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06-29-2011, 12:52 PM   #1
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## Deck footing question

Hello all

I have a 16 X 16 attached deck I'm going to be building. The building inspector gave me a formula to work with to calculate the size of the footings based on the weight of the deck.

The beam will be 14 feet out from the house with 4 footings spaced at 4 feet.

He said the ground is rated for 2000 psf which comes out to about 14 psi.

He calculated that the beam will be supporting 7200 pounds of maximum weight (16 feet of width X 9 feet of depth X 50 pounds per square foot).

This means that each footing is going to have to support 1800 pounds. Based on calculation, it would take 14" diameter footings to carry this load. The power auger I can rent only goes up to 12" diameter.

14" footings or 5 12" footings to me sounds like overkill but I know it all comes down to what the inspector requires. Does that sound right to you?

I'm in WI so the footings have to be poured 48" deep so we're talking about a lot of bags of concrete.

06-29-2011, 01:54 PM   #2
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If the Inspector says it's the way it is. It sounds right to me.
He knows the soil conditions better then I do.

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Ron
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 06-29-2011, 04:39 PM #3 Member     Join Date: Oct 2006 Location: Minneapolis/St. Paul, Minnesota - Latitude 45.057 Longitude -93.074 Posts: 4,068 Rewards Points: 2,626 Auger the 12" holes and put in the Sonotubes (5 or 6 or whatever). Set them vertical and get the inspection and go on. When they are done, order a small load of concrete and use any excess where you want (sidewalk, etc.) or force the supplier to take it back and get rid of it. Bags can cost far more than ready-mix in the end if you consider everything like time to pick up labor, cleaning a mixer, fuel and returning the excess bags, if possible. Dick

 06-29-2011, 04:46 PM #4 Civil Engineer   Join Date: Mar 2009 Location: Boston Posts: 5,641 Rewards Points: 4,862 The figure of 2000 psf is known as the presumptive capacity of the soil. This is a figure pulled from a table based on reasonable assumptions about the local soil conditions. The true soil capacity is likely much higher, but in order to demonstrate that to the building inspector, you would need to hire an engineer and run some soil tests. So it is much less costly to somewhat overbuild the footings, and avoid the need to run any tests, and hire an engineer.
 The Following User Says Thank You to Daniel Holzman For This Useful Post: Tom Struble (06-29-2011)
 06-29-2011, 06:52 PM #5 Member   Join Date: Dec 2007 Location: Wisconsin Posts: 452 Rewards Points: 462 I don't doubt his calculations but originally when I talked to him about 3 footings instead of 4 he did some tapping on the calculator and said "You might need to go with 10 or 12 inch footings" so he wasn't clear about it. Based on the formula he gave me I later did the calculations and concluded that I needed 4, but they come out to 14" to meet the code. I guess I will have to run it by him and see what he says. Thanks

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