A few things here:

1. Your substrate may not have been flat to within manufacturer requirements.

2. Bostik's Best does not hold trowel ridges as well as some other brands.

3. Even the best professionals can produce engineered wood floors that have a few hollow spots...that is why a repair kit is made to fix problem areas. Bostik makes one; the added adhesive to fill the hollow areas is injected through a small hole drilled into the flooring.

I presume it was installed over a concrete substrate. If so , minor dips to the subfloor can give it a hollow sound in spots. have no worries, it should be fine.

Oh wow, I really do hope that is the case, echoflooring! How does one determine if the hollow sound is due to dips, or if it is due to adhesion problems?

glennsfc, our substrate was flat to within the manufacturer requirements. The installers used a laser, and leveled areas with the appropriate concrete filler.

If we were to use the injection technique to fill, where do we stop? There are seemingly hollow spots in MANY areas, not just one or two. There are so many spots - I'm guessing 30 or more.... thoughts?

Well...echoflooring is right in saying "it should be fine." If you can live with the floor as it is, then maybe that is what you should do. If you can't ignore the occasional hollow sound, then you've got a problem there. Doubtful that the floor is going to do anything to cause any issue other than these hollow sounding spots.

I called a flooring store, they gave me an estimate of $1200 to inject the hollow spots in the floor with 3m epoxy shots. There are 700SF total. Thoughts on this?

That is your call? I would be anxious to hear if it remedied the problem. 30 spots over 800 sq ft. Thats a lot. It is difficult to know what went on their without examining. There are so many variable that could have caused some of this.

The open time on the glue could have been too long causing it to skin over before the flooring was laid.
Anyway you look at it the installer is responsible not you.

PS: Glen knows what he is saying!

There will always be hollow spots here and there with gluedown floors. It's the real problem areas that make noises that can be fixed with a glue injection kit. The video below does a pretty good job of explaining the method.

Here is a link that might be useful: DriTac Repair Kit

Jojogirl11,

Thank you for sharing your problem with a glue down floor over concrete. I had thought only hollowness came from floating a floor over concrete. I leveled the floor myself before the installer came so before my water damage, my floated engineered wood floor felt like a solid floor. I hope the installers fix the problem since they should have told you that they needed to level the floor. Please keep us updated.

Can these hollow wood floors allow moisture underneath and begin to bubble up? I am not concerned with hollow sound, just further damage.

Did your issue get worse? I am having this problem right now. Thanks in advance.

I googled your dilemma. The concrete must be exceptionally flat.

https://www.google.com/search?client=safari&rls=en&q=could+divots+in+concrete+floor+cause+a+problem+for+a+glue+down+engineered+hardwood+floor%3F&ie=UTF-8&oe=UTF-8

Because, the most common issue includes "hollow" or popping sounds, because of

separation of the wood from the adhesive.