefit from a layer of foam underneath. I used to live in Florida and I can tell you that slab felt very cold in winter (slab edge insulation would have helped but underslab is better) and in most seasons it was a source of heat loss/gain. Even if you assume that a Florida slab doesn't need foam underneath, why would GBA show a drawing that works for Florida but not other parts of the country?


  22. GBA Editor
    Martin Holladay | | #22

    Response to William Geary
    You're right; I misunderstood your question. My apologies.

    GBA has included many details and articles discussing the use of a continuous layer of rigid foam under slabs. Some, but not all, of our details show sub-slab foam.

    The energy saved during the cooling season in Florida by not installing insulation under a slab is worth more than any energy saved during the brief Florida heating season by installing insulation under a slab.

    GBA is often accused of a northern bias. You have complained that some of our details show a southern bias. Your complaint is duly noted. No single detail can satisfy readers in every North American climate.

  23. GBA Editor
    Martin Holladay | | #23

    Response to William Geary
    The title of your last post was, "Still Didn't Address the Question." I have re-read your posts, and I'm not sure what your question is.

    If your question is, "Is it acceptable to fill the area between the footings with crushed stone up to the top of the footings, and then to install a continuous layer of rigid foam, with the foam resting on top of the footings?" -- then the answer is: "Yes."

  24. user-696154 | | #24

    perimeter drain location
    From a drainage perspective, put it next to the footer. Do as the detail shows--put aggregate around the pipe. Backfill over that with what you planned to backfill with (probably native soil), then 6" of low permeability soil (we call that clay in this part of the world) sloped away from the wall.

    Compact the backfill--not with a tractor (the wall should hold the compacted fill--but not a tractor!).

    The drain line can be level as long as it drains to daylight (there won't be a large water flow, so friction isn't an issue).


  25. Billy | | #25

    Thanks, Martin

    Thanks for your reply.

    The discussion on this thread has focused on where to put the footer drain so the bottom of the slab doesn't get wet.

    The point I am trying to make is that when foamboard is placed on the footer and the slab poured on top of the foamboard, the bottom of the slab is raised by the thickness of the foam. This means, I believe, that placing the footer drain on top of the footer does not risk wetting the slab to the extent shown in the GBA drawing where the slab is poured directly on the footer -- especially if there is foam board (2", 4" or?) on top of the footer and polyethylene on top of the foam board.

    The point was to focus the discussion on typical green building practices where the bottom of the slab is higher than the footer instead of on the GBA drawing where the slab is poured on the footer.


  26. user-871583 | | #26

    Drain location
    Billy, I understand your comment re the drawing you posted and it is still better to have the drain down beside the footer to evacuate the water away from the footer, This helps reduce rising damp but also helps keep the bearing soil dryer for regions with expanding clay.

    I will actually be trying to get my engineer to approve a drain that is below the footer located in compacted gravel so that the entire basin below the 4" of compacted gravel covering the whole build site stays predominately dry.

  27. user-871583 | | #27

    Thermal Bridging
    Just one more comment Billy - if your foundation felt cold, then that probably meant it was not insulated on the exterior or interior, not because there was no insulation below the slab. You commented that insulation below a slab is better than slab edges and this just is not true. Slab edge losses represent the largest thermal bridge in any slab installation whether that slab is below grade, at grade, or even suspended. Sandwiching even 1" of rigid foam between the slab edge and foundation will go a long way to reducing thermal bridging and should be the number one location addressed. Of course doing both the perimeter edge and between the footing and slab under-surface is even better. I will place 2" under my entire slab and increase that to 4" for the outside 4ft around the perimeter of the foundation and will also try to get the engineer to accept 2" UNDER the footing.

  28. user-460363 | | #28

    Thermal bridging
    Sean: I looked at some of the GBA discussion about bridging and losses through the footing. And the result seemed to be that either one goes through heroic & non-"conventional" methods (like foam under footings), or one just accepts the loss of some heat in this area. Yes, 1 or 2" around the edge of the slab is better than nothing.

    My question/follow-up is: [in Ontario/areas with frost] when you're down to the minimum frost depth, it's at least 4' down. And at that level, (I'm told) the ground is a "constant" 55-60 degrees pretty much all year long. If you expect your basement to be around 68 degrees (also all year long), then we're not talking a huge disparity in temps. Whereas at the top of the foundation, where the concrete walls here (Ontario) are typically exposed to the exterior, the temps can be 0F or less. I would make more sense to spend (proportionately) more effort to deal with the exposure to 0F, than to 55-60F. Right?

    I'm wondering for myself whether to get all worked up about this bridging effect at the footing, or just be ok with it, because I'm concentrating on areas with greater vulnerabilities. Or am I missing something?

