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I'm thinking of putting a small shed (something like this) and wondering what I need to do to prepare.

I was thinking of getting a load of gravel, and having a bed of 4-6" of gravel, with deck blocks on top of that. The area under consideration isn't quite level, and gets a bit damp in the spring.

Or would just putting down the deck blocks (digging them in a bit to ensure they're all level) be sufficient?

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2 Answers 2

I looked at your profile, but don't know what part of the country you are in. Where you are makes a lot of difference. The building you are considering is a wooden structure, fairly heavy and can hold some cargo. Any building that size deserves a decent foundation. If you don't support it right, it will warp, shift and bend with the seasons, resulting in screwed doors and separations on the siding.since you said the ground can be wet in season, I would recommended a foundation that keeps it off the ground, a vapor barrier to protect he floor and is deep enough to withstand ground heaves if you are in an area where the ground freezes in winter. If you are in a non-freeze area, I think you may want to at least support the corners and side walls with blocks at least 12 inches in the ground. Any and all water you can stop from attacking your new building will increase it's life span. Your idea of a gravel base with supports would be a minimum. Let me know where you are, and I may be able to give you better advise.

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I'm in New Brunswick, Canada. Long winters with lots of snow. –  chris Mar 22 '11 at 0:25
    
I'm in Maine and we typically put in 4 foot sauna tubes or pyramid preformed porch posts to get below the frost line. –  shirlock homes Mar 22 '11 at 3:40

Long, cold winters means the ground will freeze deeply. The frost will heave your structure. Snow is a good insulator, which will reduce freezing. But a wet autumn followed by a cold, dry winter will make the frost heaves deep and strong. Your local building department can tell you the specified frost depth, which leaves a suitable safety margin.

The conventional way to address this problem is to pour a concrete foundation that reaches below the foundation, resting on a footing that is wide enough to distribute the load so it doesn't sink.

A rubble trench foundation can save you the trouble of pouring such a deep foundation. Basically, you ensure that drainage is so good that there won't be enough water in the soil to frost heave. See http://en.wikipedia.org/wiki/Shallow_foundation#Rubble_Trench_foundation

You could use concrete piers. You only have to go deep in a few places, instead of all around the perimeter. Sonotube is a common product for this purpose. Similarly, you could put pressure-treated posts in the ground.

You could use adjustable pier blocks on grade (just remove the topsoil). They will heave and settle. Every spring you adjust them to restore them to level.

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