Concrete Overlays on Wood Surfaces
Cementitious overlays can give new life to a wood subfloor. However, special precautions must be taken to ensure that movement and moisture don't damage the topping. When finished, these toppings can be saw-cut and stained, dyed or stenciled.
by David Thompson

Polymer-modified cementitious overlays are well known for their ability to put new faces on old concrete slabs. Less known is that many of these same products can put brand new concrete faces on wood surfaces, both old and new. Overlays can be used to create concrete surfaces that run up wood walls, down wood stairs, over wood countertops, and — more often then not — across wood floors or decks.

Whatever the substructure is, it has to be well constructed to successfully support a cementitious overlay. Wood, with its constant swelling, shrinking, shifting and settling, is already a moving target as far as a cementitious topping is concerned. Flimsy construction or loose floorboards won't do, since an overlay is only as strong as its base. For outdoor decks — one of the most popular areas for cementitious overlays — this means plywood should be at least ¾ inch with joists spaced every 16 inches. The preference is for tongue-and-groove joints with the plywood securely glued and screwed into place. At the very least, joists should be blocked where plywood seams meet.

Cementitious overlays can run from feather thin to several inches thick. In cases where the overlay will exceed 1/2 inch, contractors should consult with a framer to ensure the structure can handle the weight, according to Gordon Pennington, president of Renew-Crete, a manufacturer of overlayment materials that are often used over wood. "You may have to add extra joists to support the overlay material," he says. "Usually that's not the case as long as it's not more than a half-inch thick."

Not all cementitious toppings on the market are recommended for wood. Toppings that are too brittle, particularly some self-levelers, will fail. If the manufacturer hasn't specified that a topping is designed for use over wood, that's a pretty good indication that the product may crack like an egg if you try it.

Even the most flexible toppings, though, aren't designed for direct application over wood. "I don't know of any manufacturer who would recommend using any kind of polymer cement overlay directly over wood," says Gwynn Stegen, chief operations officer for Excellent Coatings. "If you do that, all bets are off and the product might very well fail."

What the heck do you do, then?

"Take the wood out of the equation entirely," Stegen says.