    This also dovetails into the concept of frost-protected shallow foundations, where they design IN heat loss at the perimeter, specifically so that the ground around the edge does NOT freeze. If you insulate too much, you no longer have an effective FPSF, and it could be disastrous. And FPSF are "green" and "efficient". So how could something 4' down be such a big deal? (Again, just thinking out loud - I don't have a position on this just yet)

    Of course in temperate Vancouver, it's a different story. At least the freezing part. I would think that footing heat leakage would be even less of a concern. But my thinking could be totally off.

  29. user-460363 | | #29

    Sean: please update us on your project
    I am greatly interested on the "debate" you will have with your engineer and building dept about the crush under the footing and the placement of the tile below the entire plane of the footings. Can you keep us updated? On this page? Or somewhere else? Please?

  30. GBA Editor
    Martin Holladay | | #30

    Response to Zenon Tymosko
    You wrote, "This also dovetails into the concept of frost-protected shallow foundations, where they design IN heat loss at the perimeter, specifically so that the ground around the edge does NOT freeze. If you insulate too much, you no longer have an effective FPSF, and it could be disastrous."

    You are repeating a common misunderstanding concerning frost-protected shallow foundations. I cleared up the misconception in an article I wrote for Fine Homebuilding ("Frost-Protected Shallow Foundations").

    In that article, I wrote, "Fortunately, these shallow foundations don’t depend on leaking building heat to keep the soil warm. Instead, horizontal wing insulation extending from the bottom edge of the slab helps to retain the natural warmth of the earth."

    By the way, you are correct that the most cost-effective way to insulate a building is to start with the building components that face the greatest delta-T (temperature difference between the indoors and the outdoors). Once these areas are addressed, you can proceed to insulate the areas with a smaller delta-T -- but only if your calculations convince you that the investment is worth it.

  31. user-460363 | | #31

    Makes sense
    Martin - yes, that makes sense - mostly. I got my perception by studying the Legalett FPSF system, and when you watch the videos and see the thermal diagrams, it certainly occurs like they rely on some building heat to keep the area free of frost. And I think they design differently if you intend to have periods of NO HEAT through the frost season (like a holiday ski chalet). So I assumed that the building heat plays at least some role in the workings of a FPSF.

  32. user-1140531 | | #32

    Water and Dampness
    I see four different conditions that bring water and/or moisture issues near foundations:

    1) Water draining down from the surface.
    2) Water moving horizontally from some distant source.
    3) Water rising up as a fluctuating water table.
    4) Dampness dispersed throughout the soil due to its sponge-like absorption and dispersion of liquid water landing on the surface from rain or snow

    Items #2 and/or #3 may or may not be present on a given site, depending on site relationship to surrounding elevations, soil conditions, etc.

    Items #1 will always be present with any undeveloped site, but it can be eliminated by site development measures and details.

    Item #4 will always be present whether site is developed or not.

    From the discussion, it sounds like there is a general consensus that the best approach is to incorporate site development details that anticipate and resist all four of these conditions as though they are all likely to occur. The general solution to the problem is encasing the foundation in a waterproof membrane with seams sealed, and then having a drain tile to prevent water from rising around the foundation, which would create hydrostatic pressure that could aid water incursion through any small breaches in the membrane.

    I would avoid a site with items #2 & 3, and provide remedies to items #1 & 4. Item #1 can be eliminated with proper backfilling and properly pitched surface membrane. Item #4 cannot be eliminated, but the dampness can be prevented from entering and wicking through the concrete. So, I would isolate the all of the concrete from the soil with a heavy duty polyethylene vapor barrier.

    This would be far easier to accomplish than isolating the concrete from liquid water. Water can get through cracks, spread out, wet the concrete, and feed absorption into the concrete over a large area. Dampness can also go through a crack, but it does not spread out over the surface of the concrete like water does. So a breach in the vapor barrier is not nearly as vulnerable to the intrusion of dampness alone as it would be to dampness intrusion being fed by the intrusion of water spreading over the surface of the concrete.

    And even if the source water continues to fall to the drain tile, if it finds a small breach in the barrier, and spreads out on the wall or footing surface, it will aggressively feed dampness into the concrete. So a barrier that actually waterproofs the wall needs to be far more robust and sophisticated than a damp proofing barrier. There is a big distinction between damp proofing and water proofing. Traditional damp proofing measures do not prevent water intrusion. They are not intended to.

  33. user-871583 | | #33

    Response to Zenon
    Absolutely agree to hit the higher Delta T locations before worrying about sub footing or even sub slab insulation. Sub slab only makes sense when the rest of the envelope is very high performance or if you have radiant heating in the slab. Slab edges however should be part of your first round locations requiring attention regardless if the slab is below, at, or above grade. I lot of attention is being paid to slab losses in multifamily balconies now. That is why products like are becoming more common.

    As for updates on my personal build. Subscribe to and you will get every juicy detail.

