Expert: Michael Curry - 8/20/2007

Question
Hi Mike,

I am also and "all experts" in the electrical division, but carpentry is way beyond me.

I have a 6' X 8' patio with a cement floor that does have a stucco roof. I wish to close it in, not as a patio, but an extension of the existing bedroom.
The problem is, the cement slab of the patio is about 4" lower than the floor of the bedroom, so I'll have to build it up. I know I shouldn't lay lumber directly on the cement floor to raise it up, so my question is, what should I put over the cement floor first, before installing the floor joists?
I have heard that a vapor barrier over the cement and then wood on top of that will create a moisture problem. Is this a fact or an old wives tale?
Also, should I use pressure treated lumber for the floor joists? I'm confident I can build it correctly, I'm just unsure of what special considerations I have to take into account since I am building on a cement slab.
Any guidance or suggestions will be greatly appreciated.

Don Beck

Answer
Hi Don,

You should use PT lumber and plywood for this and forget the vapor barrier as it can contain moisture which leads to mould. If you are in a warm climate I would even vent this floor area. In a cold climate use Styrofoam for insulation and remember that pad will move with frost - so don't attach it in a permanent way - so it can move.