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Community and Q&A

Backfill for Walkout Basement

Seeking_Wisdom | Posted in General Questions on

Bottom Line:  About to build new house. Architect and Soils Tester says that I need to use clean rock (1.5 limestone) as my back fill around our walkout basement – filled to top of existing grade and then covered with 6 to 12 inches of clay/dirt and grass graded away from house. The boundaries of rock/soil need to be separated with filter fabric.  Excavator / Septic Tank contractor disagrees. Says to use only 18 inches of clean rock then back-fill to the top with dirt. So one side is calling for all rock/ the other is calling for just partial rock.  Of course, we will have an exterior and interior perimeter drain, sump pump and will have the foundation coated with a very good water proofing mastic and have 3/4″warm-n-dri”  fiberglass boards around the exterior below grade.

Background: Wife and I are building a new house on our 100 acre farm in south-west Illinois.  The dig for the walkout basement begins in a few days, weather permitting.  The house in going into a ridge that drops 30 or more feet to a 2 acre pond that was dammed many years ago.  The downward hill (valley) continues to fall beyond the dam.    Our house in on level ground until this pond  valley starts and that is why we can have a walkout basement.  Everything – the pond, the dam – is well below the house. The soils engineer says that I have 3 layers of soil that will be penetrated by this basement and that the soil boundaries are transporting ground water.  I need to have my basement backfilled with rock to prevent hydostatic  pressure.  What doesn’t transport through the rock will fall to the french drain and be carried out to daylight to find its way to the down-hill pond.  The architect agrees. The excavator and the foundation guy say they have done this for many years and have never seen this done before.  The excavator thinks that it might backfire and actually trap more water around  the house and cause bigger problems. I seek advice and opinions ASAP — Please and Thanks!

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  1. joshdurston | | #1

    Can you provide some more details on what sort of drainage system is going to be against the vertical concrete basement walls beyond the mastic you mentioned?

    Preventing hydrostatic pressure against the house is a big deal. I wouldn't recommend compacting a couple feet the soil directly against the mastic/concrete even if it has 18" of clean rock beneath it.

    Properly installed the soil engineer's solution seems more robust, the water can get down and out the drain to daylight. But the foundation guy's solution could trap water against the foundation above the drainage plain if the soil is impermeable.

    WITH a proper waterproofing AND a drainage plane system, the foundation guys method might work fine too though.

  2. Seeking_Wisdom | | #2

    Josh - thanks for the reply. The exterior drain is the std perferated pvc french drain around the perimeter with exit to day light. It will be covered with clean rock and filter fabric no mater what. The interior drain system is also standard - perf pipe around footing and falling towards the sump pit. The exterior concrete walls will have a lifetime mastic applied by specialty company and will have 3/4″warm-n-dri” fiberglass boards around the exterior below grade to footing.

  3. Expert Member
    Peter Engle | | #3

    The engineer is right - bring the rock up to within a foot of the surface, then cover with fabric and cap with soil/clay. The entire rock backfill then acts like a curtain drain. Any water leaking out through the soil layers drains through the rock and into the perf. pipe drain, then out to daylight. Just make absolutely sure that the pipe is pitched slightly downhill and that it doesn't get crushed during construction. Include one or more cleanouts at surface level on the uphill side of the house, just for good measure.

    This is the bulletproof solution that lasts the life of the house. The foundation & septic guy's solution might work too, or it might not. The risk with using soil for backfill, especially if it's got clay in it, is that water from the intermediate layers can "perch" in the backfill and eventually find its way into the house. The warm 'n dry insulation board does act as a drainage board and the waterproofing should still do its job, so this approach can certainly work. It is just not as robust a solution. One concern is that over time the warm 'n dry can get crushed and filled with soil fines. That won't happen with the stone wrapped with fabric.

  4. Seeking_Wisdom | | #4

    Peter - than's for responding. I like the idea of clean-outs. As far as the warm-n-dry, it is between the clean rock and the mastic. The clean rock is covered with fabric on three sides - top, bottom - dirt side. My concern is our waterproofer doesn't use dimple mat - says it is not necessary when using warm-n-dry fiberglass. He says he doesn't mind selling it but it is absolutely unnecessarily. The rock fill is my curtain drain. His product (tuff-n-dry) is time tested. Its all guaranteed for a life time. Sounds good but are there extra precautions I should take?

    1. lukievan | | #8

      Hi - possible to get a follow-up on what you ended up doing with the waterproofing and backfill and any observations on how it's all working thus far?


  5. Peter Yost | | #5

    Seeking Wisdom -

    You don't include in your description what sort of waterproofing system you will use against the exterior of the foundation.

    I recommend you waterproof rather than dampproof (good resource on the difference here: and consider a dimple mat system (


  6. Zdesign | | #6

    +2 on the dimple mat. Typically on my drawing sets I include notes along the lines of fill with sand or bank run gravel only, no on site soil to be used. Those two soil types will allow for proper drainage similar to rock down to the footing drains. Keep in mind, what is on the stamped drawings is required.

  7. user-7022518 | | #7

    I would like to know more about the "warm-n-dry" system.


  8. Seeking_Wisdom | | #9

    Lukievan asked for a follow-up. This is what we did and it has worked out very well:

    1. We went with the Warm-N-Dry system, installed by Champion Waterproofing company. They sprayed a hi-tech mastic like substance on the foundation then installed 3/4" fiberglass panels over the spray. The cost for this system was approx. $2500.00 for a 3000 s.f. footprint with 9 ft. pour. A dimple matt would be better but I couldn't find anyone to do it and had too much on my plate to do it myself.

    2. My son & I installed two separate drains on the outside and a third one in the inside. We used schedule 30 perf 4" pvc below the top of the footing. We used clean rock and lots of heavy duty filter fabric to keep them clean. This was overkill and we probably only needed one outside drain.

    3. We used clean 1 to 1.5 inch limestone to back fill, just as Peter Engle suggested in his posted reply. I have my own skid steer so it wasn't too hard - except in those areas where it was too tight for a machine. Then you must resort to wheel barrows and buckets. Lots of filter fabric on top. Slope away from foundation. Grading is key. Note: careful with the limestone. It must be installed slowly and make sure you have your floor joists and subfloor down before going higher than a couple of feet.

    4. We put down lots of rock and a sheet of 10 mil plastic, just for basements and concrete floors, before pouring the basement floor. We went with a 6 sack fiber mesh 4.5 sump concrete to minimize shrinkage cracks. We also had rebar 2 feet on center and poured the floor a true 5 inches thick. A year later, my wife caulked the separation between the floor and walls in the basement with a polyurethane caulk made for concrete.

    Knock on wood, no problems to date.

  9. lukievan | | #10

    Thank you very much for the update!

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