Orinda Slab Repair, October 2024

This Orinda job was pretty straightforward. Part of the slab in a washroom had been demolished, and the homeowners wanted it patched and covered with tile to match the rest of the floor: standard six-by-six-inch restaurant tile in a terracotta color.

It helps to know the difference between three materials people often use interchangeably:

• Cement is the binder, the powder that hardens when mixed with water. It’s an ingredient, not a finished material.
• Concrete is cement mixed with sand and aggregate (gravel or stone). The aggregate makes it strong, so it’s typically the best choice for structural work like slabs and footings.
• Mortar is cement mixed with sand, without the coarse aggregate. It’s better at bonding than at bearing loads, which is why it’s used to set tile and stone.

Some are better at binding, some are better structurally, and a good repair uses each for what it does best.

When new concrete meets old, it usually needs a bonding agent. Fresh concrete doesn’t bond well to cured concrete on its own, and without one, the patch can crack loose at the seam. The exception is a patch that’s held in place by gravity and won’t carry heavy loads, like the fill in a floor slab. That was the case here. I don’t believe we used a bonding agent on this one. We patched the demolished area with concrete made with sand aggregate, filling the void and bringing it up to the surrounding slab.

Next came a layer of mortar to set the tile, and finally the tiles themselves.

That last step was the delicate part. Six-inch tiles are small enough that every one shows if it’s out of line, and the new tile had to match the level of the existing tile on the slab around it. Everything had to be dead level, so the patch disappears into the floor without a ridge, a dip, or a lip where old meets new. Getting the concrete patch at exactly the right height made that possible. Too high or too low, and the mortar bed can only do so much to compensate.

The result is a washroom floor where the repair is invisible: the same terracotta tile, at the same level, edge to edge.