I live in Fairfax, VT, zip is 05454. I'm building a deck, and I'm going to
be pouring concrete into sonotubes. Everything I've read states that the
bottom of the sonotube must be below the frost line. Nowhere does it state
how deep that might be. I assume that the USDA or the department of the
Interior must have a map on this kinda thing, but for the life of me, I
can't find it. Any ideas?
The USDA Zone map might be a good guideline. I am in Zone 6 next door to
you. Bottom of footing 18" below finished grade here.
Your local building inspection office can tell you for sure what it is in
BTW, around here at grade decks just float. Not sure what the rules are for
those over 8' off the ground.
You need a local source, building inspectors,
contractors, etc. Depth of frost can vary
tremendously over a short distance. And those
that use the published depths may end up with
major repairs during an extreme year. For your
purpose it is probably not that critical.
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