Frost-Protected Shallow Garage Slab

The details in the IRC come from ASCE Standard 32-01, which also details how a frost-protected shallow footing can be used under an unheated structure. In short, I had to insulate under the slab (which I planned to do anyway) and the footings and 4 feet beyond the perimeter of the building with 2-inch rigid foam. The foam needed to be laid over a 6-inch-deep layer of crushed stone for drainage, then covered with at least 10 inches of soil.

Once the utilities passed inspection, I backfilled and compacted all the disturbed areas, then spread a 6-inch-thick layer of 3/4-inch crushed stone everywhere, which I also compacted.

The forms were a piece of cake. I could have used 2x12s, but green 5/4?x?12 rough-sawn poplar from a local sawmill was cheaper. I ripped 5/4?x?4 whalers from the wider stock, and braced the forms back to stakes.

The concrete work was no different than any other slab I’ve ever done — a sleepless night followed by a hard day’s work. I used 3,500-psi concrete with fiber. Because it was a hot day, I ordered the concrete with retarder so there’d be more time to finish it.

The framed garage walls are supported on an 8-inch block knee wall.

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