Expert: Dan Griffin - 4/3/2009

Question
I've already completed framing and wanted to confirm whether the room has been framed correctly. My contractor is using 2 X 4 and the floor is built on an existing concrete pad which did have a 2 season room which I tore down. 2 of the walls are the house and the other 2 face the elements. The floor is 2 X 4 with pressure treated 2 x 4s on the outside. I got the spray foam insulation done on the floor.  My concern is that water will come in from the sole plates on the 2 outside walls.  Anything I can do to ensure I don't get water coming in?

Answer
Lon, this existing slab needs to have a foundation.  It will be absolutely necessary if you have a building permit or will ever need to get one to legalize the work being done.  It sounds a bit like you are closing in a patio.

If the patio/slab was at ground level, you can't win.  Code demands a minimum of 6" of exposed concrete below the siding, and the siding has to lap over the gap between the slab and the base plate.  There is no way to smear enough goop on the outside to hold out water for long.  It may work for year, but will not last.

It is usual to put down a sill sealer to stop infiltrated air, but it sounds like you are hoping to keep out water and bugs.  If this was a patio, it also has a pitch to shed water, usually 1/4"/ft.  I would guess you corrected this with 2x4 floor joists cut to fit.  The concrete slab would not have had any type of vapor barrier under it, concrete is NOT waterproof, and conditioned space will draw ground water like crazy.  Anything in contact with that concrete would have to be treated and is very subject to rot and mildew.

I wish I had better news to give you.  If I have not understood your design, please feel free to write again.