Movement Joint Silicone Sealing

A movement joint exists because a building moves. Thermal expansion, structural settlement, wind sway and shrinkage all have to go somewhere, and the joint is the designed place for it. Sealing one is a different job from running a bead round a bath: the material has to accommodate repeated movement for years without tearing, debonding or dragging the substrate apart.

This is the single thing that decides whether a movement joint survives. The sealant must bond to the two opposing faces of the joint and be free on the third, the back. If it also bonds to the back of the joint it cannot stretch: it is being pulled in two directions while anchored in the middle, and it tears, usually cohesively down the centre in the first cold spell.

That is what backer rod is for. Closed cell rod of the correct diameter, set to the correct depth with a roller rather than pushed in with a finger, both breaks the bond at the back and sets the depth of the sealant so the width to depth ratio is right. Where there is no room for rod, bond breaker tape does the same job. A joint sealed without either is a joint that will fail on schedule.

Sealants are rated for a movement capability expressed as a percentage of the joint width. That is why joint width is a design decision rather than whatever the two substrates happen to leave: a joint too narrow for the movement it has to take will over-extend the sealant no matter which product goes in it. Where we are asked to seal an existing joint that is plainly too narrow, we will say so before we start rather than after it fails.

Silicone bonds to what it touches, so it has to touch sound material. Substrates come back to bare, clean, dry material rather than being sealed over paint, old sealant or dust, and porous substrates are primed where the manufacturer calls for it. In our experience the majority of internal silicone remediation we are asked to do traces back to one of three things: preparation rushed, the wrong chemistry for the substrate, or a bead sized for the gap rather than for the movement.

New build housing, commercial fit-outs, healthcare and industrial buildings, and the structural movement joints in car parks and podium decks. It is mostly internal and mostly daytime, which makes it the exception in our work, but the discipline is the same as the joints we seal on a runway.

We are based at Little Westerhill Farm near Maidstone in Kent. Kent, London, Sussex, Essex and the wider South East are covered from our own yard, and longer programmes are mobilised UK wide. This sits within our silicone sealing service. For pavement and structural joints outdoors, see joint sealing.