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It's also possible to install an underlayment over plywood subfloors, but additional steps are necessary to reduce the chance of cracking. Adding a liquid acrylic modifier to the underlayment mix will increase the flexibility and reduce the chance for cracking. Typically the modifier replaces a percentage of the water in the mix.

Installing a Concrete Floor over Wood
Time: 03:16
Find out how concrete floors are installed over a wood sub-floor.

Installing a Concrete Floor Over a Wood Subfloor
Time: 01:10
Concrete floor video on how to install concrete over an existing wood subfloor.

It's best to place the underlayment over 3/4-inch-thick plywood fastened with screws at 12- to 16-inch intervals to help decrease deflection and add stability. On projects where deflection of the plywood is a concern, some manufacturers recommend applying the underlayment over two layers of 3/4-inch plywood for better rigidity.

Next, attach galvanized metal lath to the plywood subfloor as a reinforcing material. I recommend screwing down the metal lath perpendicular to the direction of the plywood roughly on 6-inch centers, overlapping the sheets approximately 2 inches. Be sure to use enough fasteners to ensure flat, uniform installation. Some manufacturers suggest applying the underlayment directly over the top of the secured metal lath while others recommend first applying a repair-type mortar as a body coat over the lath. I like to use a polymer additive combined with cement, which provides some flexibility. However, make sure whatever product you use is approved by the underlayment manufacturer. After the repair mortar cures (if using), install the underlayment to provide a smooth, level surface. When placing an underlayment over metal lath, be sure the thickness is at least of 1/2 inch to reduce the chance of the lath pattern ghosting through to the surface. If youre going over an elevated wood subfloor that requires you to build thicker lifts, consider bringing in a structural engineer to make sure the floor will support the additional weight.

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