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16x32 floor construction suggestions

Timw05's picture

I am building a 16x32 guest house with 16x16 loft in my backyard.  I am planning to get it permitted as a barn for the structure and then go back and add the iternal walls, plumbing, and electrical afterwards (not permitted).

I have a height limitation of 18'.  With an 8' gambrel roof, 2x10 loft floor joists to span 16', and a 7.5' ceiling below the loft I don't have a lot of space for the floor.

I was thinking of making a 12" deep by 6" wide (top 4" above grade) down each long side and one down the middle. 2x base plate anchored with J bolts every 4' and attach 2x6x16 16oc floor joists to span the 16' width (8' with center footer) sitting on top of the base plate. All flooring wood is treaed for ground contact.

I live in Southern California so no frost line to deal with.  Cost is an issue so looking for the least expensive but adequate building options.

Looking for any other suggestions/improvements/comments for the floor or anything else.  I am designing this and drawing up the plans to submit to the city myself.

Slab? (post #215180, reply #1 of 3)

Why not a slab on grade? Even though the treated lumber is approved for ground contact, you still need access. Use 4x for the loft joists @ 32" oc and 2x T&G with no cieling. 

Slab? (post #215180, reply #2 of 3)

I wasn't thinking of a slab because after it is permitted I need to add the drains for plumbing.  To add the drains I was planning to unscrew the floor plywood and trench the drains and then screw the plywood back down.

I can't find any span tables for 4x floor joists.  Any suggestion on the height for a 4x joist to span 16'?  I had read that it's not the thickness of the joist but the height of the joist that matters.  Any comments on that?

Does the 2x T&G reduce the sag of the joist?

Thanks for your comments!

Thoughts (post #215180, reply #3 of 3)

What jurisdiction are you in? 

If you permit it as a barn they may give you a hard time about loads on the loft and the floor. Residential 40 psf doesn't apply to a barn. This is all the more reason to go slab. Rough in the drain and vent for the toilet and shower after your form inspection. For subsequent inspections put a pile of stuff on the drains and hide the vents some way. (Don't tell anyone I told you this.) 4x joists @ 32 oc will span the same as 2x joists of the same depth @ 16 oc. I suggested 2x t&g because it looks nice from underneath. It should span 32" without any problems. In San Diego County for areas without sewers, the health department would look at this very carefully. Even though you call it a barn they might not.