How do I stop a shower tray or bath leaking at the edge?

Resealing a bath is one of the most repeated household jobs, and the repetition is the tell — it is nearly always being done the same way that failed last time.

Safety

What you'll need

• Sanitary-grade silicone with fungicide
• Sealant remover and a plastic scraper or blade
• Methylated spirit and lint-free cloths
• Masking tape and a shaping tool

Step by step

1. Work out whether the seal is actually the problem.Dry everything, then run the shower and watch. Water appearing only when someone stands in the tray points at movement; water appearing from a running tap points at the waste or the overflow, not the seal.
2. Check the tray or bath is properly supported.If it flexes underfoot, no sealant will survive. A tray on an inadequate base or a bath without a central support moves every time it is used and tears the seal open.
3. Check the waste and overflow first.They are behind the panel and account for a large share of leaks blamed on the seal. Dry them, run water, and look with a torch before doing any resealing.
4. Remove every trace of the old sealant.This is the step that decides the outcome. Silicone does not bond to silicone, so a smear left in the joint is a guaranteed failure point. Sealant remover, a plastic blade, then scrape again.
5. Degrease and dry completely.Methylated spirit on a lint-free cloth, then leave it. Any residual moisture or soap film stops the new silicone adhering. A hairdryer on low speeds it up.
6. Load the bath or tray before sealing.Fill the bath with water, or stand in the tray. Both drop under load, and sealing them empty means the joint is stretched apart every single time it is used.
7. Use sanitary-grade silicone.With a fungicide. Not decorator's caulk, which is not waterproof and has almost no flex, and not general-purpose silicone in a wet area.
8. Mask both sides for a clean line.Two strips of tape leaving the bead width you want. It gives a far neater result and it is removed while the silicone is still wet.
9. Cut the nozzle to the right size.At 45 degrees, opening to slightly less than the gap. Too large a bead is harder to tool and looks worse than too small.
10. Apply in one continuous pass.Steady pressure, moving at a constant speed, without stopping. Every start and stop is a weak point and a visible bump.
11. Tool it once and remove the tape immediately.One pass with a wet finger or a shaping tool, then peel the tape away at an angle while the silicone is wet. Reworking silicone drags it out of the joint.
12. Leave the load on until it has cured.24 hours with the bath still full, or without using the shower. Draining early pulls the joint apart before the silicone has any strength.
13. Fit a proper seal strip for wide gaps.Anything much over about 6mm is beyond what silicone handles well. A plastic or ceramic seal strip bedded in silicone is the right answer.
14. Check the ceiling below afterwards.If there was staining before, keep watching it. A stain that stops spreading means you have fixed it; one that continues means water is still getting past.

Tips

• Fill the bath before sealing. It drops under load, and sealing it empty tears the joint on the first use.
• New silicone will not stick to old silicone. Every trace has to come out — that is the whole job.
• Leave it loaded for 24 hours to cure. Draining early undoes the work.

Common mistakes

• Sealing over the old sealant — Silicone does not bond to cured silicone, so the new bead is only held by whatever it touches at the edges. It peels within months.
• Sealing an empty bath — The bath drops several millimetres under the weight of water and a person. A joint sealed empty is stretched every time it is used and splits.

If it doesn't work