Thank you Joseph for the response. There are small hollow spots here and there throughout the installation, but two or three bigger ones in higher traffic areas (diameter of 1-2 feet). I can tell there's a void because the areas feel different when walked on (hard to describe, but you can just feel they have less support), and they also sound hollow when knocked on or if you walk over them with shoes. However, they are not moving / showing deflection when walked on. I assume the void is between the underlayment and concrete, maybe because it's an area where the concrete isn't as flat as it should be? But that's just a guess, I don't know for sure. Sounds like you think I should leave it as is? FYI the floors are approximately 3 years old.

The void is probably between the wood and the underlayment. Wood is rigid, underlayment (the foam or cork versions) is flexible. Like a blanket laying over a messy bunch of pillows. The underlayment (again...assuming this is the flexible stuff and not a rigid product) will contour to the humps and bumps where as a rigid plank will simply span the gap. The only way you know there are humps and bumps is when you step on it and it feels like there is less support.

So...after 3 years this will not be under installer warranty. This is most likely due to lack of subfloor preparation. Concrete subfloors often require $2-$4/sf (I've seen $5/sf but that was NASTY '50's concrete that was wonky and pitting badly). Try to remember how much you paid for prep and how long it took.

There is one other possibility: moisture. Where is the slab? Is it suspended (you are in an apartment style condo)? Is it at grade (concrete slab sitting directly at ground level)? Or is this below grade?

How old is the concrete/house? What was the 'underlay'? This is a BIG part of the issue/question. Please tell us more about your situation.

Were it mine I'd draw out a map of the location and size of the hollow spots and determine whether they are growing. If not I'd likely leave them alone.

I agree that the void is likely to be on top of the sound control product, which would make the repair easier, but I have seen some where the floor was glued to the mat and the mat had lifted.

Thanks to everyone for the comments. SJ McCarthy, to answer your questions - I am living on the 20th floor of a condo building from 1983. I don't know much more about the concrete slab, other than that it previously had parquet (with asbestos mastic) that was ripped out for the new floor. The new floor is 6" engineered white oak, and there is a felt-like acoustic membrane between the wood and the concrete.

The new floor was put in as part of a gut renovation before I moved in, and I paid a fixed price for the renovation, so I don't know how much the prep in particular cost or how long the prep took since I wasn't living here during the gut renovation. However, my suspicion is that the subfloor was not adequately prepared. I am well out of warranty now, but the "good" news is that the hollow spots seem to have remained relatively stable over the past several years, and I'm not noticing movement (that I can tell) when I walk on them. Certain areas just feel different or sound hollow (you can just tell when your foot touches it). FWIW, I also keep the humidity between 35-55%.

I am open to repairing now or waiting and monitoring. I just worry about this failing in the future and requiring more costly / extreme repairs than if I had acted now (e.g, I'd hate to have to replace the floor when a glue injection now would have made a difference). If the subfloor were in truly bad shape, would I be having bigger issues by year 3? Or can you just not tell? In any event, chalk it up to a lesson and being more educated next time....

Thanks!

Also Timothy when you say to monitor whether they are growing - do you mean by knocking on the wood to see if it sounds hollow, or would you be on the lookout more for noise / deflection when actually walking on the floor? Which is the concern?

An area that is hollow can grow in size. Which is to say, as the glue continues to deteriorate more and more planks loosen. In other words. once one part starts to fail, we assume it all will fail - eventually. The question becomes "When will it fail?"

By keeping an eye on the size of the hollowness today, you can track it's expansion over the next few months/years. If these appear to be growing on a monthly basis then we know you have very little time with the floor. On the other hand these hollow spots could be stable - not requiring anything other than living with them.