Expert: Daniel Humphrey - 11/4/2008

Question
Mr. Humphrey,
I'm planing a 12'x12' shed in NE Pennsylvania.  I'd like to use cement piers that go down 42" (local code).  Lot's of large rock here, so I'd like to dig only 4 holes.

I'm planing on using 2x8x12 joists (16" spacing), with two support beams of 6x6x12 or 4x8x12. (will both these work?)

The way I've designed it, the spans between posts(piers) that support the beams would be 8ft all the way around. This leaves 2ft of joist overhanging the beams on two of the sides, and 2ft of beam over hanging the piers on the other two sides.  

My main question is would the amount of overhang be ok?  And is the span sufficient to prevent sagging of the joists.

Thanks,
Christian

Answer
Christian,

Yes, that amount (24") of overhang should be okay for a small building. I wouldn't go any more than that.

If you want to make sure the shed is solid, sheathing the walls with 1/2" plywood will essentially turn the walls into trusses that can span the entire 12 feet and put hardly any weight on the side walls. Most of the weight will then be on the end walls, and those will have the benefit of the two beams beneath. Make sure your rim joists along those outer walls are doubled or tripled, because they will potentially bear the weight of most of the building, and you want to spread out the bearing surfaces and keep from crushing the wood where they pass over the beams.  

The 4x8 beam would be stiffer than the 6x6, but it would have a slight tendency to roll that you should counter with blocking or angle braces.

It sounds like you have it under control.

Daniel