Q.: The plywood diaphragms under the floor of an unreinforced masonry building are warped, producing an uneven floor surface. I want to place a 3-to-4-inch-thick concrete topping to improve floor flatness, but a bonded topping would stiffen the diaphragms and change the load paths. Can I use a bondbreaker on the plywood so the topping will be unbonded?

A.: An unbonded topping only 3 to 4 inches thick is likely to crack extensively. But if it will be covered with carpeting or another floor covering that won't reflect the crack, this may not be a problem. Ask the manufacturers of cement- or gypsum-based floor underlayments for their recommendations, based upon the floor covering you plan to use.