  34. user-460363 | | #34

    Items 2 & 3
    Ron: yes, in a ideal world, the best way to ensure there are no bulk moisture problems is to pick a site that has no bulk moisture (except rain and snow).

    But to follow your prescription for #2 & 3 means that I need to sell my property and go try to find this mythical site (in these parts).

    Or I need to build with a FPSF, so we don't go deep enough to mess with ground water at all, theoretically.

    It's good advice. But practically not very useful.

  35. user-871583 | | #35

    Response to Ron
    I am commenting from the POV of a full basement as this is the only way we build in our region.

    If you have water draining through 8-10 ft of soil from the surface down to the perimeter drain, you have done something really wrong and have ponding water against your structure.

    2 & 4 are easily handled with a product like dimple board (Platon) to allow any moisture to drop down in an non-pressurized gap (capillarity break) to the perimeter where it is carried away.

    Water rising up as a fluctuating water table is the number one reason for perimeter drain in my view and if you would avoid sites with the problem, you would not be building in most of my region.

    "There is a big distinction between damp proofing and water proofing. Traditional damp proofing measures do not prevent water intrusion. They are not intended to." Totally agree, that is why more higher end homes are now water proofing because damp proofing in my region has proven time and time again to be ineffective.

    Providing a capillary break along the foundation surface and a drain at the bottom of the footing/slab is far easier to detail and more effective than ANY attempts to waterproof the concrete in my view.

  36. user-1140531 | | #36

    Response to Sean

    Regarding your comment: "2 & 4 are easily handled with a product like dimple board (Platon) to allow any moisture to drop down in an non-pressurized gap (capillarity break) to the perimeter where it is carried away."

    Are you referring to the gap between the wall of the dimple board and the concrete?

  37. user-871583 | | #37

    Dimple board gap
    Ron - yes.

    I have seen it installed with the dimples against the concrete in which case it provides a drainage plane along the concrete surface, for any water that somehow gets through it, as long as it was correctly detailed and capped with flashing to prevent dirt falling in and bridging the gap. The outer surface also acts as a barrier directing water away from the foundation before it hits the foundation surface.

    I have also seen it installed dimples out with a textile on the outside of the dimples to keep the dirt away (many products have this built in). This again provides a drainage plane that should see 0 hydraulic pressure as long as detailed right and the fabric is not damaged during installation.

    Without having researched the orientation yet within the building envelope community, I prefer the first method as it provides a ridgid second barrier away from the concrete surface.

    Sorry for the delayed response. I find the workings of this site quite frustrating in that you have to keep checking back to see if new comments and often I do not think to or have the time to check. I much prefer the functionality of LinkedIn that allows you to follow a conversation and automatically emails you of new responses. If there is not the budget to upgrade, they should just set up the comment section for each blog entry in LinkedIn and automate the process. Just saying.

  38. kbrod7 | | #38

    Drain to Sump Pumps?
    As an architect in Southern NY and Northern NJ, I typically need to provide 7' - 8' deep basements so my clients can make the most use out of their relatively small lots. The vast majority of these lots have relatively flat grading. Since we are usually not allowed to drain to storm sewers, these footing drains are typically routed under the footing into a sump pit consisting of an 18" perforated pipe in a 36" dia. gravel bed. Then they are pumped up to the seepage pits with an overflow to the storm drain system (if allowed). I've have had masons look at me like I have two heads when I tell them I want to bring the water inside the basement. These homes have either natural gas generators, municipal water powered pumps, or battery backup in case a storm knocks out power. Combined with liquid membrane waterproofing on concrete walls and drainage matting over, I have never had a drop of water in any of my basements. How are others draining the footing drains? By the way, I always show the drain low point at footing bottom 2" away from the edge of footing, they slope at least 1/8" per foot. Depending on the size of the basement I have 2 - 3 sump pumps so the drain piping does not get above the slab.

  39. user-460363 | | #39

    Same in Ontario, Kevin
    That's how they do it here too. Bring it inside, then pump it out. Code says to a drywell, drainage ditch or daylight. But the discharge has to stay on your property.

    Code also only requires dampproofing, not waterproofing. But I think most tract builders are putting in the dimple board anyways, because it's just easy and smart and fewer callbacks.

    Code does NOT require back-ups for if your sump power goes out. So when there's a monster storm AND power outages, then lots of people are in deep trouble.

    Your specs are the "deluxe" version of basement waterproofing around here.

  40. Bato | | #40

    filter fabric
    Filter fabric filters the soil keeping the fines from entering the gravel. The fabric is a filter and like all filters it will get clogged. Filters in cars are changed for this very reason. If you wrap the gravel with the filter fabric the water cannot get to the drain tile after the fabric is clogged with fines. If the fabric is laid on top of the gravel the finds cannot sift down into the gravel but the water can still perk up from below.
    I know this is true from experience. I have wrap the grave with fabric and it pugged up. I had to dig it up and do it over.

