How wide a silicone joint can be

The practical range

An elastic bathroom or kitchen joint usually lives well between 5 and 15 mm wide. Under 5 mm there is almost no body: the first movement of the building peels it. Between 12 and 20 mm it is still possible if you limit depth with closed-cell PE backer rod. Beyond about 20–25 mm the mass sags and no longer recovers. A gap of 25 mm or more I do not close with silicone: a cover strip, plaster, carpentry adjustment or whatever the build needs, then a closing joint.

Flooding a hole with more paste does not make it stronger. The opposite: the deeper you gun blindly, the sooner it fails. That is three-sided adhesion: the sealant sticks to both flanks and the back, and cannot stretch.

Width and depth

Depth has to follow width. As a workshop habit, not a promise for every cartridge:

• about 5 mm wide wants on the order of 6 mm of silicone;
• about 10 mm, around 8 mm deep;
• about 15 mm, around 10 mm.

On wider joints, a common rule is depth close to half the width, with a minimum of about 6 mm. If the void is deeper than that section, backer rod goes in: it frees the third face, gives something to tool against, and stops the bead sinking.

PE rod is usually 20–30 % fatter than the gap so it wedges. Do not pierce it: gas opens bubbles in the fresh silicone.

How I do it

1. Measure width and depth. Over 25 mm: reduce the gap first.
2. Push the backer rod in.
3. Degrease the flanks. Primer if the substrate is critical or absorbent.
4. Cut the nozzle to the width. Fill from the back, without voids.
5. Tool with a spatula a millimetre wider than the joint.

In a bathroom, sanitary silicone. On floor or building joints, often an elastic hybrid with decent movement capacity — typical classes sit around 20–25 %, which is a product class, not a guarantee for that joint.

Common mistakes

Without backer rod the bead slumps, you burn a cartridge, and it tears. A 2–3 mm hairline is not an elastic joint. A 30 mm hole is not “filled” with silicone.

Exceptions

A junction that moves more than the sealant can take wants a movement profile, not more depth of paste. Backer rod does not turn a structural hole into a bathroom joint.