Laying a wood deck on a wavy slab

Q: How can I install wood decking over a very uneven concrete slab?

A: Former assistant editor Rich Ziegner replies: I encountered the same problem when I replaced a concrete back porch with a sprawling pressure-treated deck. I didn’t want to bust up the porch and haul the concrete away, so I built a portion of the deck on top of the porch. Although headroom was an issue because of a canopy at the back door, I had enough leeway to attach the decking to 2x sleepers on the porch. The porch wasn’t level, so I had to scribe and rip the sleepers to fit it. You can do the same.

With a level, check the floor for the highest and the lowest spots. Mark these spots because they’ll be important when it’s time to scribe the sleepers. Then use a builders level or a spirit level and a straightedge to figure out the difference between the high and low elevations. You’ll need to know this difference to know how wide to open the compass when you scribe the sleepers. You don’t want the compass to run off the sleeper.

Lay out the 2x sleepers on edge at 16-in. centers. Shim one sleeper until it’s level, then shim  all the sleepers level with the first sleeper. Next, scribe the sleepers with a compass. One leg of  the compass rides along the slab as the compass pencil marks the contours of the slab surface on the face of the sleeper. Find those highest and lowest spots again, and adjust the compass so that you can scribe every sleeper; the pencil should never dip below or extend above the face of the sleeper. Keep the legs of the compass locked in one position and scribe every sleeper.

Once you’ve scribed a sleeper, number it so that you’ll know where it goes after you cut it. You can cut along the scribe line with a jigsaw, and when you replace the scribed sleeper, it will sit perfectly level on the slab.

Yeah, it takes some time, but as long as you’re careful, you’ll create a level floor over which you can fasten your flooring. You could use powder-actuated fasteners, concrete screws or a variety of hardware to hold the sleepers on the slab.