Silicone sealant for bathrooms, and what the cartridge is telling you

Search for waterproof silicone and you'll find it on nearly every cartridge. It's true, and it's narrower than it sounds. The sanitary part of the standard is written for joints inside a building exposed to non-pressurised water. That's a shower hitting the tiles, a bath full of water, a splash at the basin.

Its own scope rules out swimming pools, anything underwater and drinking water. So a bathroom sealant closes the gap between a bath and a wall. It is not a fix for a pipe that leaks under pressure, and nothing about it changes because the label says waterproof in bigger letters.

Where this matters on a damp site is the wall and the ceiling below. A bath seal that has split lets water run down behind the bath, and it shows up somewhere below. If you have a stain spreading on the ceiling under the bathroom, the bead upstairs is one of the first things to look at. Resealing a bath from start to finish is its own page. And if water is coming through an outside wall, that's penetrating damp, which no bathroom sealant will reach.

Outdoor joints are a different job with a different part of the standard. For those, see sealing round a window frame.

Almost every bathroom sealant says anti-mould. The XS number is what tells you how much that means. It comes from a lab test that scores mould growth on the cured sealant from 0 to 5. Zero is nothing under the microscope. Five is heavy growth over the whole test surface. XS1 covers scores 0 and 1, so nothing you could see with your own eyes. XS2 is a score of 2, growth over up to a quarter of the surface.

Here's the part the shop grid leaves out. One maker answers the question "I used anti-mould silicone, why is there mould?" on its own website. The fungicide protects the sealant. Soap, body gel and other residue sitting on the bead can still grow mould, and so can a room with poor air flow. Even a lifetime anti-mould guarantee doesn't take the cleaning off you.

So buy XS1, then treat the bead like any other surface in a wet room. Wipe it down after a shower and get the steam out of the room. If black spots are already in the bead, whether it cleans off or has grown in decides whether you clean it or cut it out.

The joint round a bath moves as the bath fills and empties, and the bead has to move with it. A sanitary sheet gives a joint movement figure for that, 20% on the acetoxy sheet read for this page. The same sheet tells you to fill the bath half full before sealing so the bead cures at the midpoint of that movement, and to empty it once the sealant has fully cured. It cures about 3 mm a day at 23C, so a thick bead takes longer.

Two more lines from it are worth knowing. A deep gap gets filled with backer rod or foam first, so the sealant is only half as deep as the joint is wide. And the surface must be clean, dry and free of grease and loose material, which an old bead never is. Cured silicone only comes off mechanically, so getting the old silicone out comes before any of this.

Also not a sanitary silicone: decorators caulk. It's paintable and silicone isn't, and where each one belongs is worth a read before you grab the cheaper tube.

How to run the bead itself is on applying silicone sealant. And if half the cartridge is left over, keeping an opened tube usable covers how to stop the rest drying out.