What actually makes a shower waterproof

Almost every shower failure we are called to look at has the same cause. Somebody treated the tile as the waterproofing. It is not, and it never was.

Tile and grout are not a barrier

Porcelain absorbs very little water, which is why it is a good surface for a shower. But a tiled wall is not a continuous surface. It is a field of tiles separated by grout joints, and cementitious grout is porous by design. Water passes through the joints, reaches whatever is behind them, and either drains away safely or does damage.

This is normal and expected. The assembly is designed on the assumption that water gets behind the tile. The question is what it finds when it does.

The layer that does the work

Behind the tile there should be a continuous, bonded waterproofing plane: either a sheet membrane bonded to the substrate, or a liquid-applied membrane built up to the manufacturer's required thickness. Its job is to catch that water and direct it back to the drain.

Continuous is the operative word. A membrane with a gap at one internal corner is not a membrane; it is a funnel that happens to cover most of the wall.

• Every internal corner needs banding, bedded into the membrane on both faces.
• Every pipe penetration needs a collar, not just sealant around the pipe.
• The drain flange has to be bonded to the membrane so the two become one plane.
• The membrane must extend far enough beyond the wet area to catch splash and overspray.

Slope, and why it matters twice

Water needs to reach the drain from the surface and from the waterproofing plane underneath. If the membrane is flat, water that gets behind the tile sits there. Over years that produces the smell people describe as 'the shower going off', long before anything visible fails.

A pre-sloped pan solves this cleanly because the fall is formed in the factory rather than floated by hand at the end of a long day. Where a pan is not practical, the slope is built in mortar and checked with a level in several directions before the membrane goes on.

Movement has to go somewhere

Buildings move. Tile does not. Where a tiled surface meets another material, or changes plane, or crosses a structural joint, there needs to be a flexible joint rather than grout. This is why the corners of a well-built shower are sealed with a flexible sealant and the flat fields are grouted.

A shower grouted rigidly into every corner will crack. It is not a question of whether.

Test before you tile

Where the assembly allows it, a pan should be flood-tested before tiling: plug the drain, fill it, mark the level, and leave it. If the level drops, you have found the problem at the cheapest possible moment.

It costs a day. Finding the same fault after the tile, the glass, and the fixtures are in costs considerably more than a day.