Footers for Lightweight Roof Over Patio

natewallMay 5, 2010

I have an existing 8 X 9 foot concrete patio at the back of my house, the patio is about 15 inches above grade. It consists of a 4-inch thick slab of concrete supported by a brick foundation. I have no idea what rests below grade under the bricks, in terms of a foundation, and I plan to dig at a corner of the patio to find out. I also do not know if there is a hollow cavity under the patio of if it is filled in. The house foundation is concrete and 15 inches deep and maybe a foot wide, if that helps. The patio was built about 1968, part of a room addition, I think.

I wish to build a lightweight wood frammed, corrugated plastic or plywood with roll roofing cover (roof) over the patio. Upon talking with my County's building inspector (Fairfax County, Virginia), I would need a 4-post cover, since there is no way to properly support the house side of the cover (roof) by attaching it to the eave of the house.

I was told that I need 24 inch deep footers at each corner of the patio, under the location of the posts, to support the patio "roof" to meet code. The footers need to be 18 inches square. The design load (from the Virginia Residential Code that the County uses) is about 2000 pounds for snow load and whatever the roof (plywood and roll roofing) is, so I would guess about 2200 pounds, maximum load for the roof. Divide by four, then the greatest weight each 4X4 post would be designed to bear woulf be 550 pounds. I just cannot fathom the need for an 18-inch, 2-foot deep footer for a 500 pound load. I understand the 24 inches is needed for the frost depth, but 18 inches square? Does this sound correct?

If it is required to dig under the patio and place these, what I consider excessive footers, I will scrap the plans for this project.

Opinions?

Thanks.

--Nate

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mongoct

The footprint of a footing can depend on a couple of things; the weight-bearing capacity of the soil below versus the load above that the footing is expected to carry, PLUS the footing needs to be wide enough to resist any rotational moment from a load that might not be centered on the footing.

So footings can often seem wider than you'd expect them to be. The frost depth, yeah, that is what it is.

    Bookmark   May 5, 2010 at 4:32PM
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