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Underground Roof Depth Clarification

brrowle | Posted in Green Building Techniques on

Hello GBA,

My house sits on a slope with one long side of the house within 6ft of a hillside that just piles on the water… perhaps not surprisingly the basement cove joint leaks on that side!

I’ve read through the underground roof and gutter articles, searched the forums, and believe that an underground roof that terminates in either a swale or shallow French drain (draining to daylight far from the house) would be help relieve the water load.  To be clear, the swale or French drain would sit at the base of the hill and the base of the proposed “underground roof”.

My question is, since I am not looking to grow anything on that side of the house, would it be acceptable to make this a “surface roof” rather than an underground roof?  In other words, lay the membrane just an inch or two beneath current grade, then backfill all the excavated portion with crushed stone?  The soil beneath would follow the recommended grade and I’d backfill near to level with the stone.

Would I be compromising the function by putting this so close to the surface as opposed to the 8″-12″ recommended?

Thank you!
Brian

https://www.greenbuildingadvisor.com/cad/detail/underground-water-barrier-retrofit-rubber-membrane

https://www.greenbuildingadvisor.com/article/an-underground-roof

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Replies

  1. Expert Member
    MALCOLM TAYLOR | | #1

    Brian,

    I don't see why it would make any difference if the underground roof was moved to grade. It would function much as hard surface patios or walkways do to divert surface water way from the foundation. I suspect the reasons it is usually below grade are to protect the materials used, and for looks.

    A caution though. They are only effective at stopping surface water, and on sloped sites much of the load on high-side foundation walls comes from sub-surface flows which get interrupted on their way down the hill by the house. The usual remedies for that are trench drains to divert it before it can get there, and a combination of waterproofing and dimple matts on the wall itself.

    1. brrowle | | #2

      Thanks very much for the reply, Malcom. I'll look into the trench drain and dimple mat solution as it sounds like I'm addressing the wrong issue with the underground roof.

  2. Expert Member
    PETER Engle | | #3

    Trench drains are also sometimes referred to as "curtain drains." To be most effective, the trench would be dug as deep as your foundations. It then gets lined with geo fabric and filled with clean stone. If the trench extends to the surface, it will work to capture both surface and subsurface water. Just as with interior drains, there is perforated pipe in the bottom of the drain and the pipe drains to daylight.

    Do be careful when installing it. With a slope that close to the house, you could destabilize it and have the whole thing sliding down. The risk decreases with the angle of the slope and lots of other factors. You would definitely want to wait for a dry stretch before doing the work.

    1. brrowle | | #4

      Hi Peter,

      Greatly appreciate the response and your insights. I think I understand now what must be done. My understanding is that the trench/curtain drain is essentially what is being described in the following article:

      https://www.greenbuildingadvisor.com/green-basics/foundation-drains-2

      Is this correct?

      Thanks again!
      Brian

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