How do I tile above a bath so the bottom row is right?

The bottom two inches of a tiled bath wall cause more trouble than the rest of the wall put together. The bath is not level, the bath moves when it is used, and the joint at the top of it is the one place water reliably tries to get into the wall — so all three have to be dealt with deliberately rather than hoped past.

Safety

What you'll need

• a straight timber batten and screws
• spirit level or laser
• sanitary silicone, matched to the grout
• masking tape and a smoothing tool (optional)
• tile adhesive rated for the substrate

Step by step

1. Check whether the bath is level and properly supported first.A bath that rocks or flexes will destroy any joint you form at its edge. Adjust the feet, add support to the cradle, and only then tile.
2. Gauge the wall vertically from the bath rim.Hold a gauge rod on the rim and see where the horizontal joints fall — at the ceiling, at the window, at the shower valve. Adjust so the top row is not a sliver.
3. Fix a level batten a tile's height above the highest point of the rim.The highest point, so that the bottom row will be a cut everywhere rather than needing to be bigger than a full tile at one end. Level it with a spirit level or a laser, not by measuring off the bath.
4. Tile upwards from the batten, working out from your centre line.Full tiles across the field. The whole wall is now square to gravity rather than to the bath.
5. Let the adhesive set fully, then remove the batten.The data sheet gives the time. Removing it early lets the whole wall settle.
6. Cut the bottom row individually to the line of the bath.Measure each tile at both edges. The cut will taper along the bath, which is fine and invisible, because the eye reads the level joints above.
7. Leave an open joint of a few millimetres between tile and bath.Not a hairline. A sealant bead needs depth and width to flex; a bead squeezed into a hairline gap has nothing to stretch and splits.
8. Fill the bath with water before sealing.A bath deflects on its feet under the weight of water and a person. Sealing it empty means the joint is formed at its highest position, and the first bath opens it.
9. Apply sanitary silicone with the bath full, tool it, and leave it to cure.Mask both sides if you want a crisp line, gun a continuous bead, tool it concave so water runs off, remove the tape immediately, then leave it alone for the full cure time before emptying the bath.
10. Never grout the tile-to-bath joint.It is a movement joint between two materials that move differently. Grout there cracks, and the crack lets water into the wall.

Tips

• If the bath is acrylic, expect more movement than with steel or cast iron, so the joint size and the fill-before-sealing step matter more.
• A shower over the bath makes this a wet area: the wall behind needs the right board and a tanking membrane, not just a good bead of silicone.
• Sanitary grade silicone contains a fungicide. Ordinary silicone in a bathroom goes black much sooner.
• If the gap at one end of the bath is very large because the wall is out, a bath sealing strip or a quadrant trim is a more durable answer than a huge bead of silicone.

Common mistakes

• Resting the bottom row of tiles on the bath rim — The rim is not level, so the whole wall runs downhill from the start, and the tiles are then loaded by the bath every time it deflects. It also removes the possibility of a proper sealant joint, because there is no gap to seal.
• Sealing with the bath empty — The bath drops several millimetres under the weight of water and a person. A bead applied at the empty position is stretched every time the bath is used, and it tears away from one side — usually the tile side, where you cannot see it happening.
• Grouting the joint between tile and bath — Grout has essentially no elasticity. In a joint that moves with every bath, it cracks within weeks, and the crack is a direct path for water into the wall and the floor below.

If it doesn't work

Questions people ask