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I'm a novice constructing a 8x12' saltbox style green house made from recycled single pane windows in climate 5 & 4 Marine. It does get freezing here in winter with maybe an inch to three of snow a year.

I plan to half lap treated 6x6's with 2x6 sills leveled on gravel with rebar anchors. Since this is a permit less structure (< 200sqft in my county) I'm concerned about possible frost heave and want to know if this foundation is sufficient for such a structure. Would I need to find my frost depth and put in say, 4 or 6 sonotubes/pads to anchor my 6x6's?

my greenhouse plans

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The short answer is: No, don't worry about it.

Your building is basically a yard barn and no building code I have seen requires frost footings for a yard barn.

I live in a Zone 5 with plenty of freezing and snow and separate buildings like garages are only required to have a 12" slab on grade footing. Yard barns can be place right on the grass if you want. Although, most people scape off the topsoil and place gravel to level the barn and provide some moisture reduction.

I don't think your building is large enough to be affected by frost heave in any appreciable way.

All that said, if you are an ovebuilder and want this to be permanent then a deep sonotube footing in each corner would certainly keep it from going anywhere. Anchoring it to these footings would also give you some wind stability.

Good luck with your project!

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  • Would scaling to 10x12 or even 10x16 make this a concern in your opinion?
    – Vetsin
    Jul 23, 2017 at 19:35
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    Not really. Small buildings (and 10x16 would still be pretty small) that essentially float on the ground are free to move up and down with the freeze and thaw of the soil. I would still remove the topsoil and stabilize the area with compacted sand and then pea-stone or crushed stone. The only other concern is mounting your panes of glass so they have a little room to move, expand and contract. If the building does flex slightly you don't want your glass held so rigidly it can't move without cracking.
    – ArchonOSX
    Jul 24, 2017 at 9:15

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