Q: Most of the homes I work on are slab on grade. Occasionally I need to break a large hole in the slab for a drain or to repair a hole where some penetration such as a drain has been removed. What is the best way to repair a hole in a concrete slab before tiling over?
A: Michael Byrne, a veteran tile installer and consultant, and the moderator of JLC’s Ceramic Tile forum, replies:
With a hole in the slab like you describe, you have one of two conditions: The hole is to be covered up by something (such as a shower pan), or it will be covered directly with floor tile. Either way, dig the hole down, removing soil to a depth of 10 to 12 inches from the top of the slab. Then add back 3 or 4 inches of gravel or pea stone so that the hole is around 6 inches deep from the top of the slab to the top of the gravel. If you have excavated under a drain in the hole, be sure that the gravel fills the space below the drain and supports the drain completely.
If the hole you’re repairing is around a drain, be sure that the pipe is high enough to clear the top of the slab. If not, add a stub and then cover the pipe to keep out debris. Prepare the hole by scrubbing away any soil from the concrete along the inside of the hole, then rinse with clean water. When the rinse water has drained away from the concrete, but the concrete is still damp, spread a thin layer of thinset mortar over the concrete. Pour in the fresh concrete immediately to fill the hole, and let that cure. The thinset has better adhesive quality than regular concrete and that thin layer helps the new concrete knit with the existing slab.
If the hole you repaired is in the middle of a floor that you are tiling over, install a crack-isolation membrane over the entire floor before putting down the tile. If the area around the repaired hole is part of the footprint of a shower stall, wait until after the shower walls are framed. Then clean the concrete in the shower area with a sponge and clean water. While the concrete is still damp, spread a thin layer of thinset mortar over it, and immediately float the mud layer for the shower floor, sloped 1/4 inch per foot toward the drain. Again, the layer of thinset helps adhere the mud layer to the concrete slab. When the mud layer cures, install the shower pan, water test it, and you’re ready to tile.