Installations where flooring products are removed prior to the installation of new flooring can always bring sight-unseen challenges. When these challenges occur, the time and money for floor preparation can be extensive. But not addressing them on the front side of the installation—which protects you and your company—may have even worse consequences post-installation.

Let’s talk about an installation which had tile as the original flooring. The tile was removed and rubber tiles were installed. The installers, wanting to make sure they had a smooth and flat surface to install over, chose to skim-coat the area of installation. They used a buffer with some sandpaper to scratch the slab. Nothing wrong with that, right?

Here’s where it gets complicated. The joints—some control joints, some trench cuts and stress fractures—were filled with a urethane caulk; the installers’ skim coat was applied directly over the urethane-filled joints (Photo 1).

A call was made several months after the installation from the end user, saying that the floor was bubbling (Photo 2). The manufacturer of the patching compound was called in to see if their product was defective. I was called in to consult on the project and met with the installation team. When we removed a tile plank, the floor patch used to skim-coat was stuck to the back of the rubber tile but released easily from the concrete substrate. When we pulled up the rubber tile, joint filler was attached to the back of the tile as well.

I applied some water to the surface of the concrete to check porosity; after five minutes, the water was still beaded on the surface of the concrete. Next, we checked the bubbles, which were occurring along the filled joints. Digging into the patching compound, I found that the joints were filled with a pliable urethane filler and the floor patching compound that had been used to skim over the joints crumbled easily (Photo 3).

So, did the soft joint cause the bubbles at the joints? No, but it didn’t help either. There was some new concrete work that had to be done near the area where the rubber tile was installed; the further away we moved from the new slab, the less noticeable the bubbles. Moisture was the culprit, but not addressing the soft joint at the front end of floor preparation, and not taking into consideration the porosity of the slab to make sure the skim coat would bond to the concrete, did not help matters. Everyone initially thought this was a floor patch product failure but that was not the case.

While the installers did abrade the slab, they should have checked the porosity to make sure they had abraded enough to allow for bonding of their skim coat. In this case, a diamond grinder would have been a better choice.

So how do you remove the existing filler? A crack chasing blade is one way, as it removes loose concrete and can clean out the joints (Photo 4). Make sure to use proper dust containment and remember to address the concerns on the front side of the installation. Just because the filler worked under a ceramic tile installation does not mean it will work for soft floor coverings. And since the slab had such a low porosity, I would imagine the tile came up pretty easily.