How do I seal an internal corner in a shower or wet room?

Corners are the hardest part of any sealing job and the part most likely to be done badly, because two beads have to meet, water concentrates there, and a finger is the wrong shape for the profile you want. The order matters as much as the technique: get the overlaps the wrong way round and water running down a vertical corner is delivered into the end of the horizontal bead rather than over it.

What you'll need

• Sanitary-grade silicone with a fungicide
• A silicone finishing tool with corner profiles
• Low-tack masking tape
• A sharp trimming knife and a silicone remover
• Methylated spirit or isopropyl alcohol
• A caulking gun with a smooth action

Step by step

1. Confirm the joint should be sealant, not grout.Wall-to-wall internal corners, the wall-to-tray or wall-to-bath junction, and any junction between two different materials all move relative to one another. Grout is rigid, so it cracks there every time, and the crack then holds water and mould. All of these belong in flexible sealant.
2. Remove everything that is in the joint now.Cut both edges of any old silicone and peel it away, then dissolve the residue with a solvent remover. Rake cracked grout right out of the corner back to the tile edges. Vacuum the joint. Any old material left in there is what the new bead will fail against.
3. Treat and dry the joint.Apply a mould remover to the bare joint, leave it, rinse and then dry the joint completely — a hairdryer on low, or a day with the room ventilated and the shower unused. Finish with a wipe of methylated spirit to remove any remaining film and let it flash off.
4. Load the tray or bath before you start.If the corner runs down to a bath or a tray that moves at all under load, fill the bath or stand in the tray while you seal, so the joint is sealed at its widest and is only ever compressed afterwards rather than stretched.
5. Mask both sides of every joint.Tape on both faces, leaving a gap the width you want the finished bead to be. In a corner this takes patience and it is what produces a line that looks deliberate rather than smeared. Run the tape past the ends of each joint.
6. Cut the nozzle to a smaller opening than you think.Cut at 45 degrees, to slightly less than the joint width. A corner needs a controlled small bead — a large one is impossible to tool cleanly and squeezes out everywhere.
7. Run the vertical corners first.Start at the top of a vertical corner and pull down in one continuous movement, keeping the gun moving at a steady speed and the nozzle at a consistent angle. Do the vertical corners before anything horizontal.
8. Then run the horizontal tray joint, over the ends of the verticals.The horizontal bead at the tray or bath goes on last and passes across the bottom end of each vertical bead, so water running down the vertical corner is delivered onto the top of the horizontal bead rather than into its end. This overlap direction is the whole point of the sequence.
9. Tool each bead once, with a corner tool.Use the corner profile on a silicone finishing tool, held at a consistent angle, drawn in one steady pass from one end to the other. One pass. Going back over a tooled bead drags it and leaves ridges, and in a corner it pulls silicone out of the joint entirely. Wipe the tool clean between passes.
10. Pull the tape immediately, while it is wet.Pull each strip away from the bead at a shallow angle, as soon as the tooling is done and while the silicone is still wet. Tape left until the silicone has skinned takes the edge of the bead with it.
11. Leave it the full cure time, with the joint held at its width.Twenty-four hours minimum before any water, longer in a cold or unventilated room. Keep the bath filled or the tray loaded until it has at least skinned properly, then drain slowly.

Tips

• Do the whole shower in one session rather than a corner at a time. Beads applied on different days do not bond to one another and the junctions become the weak points.
• Ventilate rather than heat while it cures. Silicone cures by reacting with moisture in the air, so a warm dry room is not faster than a normally humid one.
• In a wet room, the corner detail is only the visible layer — the tanking membrane behind the tile is what is actually keeping water out, and no amount of good sealing substitutes for it.

Common mistakes

• Grouting an internal corner — It is a movement joint. Grout is rigid, so it cracks, and the crack holds water and grows mould.
• Running the horizontal bead first and the verticals down onto it — Water running down the vertical corner is then delivered into the end of the horizontal bead instead of over the top of it.
• Tooling with a wet finger in a corner — A finger is the wrong radius for a corner and the water on it interferes with the surface cure. A corner profile tool costs very little.
• Going back over the bead to tidy it — The first pass is the only pass. A second drags silicone out of the joint and leaves ridges that then hold water.