Why showers leak: the layer nobody sees and everybody skips

Water goes through tile

Every tiled shower is wet behind the tile. Grout absorbs water, tile edges let it through, and the mortar bed holds it. That's expected. The assembly is designed so the water that gets past the tile hits a waterproof layer, runs down it, and exits through the drain's weep holes.

When that layer is missing, badly lapped, or punctured, the water keeps going — into the mortar bed, the backer board, the framing, and eventually into the ceiling of whatever is below.

What a correct assembly looks like

• A substrate with the right slope toward the drain, built before waterproofing, not corrected with extra thinset afterwards.
• A bonded waterproofing membrane — sheet or liquid-applied — carried up the walls, over the curb, and into every corner with the manufacturer's detail for inside and outside corners.
• Pipe and valve penetrations sealed with the fittings made for them.
• A flood test: plug the drain, fill the pan, leave it, and come back to check the level and the ceiling below.
• Only then: mortar, tile, grout, and sealant at the changes of plane rather than rigid grout that will crack.

The shortcuts that cause callbacks

The common ones are easy to describe and hard to see once the tile is on: waterproofing painted on too thin to reach the required film thickness, corners left without the reinforcing detail, a curb wrapped on three sides but not the top, a niche cut into the wall after the membrane went on, and no flood test because it costs a day.

Every one of those saves a few hundred dollars during construction and costs five figures when the ceiling below gets opened up.

What to ask before the tile goes on

Ask which waterproofing system is being used and whether the installation follows that manufacturer's details. Ask whether the shower pan will be flood tested and whether you can see it. Ask to look at the shower after waterproofing and before tile — one photo on your phone, taken that day, is the entire record of the part of your bathroom you can never inspect again.

A contractor who does this properly will be glad you asked. It's the part of the job they're proud of and nobody ever sees.