  41. user-1140531 | | #41

    I would wrap fabric
    I would wrap fabric completely around the top, sides, and bottom of the rock drain bed. Water will approach the tile from all directions and carry fines to the tile. The size of the drain bed also is a factor. The larger the bed, the more surface area of fabric it has; so the longer it will remain unplugged as a filter.

    But no matter what you do, if the system is intended to routinely drain water, the fabric will either plug, or the tile will fill with sediment and plug. That is why I would build it in a way that the tile is only a backup and never sees water under normal conditions.

    I think it is mistake to set up a sytem that routinely drains surface water down to the tile when that surface water could just be drained away while staying on the surface. There are sites that need continuous draining of underground water, but that can be done with a system that does not also include surface water draining into the tile.

  42. Billy | | #42

    Response to Sean

    I'm just catching up on your comment about slab edge versus underslab insulation. I generally agree with you that slab edge insulation can be better, but I suspect that the advantage is greater in cold climates with large temperature differentials than it is in the Florida example.

    Most of this discussion has focused on basement foundations and not slab on grade, and I believe foam under a slab and around the edge of the slab and up the interior walls as in the Building Science drawing works well.

    Thanks for correcting my slab edge insulation comment.


  43. nipsip | | #43

    Question on Slab Edge Insulation
    Anyone got any detail of how to cover the 2" of edge insulation of the slab if the homeowner wants to stain the slab and not add flooring. Cut the edge insulation at a 45 degree angle? Stumped here.

  44. GBA Editor
    Martin Holladay | | #44

    Response to Lee Hibbs
    You got it. Cut back the top of the insulation at a 45 degree angle.

  45. user-871583 | | #45

    Storm never goes to perimiter
    Completely agree with Ron - storm water should never go down to perimeter system. In BC it is a code requirement to have storm water piped separately from the perimeter.

  46. STCook | | #46

    Foundation Design Handbook
    The link provided in the article is old, the new location is:

    Foundation Design Handbook

  47. GBA Editor
    Martin Holladay | | #47

    Response to Stephen Cook
    Thanks very much for your helpful comment. I have updated the links in the article, as you suggested.

  48. graystones | | #48

    I read the entire post and lengthy comment thread, and it seems the entire discussion was about new construction. I am hoping there is more alignment on the best practices for retroactive foundation drain installations and waterproofing.

    I have an existing raised foundation with no concrete slab in the crawl space (just dirt) and the floor of the crawl space is about 18-20" below grade. There are several cracks in the concrete from about 70 years of life, and plenty of water finds its way in every time it rains. My plan is the excavate to the footer, install waterproofing on the positive side of the foundation and footer, and install a drain tile system.

    In this situation, would it be best to install the drain tile to the side of the footer, or on top of the footer? I'm considering either PVC drain pipe with 1, 2, or 3 hole patterns, or using something like the J-Drain system. Is there best practices or a consensus on what the best option is here? I can't find much in terms of reviews online for the J-Drain system.

    The landscaping will be graded away from the structure with a poly sheet, and the crawl space will get a vapor barrier installed after I'm sure I've solved the source of the water, but I'd really like to make this not so perfect foundation dry, keeping water out of the crawl space.

  49. GBA Editor
    Martin Holladay | | #49

    Q. "Would it be best to install the drain tile to the side of the footer, or on top of the footer?"

    A. As long as the pipe is below the level of the dirt on your crawl space floor, it's low enough to work. Of course, the pipe has to terminate at daylight for drainage.

    Q. "I'm considering either PVC drain pipe with 1, 2, or 3 hole patterns, or using something like the J-Drain system. Is there best practices or a consensus on what the best option is here?"

    A. I would stick with ordinary 4-inch perforated pipe, which has a larger diameter and is therefore less likely to clog than the J-drain system.

  50. graystones | | #50

    Martin, your answers are well taken.

    With respect to terminating at daylight, that would be great! It would be much more simple if my house was built on a slight hill, but here in Southern California most houses in my neighborhood are at street level. My footer is below grade by about 18-20" to the top of the footer. Consequently, I'm thinking that some sort of dry well or pump situation is required, although I'd much prefer to solve this with methods that do not have potential points of electrical failure. I only have about 15 ft in front of the house before I get to the sidewalk, and I'm guessing that's not far enough away from the foundation to be directing a bunch of rain water from the back and sides of the property.

  51. h_mott | | #51

    With a FPSF where should the drainage pipe go? In the attached detail, should the pipe go at location A or B? Both locations are below the slab. Thanks!

    1. Expert Member
      MALCOLM TAYLOR | | #52


      If the slab is above finished grade, you don't need a perimeter drain at all.

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