The sealant line is where the water gets in

Silicone is the last thing done in a bathroom and the first thing to come back. It is a five-minute item on a snag list before handover, and a far more expensive one once water has been tracking behind a bath for six months.

What to check

• The junctions the standard names. Where wall tiling meets the bath, and where it meets the shower. Those two are the ones with a published requirement behind them.
• The junctions it doesn’t. Worktop to upstand, wall to floor in a wet room, basin to splashback. Good practice, and worth doing, but if you raise them expect to argue them on workmanship rather than on a clause.
• The basin waste. Different from the tap, and it is where the water actually gets through. Tap manufacturers seal the tap to the basin with a moulded rubber washer and say nothing about silicone — but they do tell you to bed the waste on sealant.
• Gaps, shrinkage and mould already behind the bead. Mould present at handover usually means the joint was sealed over a damp or dirty surface.
• Whether the bath was filled before sealing. An empty acrylic or thin steel bath sits higher than a full one. Seal it empty and the joint is at its narrowest — the first proper bath stretches it and it splits or peels.
• Flexible sealant where there is movement, not grout. Grout has no meaningful movement capability; it will crack.
• Tooling. The bead should be smoothed into the joint, not left proud and lumpy. Manufacturers typically specify tooling within minutes of application, before a skin forms.

Those are two separate tests, and a bead can fail either one on its own. Neat but rigid — grout, or a sealant with no movement capability — fails the performance requirement even though it looks perfect on handover day. Watertight but ragged, uneven in width, or wandering off the line fails the appearance requirement even though it will never leak. Worth knowing, because “it doesn’t leak” is the usual answer to a complaint about a poor bead, and it only answers half the question.

One thing that is not a requirement

You will see it said that the tap should be sealed to the basin or the worktop. It is worth knowing that no NHBC clause, British Standard or Building Regulation says so — the tap seals to the ware with a moulded rubber washer clamped by the backnut, and that is the designed joint. Read a tap manufacturer’s installation instructions and you will find silicone mentioned for the waste, not for the tap.

That matters in both directions. If you are raising it, raise the waste rather than the tap and you are on firm ground. If someone is raising it with you, a properly fitted tap with a sound washer is not a defect for want of a bead of silicone around its base.

The real evidence is underneath. Run the tap for a minute, then look at the underside of the worktop cut-out and the tap tails with a torch. Swollen, dark or crumbling chipboard at the cut-out edge shows up long before anything appears on the ceiling below.

Why filling the bath matters

This one is trade practice rather than a published tolerance, but the physics is straightforward. A bath deflects under the weight of water and a person. If the joint is sealed while the bath is empty, it is sealed at its narrowest point, and every use afterwards pulls the sealant beyond what it can take. Sealing with the bath full means the joint is formed at its widest — so in normal use the bead is being compressed rather than stretched.

It is worth asking the question at handover, because you cannot tell by looking, and it is the single most common reason a bathroom seal fails inside a year.

Do it the Snagga way

1. Check every wet junction in every bathroom, en-suite and WC.
2. Photograph gaps, mould and split beads — close enough to see the joint, wide enough to show where it is.
3. Assign it to the trade that owns the joint, not to “the builder”.
4. Re-check after any bath or tray is refitted — the seal is only as good as the last person who moved it.

More Site Notes: Snag the wet trades before the decorator arrives · All notes