How do I seal a gap wider than 10mm around a bath?

Silicone seals a joint by stretching between two surfaces. If it is also stuck to the back of the gap it cannot stretch, so every movement tries to tear it instead. That is why a fat bead squeezed into a 15mm gap splits down the middle within a season.

What you'll need

• Closed-cell foam backer rod in the right diameter
• Sanitary silicone and a gun
• A bath seal trim if the gap is very wide
• Masking tape and a smoothing tool

Step by step

1. Measure the widest point of the gap.Up to about 8mm, a normal bead is fine. From 8 to about 20mm you need a backer rod. Above 20mm the gap needs closing mechanically.
2. Find out why the gap exists.A bath pulled away from the wall, a wall that is badly out of plumb, or a bath that has never been secured. If the bath moves, fix that first — no sealant survives it.
3. Remove all the old sealant.Cut it out and clean back to bare surfaces with a silicone remover, then a solvent wipe. The invisible film left behind is the usual cause of a new bead lifting.
4. Choose a backer rod slightly larger than the gap.It should compress a little as you push it in, so it stays put without adhesive. Closed-cell rod is essential — open cell absorbs water.
5. Push the rod in to a consistent depth.Set it so the finished silicone will be roughly half as deep as it is wide. A blunt tool along the joint gives an even depth.
6. Mask both sides of the joint.Two lines of tape at the finished bead width make a wide bead look deliberate rather than smeared, which matters much more on a big joint.
7. Apply in one continuous pass.Cut the nozzle to the full bead width and lay it in one movement. Joining halfway along a wide bead almost always shows.
8. Tool it concave, not flat.A slightly hollow profile puts the silicone in tension evenly. Pull the tape immediately after tooling, while the bead is still wet.
9. Consider a trim for very wide gaps.Above about 20mm, a plastic or ceramic bath seal trim bedded in silicone gives a far better result than any amount of sealant, and can be replaced later.
10. Load the bath before sealing.Fill it with water, or stand in the tray, so the joint is at its widest while it cures.

Tips

• Backer rod is sold by diameter in a range of sizes. Buy one size up from the gap so it grips.
• If the gap varies along its length, use rod for the wide part and let the bead narrow naturally at the ends.
• A gap this wide usually means the bath needs securing to the wall as well — check before you seal it.

Common mistakes

• Filling the whole gap with silicone — It bonds to three sides, cannot stretch, and tears the moment the bath moves. It also takes days to cure through and often never cures in the middle at all.
• Using open-cell foam or rolled paper as backing — Both absorb water. Once wet they hold moisture permanently against the wall and the bath edge.

If it doesn't work

Questions people ask