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I'm planning a storage shed underneath a deck. I'll assume that the deck is anchored below the frost line and won't move when the ground freezes.

I'd like to build the shed on just a floating gravel pad, which I assume will rise a bit when the ground freezes.

Is it possible to estimate how much overhead clearance is necessary to account for the heave displacement, such that my shed doesn't end up bursting through the floor of the deck?

Luckily I'm in a pretty temperate area and the soil frost depth in my area is only 12 inches, so I expect I won't have too much displacement to worry about (thinking about something like 6" of overhead clearance at highest point), but just wanted to see if anyone had a knowledgeable answer.

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    I think you would've noticed by now if the ground in your backyard rose 6" every time it froze....
    – Drew
    Commented May 24, 2017 at 19:26
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    Yeah, not much of a concern. Frost doesn't usually move stuff that rests on the surface very much anyway. Frost "jacking" is a bigger problem.
    – isherwood
    Commented May 24, 2017 at 21:27

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Think outside the box.

If you are trying to minimize the distance to the deck to have max headroom, then attach the shed roof to the deck, so that water coming through the deck runs off.

Now build your shed but stop the walls a foot from the suspended roof.

From the roof hang 18" walls that have 1" clearance from the sides of the shed. Stuff with insulation. Now if the shed moves up and down, the walls frm the floor telescope with the walls from the ceiling.

Edit in response to comment: If you use metal roofing, you can screw the roofing to short chunks of 2" dimension wood cut in various widths to create a slope to one side. this will add 1-2 pounds per square foot to the deck.

Note that this won't work well if you have support posts in the area that you want to use as a shed.

Downsides of this sort of construction: Debris from leaves, seeds, spilled food that falls between the cracks accumulates on the metal sheeting. Since the sheeting is attached to blocks on the under side of the joists, this stuff can get trapped, and build up. It also is a great nesting spot for wasps that is difficult to access.

You may want to just build a shed under the deck and use 2x4 or 2x6 rafters. Because of the deck protection, it can be pretty flimsy. It will never have to carry a snowload, or the weight of anyone walking on it. Metal roofing is routinely used to span 4 feet on center purlons. The difficulty with this approach is attching the metal to the frame, unless you build the shed before you build the deck. If it's a small shed you may be able to build the roof on the ground, and carry it into place with 3 friends.

There are deck coverings that are waterproof, but these generally require that you deck with 3/4" T&G plywood or OSB, then put a glue down membrane, then a protective coating. They tend to be pricey. However they do clean up nicely, and don't trap crud between the planks.

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  • Maybe, but now you have to be sure your deck is up to supporting a shed roof in addition to its normal duties.
    – isherwood
    Commented Jun 30, 2021 at 19:51
  • True, but this is pretty trivial weight. Deck is usually spec'd to 30-50 lb/sqft live load. Commented Jul 2, 2021 at 11:36
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The best defense of frost-heave is to eliminate the ingredient for occurrence.

Frost-heave needs three ingredients to work: 1) freezing temperature, 2) water, 3) path for water to rise through capillary action.

We can't control the weather, and often the water, but the third ingredient is something we can eliminate by placing coarser backfill beneath to cut off the flow path. Luckily you are located in a warmer region with such shallow frost depth. I suggest digging the ground down 18" to ensure the base starts below the frost penetration depth, then fill it with compacted gravely fill.

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