adding a subfloor

[cayvman]

We had a remodel done extending the dining room onto part of the cement porch. The extension is approximately 12 x 6. I need to add a subfloor to the extended area so that I can install hardwood floor. My plan is to attach joists to the cement porch and then 3/4 plywood on top of the joists. Then lay my hardwood on top of the plywood. One of the problems is the cement porch is neither level nor flat, it slopes downward (as one would expect for drainage). The space that I am making up is approximately 4". That is from the top of the cement porch to the top of the existing hardwood floor in the original area of the dining room. So, given my hardwood floor is 3/8" and the plywood is 3/4", the joists would average around 2 7/8". Skinnier at the adjoining end and thicker at the new wall end. The problem is the fact that the porch is not flat but wavy. The long and short of it is what is the best way of building up my subfloor so that it is level with the existing subfloor with the above mentioned criteria? Any suggestions will be greatly appreciated.

[Tom Baker]

You could try joist hangers or a ledger nailed to the house. This would avoid the uneven floor. It sounds like you will have a stud wall added at the outer edge of the slab. That can be used to support a ledger.

2x6 of Southern Yellow Pine #2 grade spaced at 24 inches on center with 40 pounds of live load will span 8'-6" according to a span table here at hand. You could shim each joist seperately at a foot out from the house wall to insure bearing capacity. The playwood should provide some extra support.

Tom Baker

[default]

Be kind of tough to squish a 2x6 joist into a 2 & 7/8" vertical space, though, don't you think?

[ameijers]

Scribed sleepers on the slab, over a waterproof membrane. They showed this process in detail a couple years ago on This Old House, on the million-dollar 'remodel' of that coastal beach house.

Hope whoever extended your dining room made sure there was an adequate foundation to hold the new walls and roof extension, and didn't just nail the sill plates to the slab. If they did, you will always have mositure problems. Even if slab was ultra-heavy and solid, a course of 4" block under the wall would have been proper, to keep the wood up out of the wet area. From what little detail you gave, it sounds more like they did it as an enclosed porch, not a proper bumpout or addition. That would have included framing in a floor system as part of the wall framing, to avoid the little hump or expansion crack where the old and new floors meet. A porch slab, even a well-laid one, is 'floating' relative to the house. A finish floor that crosses from the house foundation to resting on the porch, is likely to have problems at the transition point. If the porch slab heaves up and down in winter, the floor will crack or go out of level.

aem sends....

[Tom Baker]

The joists would have to be cut down. As I read the O.P., this is a sloping surface that's not very smooth. Given that, some method of setting a level line is needed. I suppose 2 by material could be used and supported with individual shims at interals along the span.

TB