A. Michael Purser, a second-generation wood flooring contractor in Atlanta, responds: While it's reasonable to suppose that the trowel's spread rate has caused the problem you describe, it's more likely that this situation is the result of unevenness in the concrete's surface.
Regardless of how much mastic is put down, if a poured concrete slab has significant low or high spots, these are probably the source of voids between the flooring and the subfloor. The engineered product is not going to stay in place unless the surface is reasonably flat; if it isn't, the flooring breaks free from the mastic. And in this case,...
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