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Q.A subcontractor glued down 4 1/2-inch-wide plank engineered flooring on a concrete slab-on-grade using flooring adhesive applied with a V-notch trowel. But the instructions on the bucket indicated that a 1/4-inch square notched trowel should have been used instead; consequently, the spread rate was about 25 percent less than it should have been. Now the floor has developed several ‘hollow spots' where the flooring seems to be floating rather than firmly glued to the substrate beneath. The sub says he'll use a small drill and inject some adhesive into the hollow spots, but the homeowners have misgivings about this approach. What should I have the subcontractor do to fix this?

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, the flooring's 4 1/2-inch width would only make the situation worse; flooring with a thinner — and therefore more flexible — profile would be a bit less likely to separate from the glue.

Injecting glue into the affected areas is a very common solution. Although some installers resort to epoxy, DriTac (800/394-9310, makes a repair kit — consisting of an injection gun, adhesive cartridges, mixing nozzles, applicator tips, and drill bits — designed specifically for this situation.

To use the kit, you start by drilling a pair of small, 3/32-inch-diameter holes into the wood flooring, preferably in a low-profile spot like a head joint, a V-joint, or wherever there is noticeable graining. One hole is used to deliver the glue, and the other allows air to escape as the glue is injected into the cavity (more holes may be necessary for larger areas). The adhesive is self-leveling, so after the cavity is completely filled (you'll know because glue will start to come out of the second hole), you can fill the holes with a tapered dowel trimmed to fit and matching putty.

According to the manufacturer, there's no need to apply weight to bed the flooring in the new adhesive, and the flooring can be used immediately after repair.

This is not an unusual problem for engineered flooring, and it's easily remedied. Your subcontractor's solution is very typical and shows that he has successfully dealt with this situation before.