How To Decide On On-grade Or Frost-proof For Wood Shed Foundation - Building & Construction - DIY Chatroom Home Improvement Forum

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Old 08-11-2013, 09:40 AM   #1
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How to decide on on-grade or frost-proof for wood shed foundation

I'm planning on building a wood-shed near my house, 9' x 12', to hold 4 cords of wood for winter heating. We get so much winter rain (PNW) that a wood shed is essential for keep dry fire wood.

I'm unsure what type of foundation I should use. The plan is to build a simple 'post and beam' style, open sided, metal sheathed shed style roof. The roof's high side will be to the south, which is also our weather exposed side. Our frost depth is 18 inches.

The sources I've read don't agree, some say go frost free (concrete pier footings below frost level) some say on-grade is fine. I'd like to build a shed to last a couple of decades, but if an on-grade foundation will suffice, I'd happily avoid digging 2 foot holes and dropping all the cash for cement and quick-tubes.

What would you advise?


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Old 08-11-2013, 10:02 AM   #2
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The reason frost free foundations are used is to avoid frost heave in the winter. This is essential for structures designed for human habitation, but I doubt your firewood really cares if the floor (if you even plan on having a floor) is uneven. I have two similar sheds for storing firewood (I store about 6 cords under a roof in two sheds). I am in Massachusetts, where the official frost depth is 4 feet.

One of the sheds is built on a concrete block "foundation", with a total excavated depth of about 12 inches, and the blocks were laid directly on the soil. This was in 1959, when the house was built, and the shed is in perfect condition.

I built the other shed originally as a chicken coop, but we don't have any chickens anymore, so I use it for firewood. I have a few sonotubes supporting the sill beams. The sonotubes are only a couple of feet into the ground. In both cases, I don't really care if the foundation moves around a little, since both sheds have dirt floors (actually a few inches of gravel to stop wicking of moisture into the firewood).

It is worth noting that frost heave only occurs under specific conditions. Three things are required for frost heave. Freezing temperatures, moisture, and either very fine sand or silty soil. If you don't have all three conditions, no frost heave, no problem. My soil is sandy gravel, relatively little silt, so neither foundation has heaved in over fifty years of life.

If you do not have frost heave soil conditions, I would save the work and put down a block foundation. Excavate to natural mineral soil, compact a little, then add a couple of inches of gravel or crushed stone, put down the block, and go from there. If you have frost prone soils and are concerned about heave, then install sonotubes to below frost depth. A little more work, but really not much, and very low cost.


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Old 08-11-2013, 10:09 AM   #3
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I don't have a frost problem where I live....but we do have moisture and critter issues.

What ever you do, you want to make sure water stays off the bottom of the wood (the higher you get it, the better) and I would make it so that critters can't take up shelter under it.
Even if you are on the right track, you will still get run over if you just sit there.

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Old 08-11-2013, 10:17 AM   #4
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I neglected to mention that the plan for the floor is to put down some free pallets, directly on the ground, and to stack the wood on the pallets.
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Old 08-11-2013, 12:09 PM   #5
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Originally Posted by dochockin View Post
I neglected to mention that the plan for the floor is to put down some free pallets, directly on the ground, and to stack the wood on the pallets.

Wood should never be in direct contact with the ground, at least elevate the pallets with cmu's if you want them to last.
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Old 08-11-2013, 04:14 PM   #6
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I would build it on grade and not worry about it.


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