One of the most common installations we do in Manhattan is engineered wood over a concrete slab.  With an engineered hardwood floor installation of this type it's not uncommon to end up with hollow spots in the floor due to concrete unevenness ... even after spot patching and flattening of the slab has taken place.  The video below shows a hollow spot we had in an engineered wood floor installation at The Sheffield at 350 east 57th street in Manhattan.  In addition to the concrete being very uneven, the floor boards being installed were about 7' long which made the finished floor more susceptible to hollow spots. 

I've only included video that shows a bit of the prep work, drilling of holes and how to inject the epoxy.  Once all the glue has been injected into the floor, the holes get plugged with small dowels provided with the kit and then get a pre-finished filler (in similar color to floors) applied over the dowel heads.

This is a list of materials I needed for the repair:

  • Dri Tac engineered wood floor repair kit
  • Towels/rags ... it can get messy, so make sure to have a few handy
  • Sharp utility knife ... for whittling down dowels
  • Pre-finished wood filler ... make sure it's similar color to the floor
  • Blue painter's 
  • Small drill ... the small cordless Milwaukee you see in the video is completely adequate
  • Hammer ... for sinking the dowels provided into the holes
  • Plastic bag ... for garbage

A high level overview of the injection process looks something like this:

  1. Tap the floor to identify hollow locations.  You can hear the difference between a hollow and solid spot in the video below.  I taped off the area that needed the injections but I didn't think it was necessary.  Make sure to use a blue painter's tape so that you don't damage the floor finish.
  2. Drill holes over the hollow spots with the bit from the kit.  I knew there was black rubber underlayment under the wood, so as soon as I saw black rubber coming out of the hole I stopped drilling.
  3. Clean off sawdust and apply tape over injection holes.  The glue came out of the holes a little bit with pretty much all of the injections I did, so having the tape down around the hole helped prevent glue from getting on the floor. 
  4. Mix glue.  I used one of the dowels they provided but I'd highly recommend using something a little longer as glue got all over my hands.  
  5. Pull the glue into the syringe.  This is harder than it looks even though in the kit they say the glue is "thin".  After you pull the glue up into the syringe, make sure to tilt it back a bit and even keep a touch of negative pressure on it to keep the glue from dripping out.  They say you can take the syringe apart and pour the glue in, but this seemed like a recipe for (messy) disaster with only one person doing the injecting.  
  6. Inject away!  I found I had to adjust the height a little bit once I put the needle into the floor to get the fluid to flow out easily.  I think I may have initially pushed a little too far into the rubber which prevented the glue from leaving the syringe.  VERY IMPORTANT: if the location of your injection site doesn't allow the glue to flow freely out of the syringe - could be a few things like floor glue blocking it, etc. - the glue could either a) come out of the injection hole or b) comes up through the cracks in the boards.  I played around with doing injections on seams that were really tight as opposed to more open and found that I had the same issues with glue flowing out through seams in both instances.
  7. Remove tape and clean excess glue.  I'd recommend doing this immediately after doing the injections as cured glue can be challenging to clean off of wood floors.
  8. Whittle down dowels and insert in injection holes.  You'll need a sharp utility knife handy to whittle these down as they're intentionally oversized and won't fit the holes.  They provide a metal sink with the kit so that you can tap the dowels into the holes.
  9. Apply filler.  Make sure it's pre-finished and similar color to the floor.  You'll never get an exact match so pretty close is okay.


This video shows the hole drilling and initial injections. IMPORTANT: make sure not to step on any of the locations where the floor has been injected until the glue is dry as it can come out of the holes and cracks in the floor.

We have additional videos of how to do the dowel insertion and also of more injections being done.  Contact us via the form on our site if you'd like the YouTube links to these.

A few random notes on this method of hollow spot repair:

  • Keep in mind that this injection method of fixing hollow spots in a wood floor is really best used with glued-down, engineered wood floor systems.  My experience has been that it's not really effective with solid, nailed installations.  They note this in the installation instructions.
  • The Dri Tac System is not an epoxy kit which I personally like because there's minimal mixing and it doesn't dry immediately, meaning the glue will flow out under the floor and secure a larger area.  The glue also comes off your hands pretty easily with soap and water.
  • Once you do your injections, don't step on the floor where you've injected:  the wet glue can come up through the holes or seams in the boards.
  • I put the syringe directly into the container and pulled glue into it.  My preference is to pour it off into a separate container so that the angle at which you need to hold the syringe is a little more natural.  When I got closer down to the bottom of the container it got awkward (and fairly taxing) to pull the glue into the syringe.
  • The glue can come up through the cracks once the cavity you're working on  gets filled up.  Sometimes the glue actually comes up under the blue tape that you've applied to protect the area around the injection.  MAKE SURE TO TAKE THE TAPE OFF THE FLOOR AS SOON AS YOU FINISH THE INJECTION!  If you let the glue dry by the injection site with the tape still on it you can potentially have a challenging time getting the tape off the floor.

Apologies in advance for any light profanity ... this was a little impromptu and is unedited ;-)

AuthorMarc Ringel