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Q: If the plywood subfloor is already in place, what makes a better setting bed for a floor tile installation: backerboard underlayment or plywood underlayment?

A: Veteran tile installer and consultant Michael Byrne, who also moderates JLC’s ceramic-tile online forum, responds: In any comparison that pits 1/2-inch cement backerboard against 1/2-inch plywood underlayment—both properly installed over a 5/8-inch plywood subfloor—the double-layer plywood sandwich wins as the stronger of the two, even though both meet the tile industry’s minimum standards. The reason is that backerboard products do not have any structural strength.

Adding strength to the floor system is only one factor, however, when looking at the integrity of a tile installation. Another major consideration is adhesion. If you compare the two configurations using identical thinset mortars, tiles installed over cement backerboard have greater shear-bond strength than those that are installed over plywood.

In dry-area installations, backerboard installed over a double-layer plywood floor provides approximately the same overall bonding strength as does a crack-isolation/waterproofing membrane over the same double layer of plywood. But for wet-area tile installations, the edge must be given to using a membrane because backerboard doesn’t offer any waterproofing properties.

I’ve also seen membrane installed over a system of backerboard on top of the plywood layers, but at that point the backerboard becomes superfluous and just adds to the expense of both labor and materials, as well as contributing to the overall weight of the floor system.

Because there are so many variables involving the cost of plywood, backerboard, and membranes, it’s difficult to pin down what might constitute the best configuration in every scenario. But as you determine what’s best for your particular situation, consider the concept of minimum standards. The details described above—backerboard or 1/2-inch plywood underlayment properly installed over 5/8-inch plywood subflooring—both will give you a minimum passing grade, or a “D,” from the tile industry.

But who is happy when their child comes home with a D on his report card?

When you’re creating a setting bed for tile, better grades are easy to achieve. Whether you choose backerboard or membrane over the plywood layers, upsizing the underlayment to 5/8 inch will raise your grade to a C. If you also upsize the subflooring to 3/4 inch, you will raise performance to a B. To score an A, use 3/4-inch underlayment and 3/4-inch subflooring.