Sealant and caulk

Frequently asked questions

Why does a sealant joint need a backer rod?

To control the DEPTH, and to stop the sealant sticking to the back of the joint. A sealant bead stretches when the joint opens, and how far it can stretch depends on its shape: a bead that is shallow relative to its width distributes the movement across a thin section and survives it, while a deep plug of sealant concentrates strain at the edges and tears away from one side. The usual design target is a depth of about half the width, within a stated minimum and maximum, and a closed-cell foam rod pressed into the joint sets that depth in one operation. It also creates the second condition: the sealant must bond to TWO sides only. If it also adheres to the back of the joint — three-sided adhesion — it cannot deform freely, and it splits down the middle or peels off a face the first time the joint moves properly. Where there is no room for a rod, bond-breaker tape does the same job.

What does a movement class actually promise?

The percentage of the joint's own width the sealant can take, repeatedly, without failing — and it is a percentage, which is why joint width is a design decision rather than whatever gap turns up. A sealant classified at twenty-five percent in a ten-millimetre joint accommodates two and a half millimetres of movement; the same product in a twenty-millimetre joint accommodates five. So the joint is sized from the movement expected, which comes from the panel length, the material's thermal expansion, and the temperature range it will see between the coldest night and a dark surface in summer sun. The classification also distinguishes extension from compression, which are not symmetrical — most sealants take compression better than extension. This is the reason a joint that failed is rarely fixed by a better sealant: if the gap is too narrow for the movement, every product in it will be over-extended.

Why does silicone fail on some surfaces and not others?

Because adhesion is chemistry plus preparation, and silicone is the least forgiving common sealant about both. It will not bond reliably to damp substrates, to dusty or chalking surfaces, to some plastics without a primer, or to anything still carrying a release agent or a mould oil — and on porous materials such as natural stone the oils in some formulations migrate into the surface and leave a permanent dark halo either side of the joint, which is why low-modulus neutral-cure grades are specified there. It also cannot be painted, so it must be the final finish rather than a stage in one. Against that, nothing beats it for permanent flexibility and weathering. The practical consequence is that the product is chosen from the SUBSTRATES and the movement together — polyurethane where the joint will be painted or trafficked, acrylic only where there is no movement and no weather, silicone where flexibility and water matter most — and every one of them fails on a surface that was not cleaned.