Building Deck close to a concrete surface

by Doug Shannon
(Battle Ground, WA )

I would like to build a wooden deck on top of an existing concrete patio. The top of the deck will only be 8.5 inches above the concrete. How do I proceed? I don't want to have to tear up the concrete.

Editor's Comments

This is really quite simple to do. You will have to take into consideration the width of the joists and decking in order to maintain the 8.5" elevation you are looking for. However, the only thing you really should do is try and maintain perfect level across the framing which will essentially be sitting on the slab. You can fine tune the level by using some shims.

You would be wise to rest the joists on something like blocks of wood or rubber pads to keep the joists completely dry. If you do this then the structure should last a lifetime.

Lastly, if you really want the deck to never move you use some metal strapping and secure it to the concrete.

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Dec 15, 2017
The proper way to build a deck on a concrete pad.
by: Rob Markham

Pre-drill and screw 1/2"x 2 1/2" galvanized lag screws into the under-side of your joists. The head of the lags will keep your joists from direct contact with the concrete. Using a small brush treat the pre-drilled holes with a end-cut preservative prior to screwing in the lags.

Add strip of JoistGaurd or BlueSkin to the underside of your joists before screwing in the lags if water is a real concern. Use the lags as "levelers" by backing them out of the joist with crescent wrench wherever the joist do not quite come in contact with the concrete.

Space the lags a minimum of 16"o.c. apart along the joist. Using "ground contact" certified lumber is also a good idea. Make sure that water is able to easily run off the concrete; do not trap it with your rim joist.

Editor's Comments

Great tip Rob! Thanks for helping out and offering your expertise.

Nov 28, 2017
by: Don

Is 8.5" above the concrete enough spacing for ventilation?

Editor's Comments

If you are building with wood deck boards and you have 3/16" spacing between each board you should be fine. However, if building with composite deck boards you should check with the manufacturer about the amount of space they recommend as a minimum. Composite boards are denser and retain more heat so usually need some modest are flow to deal with that.

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