How to Tile Over Tile
Tile expert, Mark Ferrante has some basement tile suggestions
I’m going to install 18-by-18-inch ceramic tiles over tiles on my basement floor that were installed in the ’50s. The old tiles are secure, but do I need to use a special primer or thinset to set the new ones?
—Jim Beller, via e-mail
Have you already determined that there will be enough headroom after the new tile goes in? In some basements, a loss of even ½ inch or so could present a problem.
It’s good that the old tiles are still securely attached to the basement slab; a solid, stable base is essential for any tiling project. Even so, take a few minutes to give each tile a tap with a rubber mallet, just to make sure that none of them are loose. Mark the ones that make a rattling sound, and if there are only a few of them, chip them out and fill the recess with thinset. But if there are a lot of loose tiles, you should probably remove all of them and start fresh. Also, make note of any long cracks that indicate that the slab has shifted. You’ll deal with those later.
Now, if the old tilework is staying in place, roughen its surface with 60-grit sandpaper to ensure a good grip for the new tile. A rented floor sander can quickly cover large areas. Use a belt sander or random-orbit sander along the edges where the floor sander can’t reach. Next, remove any loose or moldy grout, either with an electric oscillating tool fitted with a grout attachment, or by hand, with a carbide-toothed scoring tool. Vacuum up the grit, wash the surface with a no-rinse cleaner like Spic and Span, and let it dry.
While it’s drying, use a long straightedge or level to check the floor for dips and humps. It doesn’t have to be level, but it should be flat. Otherwise, you’ll have a heck of a time getting the edges of your 18-by-18 tiles to line up. The Tile Council of North America (TCNA) specifies that tiles larger than 15 inches on a side require a subfloor that doesn’t vary more than 1⁄8 inch over 10 feet. If your existing floor meets that criterion, and has no cracks, you can start tiling.
If only a few low spots need to be filled, use a cementitious floor patch that can be troweled to a feather edge. But if lots of filling is required, and the floor is fairly level, then mix and pour a cement-based, self-leveling underlayment over the entire floor. As its name implies, this type of underlayment will flow to the lowest level, and the more a floor slopes, the more underlayment you’ll need to pour. For those areas where you don’t want the underlayment to go, put up dams using 2x4s and construction adhesive. The big payoff for all this extra effort will be a perfectly flat and level surface, which will make laying your tiles much easier.
Now is the time to deal with cracks, if there are any, in the original floor. The way to do that is with a crack-isolation membrane, such as Bostik’s GoldPlus (Bostik), which stops cracks from propagating through to the new tiles.
Once all the dips and cracks have been addressed, tiling can commence. This is a job for polymer-modified thinset, the kind you mix with water. Ready-mix mastics have no place here. For your large tiles, I recommend that you butter thinset on the back of each one before setting it into the thinset on the floor. That way, there’ll be no worries about tiles coming loose.
One last thing: Lining up the edges of a big tile won’t be easy, but giving extra attention to them is critical. You don’t want a floor plagued with misaligned edges, a.k.a. lippage. Using edge-aligning devices like Tuscan SeamClips (Tuscan Leveling System) can be a big help here because they eliminate lippage, and speed up installation.
Mark Ferrante is the owner of Ferrante Tile in Woburn, MA, and has laid tile in dozens of This Old House TV project houses.