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Could we use a raft slab with ICF walls on a sloped site?
Paul Fischer
Sept. 15, 2020Updated: Sept. 17, 2020
We are thinking of building an earth-bermed home cut into a hill (think modified Earthship) in west-central Alberta. Because of the sloping grade, footers below frost would require a number of different depths to stay 4+ feet below grade. I would use ICFs in lieu of the tires that Mike Renolds advocates, as I am 73 yrs old and stacking and filling hundreds of used tires is WAY beyond my capabilities. I guess what I'm asking is, would a raft slab tend to float away from the uphill side?
We would strongly advise against building a house out of old tires at any age! I think you are very much on the right path with a raft slab / slab on grade shallow foundation and and ICF block wall. But I think building the slab on grade would be more simple than you think, you can read more here if you haven’t already found this page-
Raft Slabs – how to build a slab on grade or basement on problem soils
A raft slab is engineered to spread the weight of the building evenly over the entire surface of the slab, so there is no footings needed, and no, its not going to go anywhere when done properly, and a perimeter skirt insulation is added to prevent frost heave.
So what you may be able to do, is dig out the portion of the hill that would be the back wall, create a level flat surface that big enough for the footprint of the building, and lay the slab on it. From there you would build up with your ICF walls and then backfill it and you’re done. It’s not always easy to find an independent engineer that will design a raft slab, so I would suggest you contact legalett that provides slab on grade prefab kits across Canada and the US. We used one on our last demonstration home builds, here you can see a time lapse DIY raft slab / slab on grade installation video.
And if you're interested in the definitive history of ICF Construction and ICF Blocks, that's here.