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Hillside against building: Is concrete the only way?

joenorm | Posted in General Questions on

I am building a small building where the lower 2-3 feet of the uphill side will have to be right against the earth. 

My plan was to build slab on grade. Is the only way to do this properly to pour a concrete stemware on the uphill side? 

Is there a way to detail wood framing to be able to backfill against it? For instance could you flash properly, add appropriate waterproof membrane, apply siding that is ok with moisture, then backfill with gravel?

I know a lot of people will object to this approach but perhaps it’s being done with success?

thanks

 

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Replies

  1. Patrick_OSullivan | | #1

    > Is there a way to detail wood framing to be able to backfill against it? For instance could you flash properly, add appropriate waterproof membrane, apply siding that is ok with moisture, then backfill with gravel?

    If you search for "permanent wood foundation", you'll find things like this: https://www.greenbuildingadvisor.com/article/the-bs-beer-show-permanent-wood-foundations-pwfs which also has the link to a BS and Beer video on the topic.

    I would suggest taking a look there as a starting point. You can then decide if you want to keep digging or if you'll pursue another options.

  2. Expert Member
    KYLE WINSTON BENTLEY | | #2

    Joe,

    That 2-3 ft of backfill is a lot of weight should the hill ever decide to move on you. I’m not sure how large your building is, but perhaps it would be easiest to build that section out of CMU, and then build the rest of the wall on top of it.

    Four courses with an appropriate amount of reinforcement is within reach of someone who has never laid block before. With enough YouTube and reading you can knock it out about as well as anyone else, it’ll just take more time and a few more tools.

    I wouldn’t trust an unbalanced load like that to a permanent wood foundation. It’s one thing to have a load that’s roughly the same on all four sides, counteracted by the slab below and floor system up top, vs a large load on one side of the building only.

    Not only is there a differential pressure on that section, all of the water from the hill is going to be hitting that wall. That’s a lot of water, and I’d rather have something that is easier to waterproof and have a little resistance to decay than even treated lumber.

  3. jollygreenshortguy | | #3

    You've got some good answers above. I just wanted to mention another option. If space allows, you'd probably find it cheaper to build a small retaining wall 3' away from the house. It would require just a little more excavation. But the cost of the retaining wall would be considerably less because it wouldn't be part of the building itself. You could even just use retaining wall blocks. It would be a simple DIY project.

    Another advantage of this approach is that it completely eliminates moisture issues related to having earth against the house wall. Just install a good french drain in the space between the house and the retaining wall.

  4. Expert Member
    BILL WICHERS | | #4

    I would not trust a wood foundation in any kind of high-stress application like this. I don't really trust wood foundations at all for anything. The big issue is that wood foundations are a lot more likely to fail compared with concrete, and if you have a major foundation problem down the road, your best case scenario is a very expensive repair. The worst case scenario is part of your home collapses. Concrete is a much more permanent material and better suited to heavily loaded underground structures like foundations.

    BE SURE to include some additional drainage between the hill and that wall, regardless of what you use to build the wall. You also need some extra good waterproofing on the exterior side of that wall. My recommendation would be a poured wall with continuous water proofing, AND a dimple mat over that. Drain the dimple mat into a gravel bed next to the footing, and put in drain tile with a sock in that gravel bed at or below the level of the bottom of the footing. In this way, you ensure that you never have water exerting hydrostatic pressure on that wall that can cause seepage into the interior space, and you drain any water away to protect the foundation.

    Bill

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