How deep the joint should be: more silicone is not stronger

More paste does not hold more

Depth is not a guess: it follows width and how an elastic mass deforms. Up to about 10 mm wide we use, as a trade rule, depth close to the width, with a minimum of about 5 mm of mass. Thinner than that and there is too little material to take even a small movement.

Between 10 and 15 mm wide, the silicone stays around 8–10 mm deep. The rest is backer rod, not a block. A deeper well looks “more solid”; in fact there is more mass to stretch, and that force goes into the flanks: adhesion (peel from the tile) or cohesion (tear down the middle).

When to change the depth

Narrower than 10 mm: depth ≈ width, never a hairline.

10–15 mm wide: cap the depth at about 8–10 mm.

Void deeper than it is wide: do not fill it solid. Closed-cell PE to the right height.

Gap under about 5 mm: do not force a bead. Widen or rout it; a 2 mm film splits quickly.

Hourglass, not three faces

If the mass also sticks to the back (adhesive, cement), you have three-sided adhesion. A round PE rod hides the back and forces the hourglass: flanks filled, a thinner belly, real movement. The rod is usually 20–30 % wider than the gap, so it sits under compression. If the meeting is too skinny for a rod, PE tape on the back does the same job as a bond breaker.

How I do it

1. Measure the width between flanks; that sets depth and rod diameter.
2. Cut out the old. Clean, degrease, dry.
3. Push the PE to height.
4. Check the back is covered.
5. Fill and tool. In a bathroom, OTTOSEAL S100.

Watch the knife when you strip the old: do not take the tanking with it.

Exceptions

A crack from a sinking substrate or a loose tile is not “filled”. Sealant is not glue or a prop. Width that changes along the run: look at the worst spot, not the average.