Three-sided adhesion: why a “full” joint tears

Why a stuffed joint tears

Three-sided adhesion is when the bead bonds not only to the two flanks of the joint but also to the plane behind: tile adhesive, concrete, the back of a profile. Elastic sealant needs to change shape between two faces. If the back holds it as well, movement piles up at the edges. Then it either tears through the middle (cohesion) or lets go of a flank (adhesion).

Think of a rubber band. Pulled from both ends, stretch spreads. Stick the back to a wall and pull: that point cannot extend. All the load goes to the ends. In a bathroom joint, the typical movement class of an elastic silicone — on the order of 20–25 %, not a guarantee for that joint — collapses. You are left with a few per cent.

A void packed with silicone is not “stronger”. It has more contact faces and fails sooner.

When to use it — and when not

Yes, backer rod of closed-cell PE when there is depth to spare. PE does not bond; it frees the back and sets the geometry.

If the joint is too shallow for a rod, PE tape or foil on the back is the bond-breaker.

More paste will not save it if the movement is slab settlement or a badly framed partition. Stabilise the build first. Backer rod will not hold a structural crack.

In the wet, closed-cell PE only. Open-cell foam is a sponge: water behind the bead and mould.

How I do it

1. Cut out the old joint and the hard adhesive that has squeezed into the corner. When you scrape, do not nick the waterproofing under the tiles.
2. Check width and depth; the section has to make sense.
3. Push in rod 20–30 % fatter than the gap, wedged, not crushed flat.
4. Degrease the flanks, one direction, a lint-free cloth.
5. Sanitary silicone or an elastic sealant that wets both faces. Tool it.

On tiling, if adhesive fills the internal corner and cures, that is the third plane. The tiler should leave room for the PE. If not, you scrape it before you seal.

Exceptions

A rigid mortar joint does not take elastic backer rod. A rocking shower tray is a support problem, not a bead-shape problem.