May 2015 Job of the Month:
Height would be an important detail in this particular home renovation project. The homeowner was making several changes to his kitchen and contacted us about creating a new surface for the floor. The existing concrete floor was covered by a layer of tile. Normally, we would remove the tile, clean the exposed concrete floor, and then pour the new concrete overlay on top of the existing concrete floor. We wouldn't be doing that this time. The homeowner didn't want us to pull up the tile. Instead, he wanted us to lay down a new microfinish concrete overlay directly on top of the tile.
His reason was simple. He was concerned about the height of the renovated floor. The tile, along with the thinset layer underneath it that adhered it to the concrete floor, was 1/2" tall. The microfinish overlay, once cured, would be 1/8" tall. The kitchen floor butted against a section of tile that would remain untouched by the renovations, and the homeowner wanted the renovated kitchen floor to maintain the same height as the untouched tile section. If we removed the tile and then laid down the concrete overlay, the kitchen floor would drop by a total of 3/8", meanwhile, if we lay the concrete overlay over tile, this would add 1/8". The homeowner decided that the extra 1/8" of height was the best way to go.
Placing a concrete overlay over tile isn't always an option. This process is ideal only if the tile is stable. If the tile ever loosens in the future, it would bring the overlay up with it. There are some benefits to laying a concrete overlay over tile. We didn't need to pull up the tile and then clean the exposed concrete floor underneath, so this option saves some time and expense plus you don't have any issues with the baseboards being too high off the finished floor after the tile was removed.
That said, it's not the easiest process, before we poured any of the concrete overlay over tile, we had to grind the surface of the tile layer. This allows the microfinish to stick tightly to it. If we skipped the grinding and, instead, pour the overlay onto unprepped tiles, the overlay would stick about as effectively as glue would to glass, i.e., it would but not very well, and it could be more likely to peel up later. We got to work by grinding the tile. The first photo shows the original tile floor before while the second shows the tiles after we ground them.
If you look carefully you will see something that there is a cement like material between each tile that is called grout, it fills in the gap between the tiles. We have to seal this off and we use an epoxy to do so. Epoxy is a plastic-like material that, once cured, will become as hard as rock. The epoxy also helps prevent the thin microfinish overlay from sinking into the grout lines and creating a shadow of the tiles in the finished floor. It also prevents moisture from coming up through the concrete slab and into the concrete overlay.
After all that prep we laid down a skim coat overlay layer. In a microfinish concrete project like this one, a skim coat acts as a bond coat between the upper microfinish layer and the layer beneath it. The skim coat completely covered the tile, as shown in the third photo but it is thin enough that some "ghosting" of the tiles are still visible. The epoxy we'd placed inside the grout lines had filled in most of the lines but not all of them. Without the epoxy, however, the ghosting would have been more prominent.
Next, we laid down a layer of microfinish overlay. This layer filled in the ghost images of tiles, creating a smooth surface. To wrap it up we added a color stain that complemented the new kitchen countertop to the new concrete overlay over tile and sealed it with a high gloss polyurethane. The finished kitchen floor was a virtually unnoticeable 1/8" taller than the section of tile adjacent to it and the homeowner was ecstatic about his new stained concrete floor.