Movement Joint Sealants & Waterproof Detailing

Buildings move. Concrete expands and contracts with temperature, structures settle, and different materials move at different rates. A movement joint is designed to accommodate that movement — and the sealant is the part that has to stay flexible while it does.

Most sealant failures are joint-design failures, not product failures. The sealant needs the right width-to-depth ratio, a backer rod to control the depth, and a bond breaker so it adheres to the two faces and not to the bottom of the joint. Without that geometry, even a good sealant is stretched beyond its movement capacity.

The product has to match the movement. Polyurethane sealants suit most external construction joints and have good movement capacity; silicone suits glazing and some facades; polysulfide suits water-retaining and fuel-resistant applications; acrylics suit interior and low-movement joints. Choosing by price rather than by movement and exposure is how joints fail early.

Preparation decides adhesion. The joint faces have to be clean, dry and sound, with primer applied where the manufacturer requires it. Sealing over dust, old sealant or damp concrete produces a joint that looks finished and fails at the first movement.

At waterproofing transitions — where a membrane meets a wall, a drain or another system — the detail has to allow movement without breaking the waterproof line. We treat those transitions as part of the waterproofing design, not as a bead of sealant applied at the end.