Expert: Daniel Humphrey - 12/12/2009

Question
I was wanting to build a deck over my patio without having to rip out the concrete.  The problem with this is that there is only 4" gap from the concrete to the bottom of the door.  Do you have any suggestions on how to go about doing this?

Answer
Chris,

The usual way of doing this is to lay treated-wood sleepers onto the concrete slab; these would serve as joists to screw the decking into. The sleepers are fastened into the concrete periodically to keep the deck from moving.

In your case, the sleepers would probably be ripped from 2x4 material to about 1-3/4 in so that you get two usable pieces. Where the concrete dips or has a bump, shims or grinding would keep the sleepers straight and level.

Since the ordinary gaps between the decking boards would allow things to drop into that tight space and be stuck there, I put the deck boards tight together. Starting with wet treated wood or wet cedar, swelling from rains has not been a problem.

If you live in an area prone to frost heaving, you should put some foam insulation down between the sleepers and cover it with the decking boards.

--Daniel