Deck Load and Soil Capacity?

by Deb
(Illinois)

My husband built a deck around our above ground pool. Problem is we had no permit. We are now trying to obtain one and the township wants to know the PSF. How can we calculate that on an already existing deck??

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May 04, 2010
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Load for a deck and the soil
by: Editor - Rich Bergman

They are probably referring to the design load capacity of your deck. Decks are supposed to handle a minimum load of 50 psf. That is 40 psf of live load and 10 psf of dead load. The dead load is the weight of the structure and live load is the weight of the things that go on the deck - people and things.

This is a minimum. As you can imagine, it would have to be a lot stronger if you intend on sitting a 6000 lbs hot tub on it.

To calculate that requires reviewing your species of lumber you used, the size and spans of your joists and beams and then being able to understand the actual physics of the structure. Needless to say, working backwards like this is pretty much impossible unless you are an engineer with specialized data and tables.

Usually 50 psf is not a hard target to meet and almost all decks do.

You should probably make a quick call to a local structural engineer who, if he is worth his salt, can take a quick look and be able to know right away if it meets that minimum threshold.

Now, the last issue for psf they want could relate to the bearing capacity of the soil. This is something that soil engineers do after you excavate and you have exposed the soil on which you are going to place footings.

They use a device that is kind of like a needle and depresses the soil. That little device and some basic knowledge of the type of soil it is allow them to say it can support 1500 psf, 2000 psf or 3000 psf or more.

Based on that you can then find out how much surface are of your deck is supported on each footing. That amount must be equal to or less than the bearing capacity of your soil.

Hope this helps a bit.

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