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Community and Q&A

Raised concrete slab?

Karl Ludwig | Posted in General Questions on

My wife and I just purchased a moderately sloping lot in northern Arizona, near Flagstaff and we’re in the initial stages of designing a roughly 1000 sq. ft. home. Due to it’s 6000+ft elevation the area can get significant snowfall in the winter. Since the lot is downslope from National Forest land we have concerns about drainage thru the soil, particularly with spring snowmelt. One solution we’re considering is a raised foundation that utilizes a minimum of footings; the idea being that a “floating” design would sit gently on the land and allow natural drainage to occur under the house as it continues downhill. Ideally we’d like a radiant heated concrete slab but realize this might not be practical. Would something like this be super expensive? And could it even be properly insulated for the cold winters?

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Replies

  1. Malcolm Taylor | | #1

    You can do it. A lot of multi-unit residential construction uses suspended slabs and there are a number of strategies for insulation them successfully, but in your situation I don't think it's a good idea.
    First, unless you have more than generalized concerns about run off, say an active watercourse or spring, there isn't sufficient reason to depart from conventional construction so radically. You can easily build a foundation that will not leak.
    Secondly, houses built on piers may in theory sit lightly on the land, but in practice the area underneath the house is usually a neither-nor space, with none of the positive attributes of either indoor space, or the natural environment around it.
    If you do decide to built that way, I would consider a hybrid system with a core built on a conventional foundation for your services and utilities, and the remainder of the house on piers. Good luck with your design!

  2. User avatar GBA Editor
    Martin Holladay | | #2

    Karl,
    Malcolm's advice is sensible. You might want to consult an engineer to obtain reassurance about a conventional foundation -- for example, a basement.

    One point to remember: in a cold climate, homes on pier foundations have no easy way to keep the pipes from freezing.

    If you have your heart set on a pier foundation, you may want to read the article on that topic in the GBA Encyclopedia: Pier foundations.

  3. User avatar
    Andrea Lemon | | #3

    We built a Passivhaus in Vermont on a pier foundation. It's not a raised slab, though -- our floor is made from 11-7/8" I-joists, insulated with dense-pack cellulose and with 4" of polyiso underneath. For details see http://www.vermontpassive.com/category/blog-subjects/foundation

  4. Bill Costain | | #4

    Karl,
    I agree with the others. I build in Colorado from 6,000 to 10,000 feet elevation and our climate is somewhat similar to Flagstaff. Not sure how the geology compares.

    Not sure exactly what you mean by "drainage through the soil", but snow melt is usually a slow and constant process. A heavy rainstorm is more likely to create water problems than snow melt.

    Proper site grading takes care of the surface water issues from upslope runoff. In the grand scheme of things it doesn't cost a lot to have an engineer evaluate your site and create a drainage plan with the recommend grade cuts and swales that will manage runoff. If you excavate for a basement you can usually use that dirt as fill to reduce the total amount of excavating and grading. Probably anywhere from several hundred to a couple thousand dollars buys a lot of piece of mind.

    A soils test will take a core sample of your site and the geotechnical engineers will be able to design a foundation that is well suited to your site. After excavation, an open hole inspection by the engineer confirms what the soil test predicted and helps to identify any trouble spots that may not have shown up in the soil test. While a soil test doesn't give a complete profile of whats underground, it's a reasonable indication that gives you confidence to proceed with a design. Around here a soil test is anywhere from a few to several hundred dollars.

    There are many construction details on GBA (proper surface contours, footing drains, underslab drains, crushed rock under basement slab etc. etc. etc.) that would help keep your house dry in most situations, unless there is an underground spring not identified.

    All engineering, design and construction represents a compromise, with economy, constructibility and durability the primary considerations. Engineers will be able to design a structure that suits the site conditions and goals. No structures are built to withstand every conceivable event that might occur--it would cost too much.

    OK, maybe NORAD here inside the mountain is designed for every event, including nuclear war, but other than that...... :-)

  5. Karl Ludwig | | #5

    Alright, thanks to everyone for all the great advice and expertise. Building this house is a longtime dream of ours and we obviously want to start off with the right foundation. We'll surely have more questions as the project progresses and we look forward to the "discussion".

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