Tying in a Frost-Protected Shallow Foundation
An FPSF can be used in place of a frost wall in cold climates where the frost depths are greater than 12 in.
The house we’re building is on a sloping lot, and I’m designing the footings and foundation. Three sides will have 9-ft.-tall foundation walls, and the downslope side will have a frost wall for an at-grade walkout. We’d usually dig and pour a 4-ft.-deep frost wall buried on both sides (our frost depth is 42 in.). However, the groundwater is about 2 ft. below the slab level during spring when we’re planning to break ground, so we’ll be swimming in a 32-ft.-long puddle to form and pour the frost wall.
The concrete contractor suggested using a frost-protected shallow foundation (FPSF) instead of the frost wall. I’m not familiar with this type of foundation. Can it be connected to a regular foundation?
—Derek C., via email.
To your question about connecting full-height foundation walls to an FPSF, Section R403.1 addresses this. It states, “Foundations that adjoin frost-protected shallow foundations shall be protected from frost in accordance with Section R403.1.4.” That section addresses minimum footing/ foundation depth and frost protection. Since the other three foundation walls will be 9 ft. tall and backfilled, the footings will be below your local 42-in. frost depth. They are protected from frost, so you can connect the walkout FPSF to the main foundation. On homes where I’ve done this, we formed 18-in.-deep footings along the walkout area to serve as the FPSF and connected them to 8-in.-deep footings that continued beneath the full foundation walls. With just a short step between the different footing levels, the excavation and forming was easy. We formed and placed all the footings in one pour using the FPSF method instead of the two-step process a deep frost wall would have required.
Illustration by Dan Thornton.
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