Foundation question: Slab on grade or monolithic?
I am going to build a small 12 x 16 Timber Frame house in the Intermountain West – Zone 6. I attached a rendering. One question I have is about the foundation. I think I am confused about the terminology that I hear. Would it be preferable to do a monolithic slab or a slab on grade with a stem wall? I know there are lots of details here and in Lstiburek’s Building for Cold Climates but I’m wondering what a pro might recommend (no basement or crawl space). I am also going to forego any tubing in this small slab since the space is going to be fairly small and I don’t want to house too much mechanical. I will enclose it with SIPs. Snow loads are significant (90#) so I’m curious to hear what a pro might recommend. I do plan on prepping the slab, doing the proper drainage, radon mitigation, insulation below the slab but I just want to use the correct terminology when I put the foundation out for bid (I may do it myself).
Basically, this structure will serve as a “shop” while I build my main house and subsequently be an Accessory Dwelling Unit.
Thanks in advance.
GBA Detail Library
A collection of one thousand construction details organized by climate and house part
Here is a rendering of the structure.
The traditional approach for a cold-climate slab would be to install frost walls on footings. The footings would need to be installed below the frost line -- probably at least 4 or 5 feet below grade. The frost walls are filled with compacted gravel, and the concrete for the slab is placed on top of the compacted gravel.
About 30 or 40 years ago, the Scandinavians showed the world that you can install a monolithic slab on top of well-drained substrate, with various configurations of rigid foam (horizontal rigid foam under the slab, vertical foam at the perimeter of the slab, and in many cases a "skirt" of horizontal rigid foam around the perimeter of the slab, beyond the footprint of the building).
The Scandinavian approach has several variants, called monolithic slabs, thickened-edge slabs, raft slabs, or frost-protected shallow foundations.
Either approach can work. I suggest that you choose a local concrete contractor and use the method that the contractor is familiar with.
For more information, see these articles:
Insulating a slab on grade
Frost-Protected Shallow Foundations
Thanks Martin. I just got word from the building inspector that "most of the time it is a footing and stem wall for the foundation." Given that, I guess it's gonna be a 2-part pour or possibly 3, right?
First will be the footer, then a capillary break applied, then the stem wall and finally the internal slab.
Or could the stem wall and internal slab be poured at the same time?
Just thought I would follow up with the response from the local building inspector which probably reflects what local concrete contractors do. Thanks for all your prompt and thoughtful recommendations.
Q. "Could the stem wall and internal slab be poured at the same time?"
A. No. After the stem walls are poured, you have to wait at least 24 hours before you pull the forms. Then it's best to wait a bit for the concrete to cure before gently backfilling at least one side for access. Then you have to haul several loads of gravel to fill the foundation. You have to install the gravel in lifts, compacting each lift with a plate compactor. Once you reach the desired height, you can then begin to prep for the slab pour.
My advice: Hire a knowledgeable concrete contractor who's done the work many times.