Sealant movement classes: why class 25 matters on a facade
A facade joint moves with every hot day and cold night. A sealant rated below the joint's movement tears. Class 25 sealant and the right joint design are what hold. Here's the logic.
A facade joint on a Chatswood tower opens and closes every single day. The sun heats the panels and they expand, the joint narrows. The night cools them and they contract, the joint widens. Over a year that joint cycles thousands of times. A sealant that cannot stretch and compress with that movement tears away from the joint face within a couple of seasons, and the weather gets in. The number that decides whether the sealant survives is its movement class, and on a facade that usually means class 25.
A facade joint is not a static gap. It is a moving target, and the sealant has to move with it.
Why facade joints move
Building elements expand and contract with temperature. A dark facade panel in full Sydney summer sun can reach well above the air temperature, then drop back overnight. Each material moves at its own rate, its coefficient of thermal expansion, and the joints between elements exist precisely to absorb that movement. A joint between two panels might open and close by several millimetres across a daily cycle, and more across the seasonal range.
Structural movement adds to it, slight building sway, deflection under load, settlement. The joint is the designed relief that lets all of this happen without the elements cracking against each other. The sealant filling the joint has to flex with every cycle, in tension as the joint opens, in compression as it closes, indefinitely.
What a movement class means
Sealants are classified by the movement they can accommodate as a percentage of the joint width, under the relevant standard. A class 25 sealant can accommodate plus or minus 25 percent movement, the joint can widen by a quarter and narrow by a quarter, repeatedly, without the sealant failing. A class 12.5 sealant tolerates half that. Lower classes, less again.
Match the class to the movement the joint actually sees. A facade expansion joint that cycles plus or minus 20 percent across the year needs at least a class 25 sealant to survive it. Fill that joint with a class 12.5 product and you have specified a sealant that will be torn past its limit on the hot and cold extremes, every year. It fails not because it is a bad sealant, but because it was the wrong class for the joint.
Joint design is half the answer
The class is necessary but not sufficient. The joint has to be designed so the sealant works within its capacity. Two details matter most. The width-to-depth ratio, the sealant bead has to be shaped, typically wider than it is deep on a facade joint, so it can flex without over-straining. A bead that is too deep is too stiff and tears. The backing rod sets this depth and also breaks the bond at the back of the joint, so the sealant adheres to only the two joint faces, not the back, and can stretch freely between them, two-sided adhesion, not three.
Three-sided adhesion, where the sealant sticks to the back of the joint as well as the two faces, is a classic failure. The sealant cannot stretch, the stress concentrates, and it tears. The backing rod prevents this. So a durable facade joint is the right movement class, the right width-to-depth ratio, a backing rod for two-sided adhesion, and a primer if the substrate needs it. Miss any one and even a class 25 product fails early.
Why this is a common remedial defect
On Sydney's 2005 to 2015 mid-rise stock, facade sealant joints were often filled with a lower-class product, or detailed without a backing rod, or both. They performed for a decade, then the daily cycling tore them past their limit and the weather started getting in. The water reaches the substrate, sits in the cavity, and either corrodes wall ties or surfaces inside. The re-seal is not a re-caulk. It is rake out the old sealant entirely, install a backing rod to set the depth and break the back bond, prime if required, and install a class 25 sealant tooled to the right profile.
What to do next
- On any facade sealant scope, confirm the movement class specified, class 25 or higher for a facade expansion joint.
- Confirm a backing rod is specified, so the sealant has two-sided adhesion and can stretch.
- Treat a re-caulk over old sealant, with no backing rod and no class stated, as a repair that will tear within a couple of seasons.
- On a leaking facade joint, expect the water to have reached the substrate. Ask whether wall-tie corrosion behind the joint was checked.
How Supcon handles this
Thomas specifies the sealant to the joint movement, class 25 or higher on a facade expansion joint, and designs the joint to suit, backing rod for two-sided adhesion, the right width-to-depth ratio, primer where the substrate needs it. The old sealant comes out entirely before the new bead goes in, tooled to a concave profile.
The class and the joint design hold the weather out together. See facade defects in Sydney apartments for where this sits in the wider facade picture, and the facade repair service page for the technical detail.
Get a remedial scope. Send through the issue.