Foundation Recommendations, zone 6
Hello,
I just had a survey done, which changed my assumed property line, and has necessitated me choosing a building site on my property that I hadn’t previously considered. Coastal Maine, zone 6. I am the owner builder and a contractor, but my expertise does not extend to foundations. I am looking for a push in the right direction so I can begin researching a foundation type.
I have designed a double stud PGH, about 1000 square feet, and prefer to build green. I have no need for a basement or crawl space. The building site is about 30 feet below a long ledge wall (30′ is the setback, the wall is the boundary), currently a mossy forest. Water comes off the ledge, and as a result, the soil is pretty wet. The soil tester thinks it might be too wet for a drain field, but we will see next week. (we might have to pump uphill) In general, its about 1.5 feet of duff above a thick clay pan. The grade at the site is about 10 degrees, sloping away.
I had previously intended to build on piers, since i was hoping to build on ledge, but am now considering a slab, or perhaps building a 2x’s over one with a slab for the ground level and half of the 2nd floor supported by piers, which is similar to the original design.
Choosing a slab type for this site is whats got me stumped.
Thanks very much.
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Replies
stephenr,
Monolithic slabs work best on flat sites. If you don't want to build up the site, or dig down and retain the grade on the high side, stem-walls and a slab would be my pick. The detailing is pretty simple and within the skill set of any builder. That's how my own house is built.
How do monolithic slabs work in frost prone areas (around 4' in Maine, though probably shallower along the coast)? Or is the presumption it would be frost protected with insulation? (I realize you advised against monolithic due to the slope here, im just curious as an aside).
maine_tyler,
It would have to be a FPSF.
Thanks.
So, essentially 8 inch wide foundation walls around the perimeter that rise say, 2 feet above grade, and are silled for the exterior walls. Interior of this would be a slab. The stem walls would go 4 feet down below the frost line and there wouldn't be any need for frost protected wings or any of that. Would the stem walls be insulated from the inside? Would the slab be insulated from underneath?
Also, for future reference, is this assembly referred to by any other name other than "stem walls and a slab?"
Stephen
Stephen,
Yes that is a good description. You have a few options on how to insulate it. These articles show the options.
https://www.greenbuildingadvisor.com/article/different-types-of-insulated-slabs
https://www.greenbuildingadvisor.com/article/insulating-raised-slab
Or by "stem wall" do you mean frost wall, that is, a below grade foundation to support a slab edge?
Stephen, stem wall is the proper term; "frost wall" is a vernacular term for the same thing--a wall that is unsupported at the top. They don't have to be 4' below the frost line; the frost line in most of Maine is 48" below grade so the bottom of your footings need to be at least that far below grade.
I design homes in Maine and do a lot of FPSFs but for a current project in Pownal with similar conditions to yours, we involved a geotechnical engineer and a structural engineer and consensus is to use frost walls over 12" of crushed stone with footing drains to open air. When I use this "raised slab" detail, I insulate under the slab and bond out a shelf in the concrete wall (sometimes called a reverse brick ledge) so the slab sits in a "bathtub" of foam.
Thanks Michael. I think i will pursue a FPSF until I reach the point where it doesn't seem viable. The cost factor of having a pump truck show up three times, at 1000$ a pop, to do the footings, stem walls and then slab is kind of driving this. I believe my site can be leveled, (it varies between 5-10 degree grade) and I would rather head in this direction. Its less concrete as well.
https://www.greenbuildingadvisor.com/article/minimizing-concrete-in-a-slab-on-grade-home#0
Michael,
I found this article you wrote several years back and it is very interesting to me. I also plan on doing pine floors and am not considering radiant heat. I like the smaller amount of concrete . Two questions, please.
1. Since this design incorporates a frost wall, is it reasonable to assume that the soil requirements of the site (grade and wetness) would be less strict than for a traditional FPSF?
2. Is there anything I need to know concerning code or permitting for this design?
Thanks?