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Community and Q&A

Sub-slab drainage details

user-6356169 | Posted in Green Building Techniques on

New construction, planning out the foundation and basement details … I’ve asked our builder to install drainage pipes under our basement slab (~1000 sq ft), and have them connect through the foundation to the footer drainage pipe. Builder says ok (even though he considers this overkill), but it’ll cost us dearly in extra gravel. Had originally planned for a 4 inch layer of gravel (topped by 2 inches of XPS, then poly.) Builder says to install the 4 inch drain pipe properly would require about 8 inches of gravel. Alternatively, he said, we could try just digging a few trenches where the gravel could reach 8 inches in depth, and set the pipes there, leaving the rest of the sub slab area with only 4 inches of gravel. Just wondering … how is this normally done? With a deep bed of gravel? I’ve heard that 4 inches of gravel beneath a slab is standard and should be sufficient, but then how do you install 4 inch drain pipes therein? What about this idea of a few trenches with 8 inches of gravel?

Also, is there any merit to the idea of sub slab drainage being somewhat “overkill”? (We’re on modestly sloping land, but don’t know much about soil profile or groundwater level at this stage.)

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  1. GBA Editor
    Martin Holladay | | #1

    I don't think that the 4-inch layer of crushed stone under the slab is overkill. And needless to say, footing drains are a code requirement.

    There are different approaches to subslab drainage, however. Some builders include perforated pipes under the slab; others assume that if the perimeter is drained (via the footing drains), then there is no need to have a perforated pipe under the slab.

    While this is a judgment call, I think it's a good idea to include at least one pipe that goes through the footing to connect the sub-slab stone with the footing drain. Note that this pipe can go through the footing -- it can interrupt the concrete. It doesn't have to go under the footing. (Check with an engineer if you don't believe me. The poured concrete wall above the footing can easily bridge the 4-inch interruption that the plastic pipe represents.)

    The wrinkle concerns radon. If you are including a perforated pipe for radon collection under your slab -- and you probably should -- you can make a valid argument that this radon collection system shouldn't connect with the footing drain system. Separating these two systems is an argument in favor of putting the drainage pipes at the perimeter of the footing, without a pipe through the footing, while keeping the sub-slab area for radon collection.

    In my opinion, either approach works. The crushed stone pad allows water to find the pipes. I like to see at least a couple of inches of crushed stone under the footing, but I know that some old-time contractors want to see the footings on "undisturbed soil."

    -- Martin Holladay

  2. user-6356169 | | #2

    Thanks Martin. To clarify … the 4 inches of gravel under the slab is not in question. We definitely need that and a pipe venting the gravel bed through the roof for radon control. One question is do we need sub slab drainage at all, and this you say is really a judgement call.

    But if we do decide to have sub slab drainage, seems we might need a deeper bed of gravel, perhaps 8 inches, so the 4 inch drain pipe can have some clearance above and below. Alternatively, builder has suggested just making some deeper gravel-filled trenches and laying pipe in there, but keeping the rest of the gravel bed at 4 inches. Seems like that should work, no?

    As far as radon goes … are the same pipes typically used for radon collection and drainage? And why is that considered a potential problem? Would using separate pipes for each system solve the problem?

  3. GBA Editor
    Martin Holladay | | #3

    Q. "Are the same pipes typically used for radon collection and drainage?"

    A. No. If the radon pipe ever gets connected to an exhaust fan (thereby creating an active system), you don't want the fan to be pulling air from the "drain to daylight" pipe. You want the radon fan to depressurize the sub-slab gases.

    Q. "Would using separate pipes for each system solve the problem?"

    A. Not entirely, but separate pipes are still recommended. More information here: All About Radon.

    -- Martin Holladay

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