Expansion and control joints are the deliberate gaps that let a building move. Concrete, masonry, and render all expand and contract with heat and moisture, and without joints to absorb that movement, the façade would simply crack. The joints do their job silently for years — until the flexible sealant that fills them perishes. Then the very line that was built to protect the wall becomes the fastest way for water to get in.
A failed joint is easy to miss and easy to underestimate. From the ground a perished joint just looks like a thin dark line, but up close the sealant has gone hard, cracked, pulled away from one face, or dropped out entirely. Because these joints run the full height of a building and sit at exactly the junctions where movement concentrates, a single failed run can channel water across multiple levels — feeding water ingress, staining, render debonding, and corrosion long before anyone connects it back to the joint.
On Sydney strata buildings, joint sealant is one of the most common maintenance items overlooked in a 10-year capital works plan. It is treated as trivial because it is small — yet a full reseal restores watertightness across an entire elevation for a fraction of the cost of the damage a leaking joint causes. Our approach treats the joint as the engineered movement detail it is, not as a cosmetic caulking line.
Our Process
- Joint Survey & Movement Assessment
- We inspect every joint run and assess the condition of the existing sealant, the joint width, and the degree of movement it accommodates. Getting the movement class right is what separates a joint that lasts from one that fails again in two years.
- Full Removal of Perished Sealant
- Old sealant is completely cut out — not overlaid. Overbanding fresh sealant onto failed material is a false economy that traps the failure underneath, so we take the joint back to clean, sound substrate on both faces.
- Substrate Preparation & Priming
- Both joint faces are cleaned of dust, laitance, and old adhesive, then primed where the sealant manufacturer requires it. Adhesion failure almost always traces back to preparation, so this step is non-negotiable.
- Backing Rod Installation
- A closed-cell or bond-breaker backing rod is installed to the correct depth. This sets the right sealant depth-to-width ratio and enforces two-sided adhesion — allowing the sealant to stretch and compress rather than tearing at a third bonded face.
- Correct-Movement Sealant Application
- We apply a movement-rated elastomeric sealant — typically polyurethane or a hybrid — selected for the joint's movement, exposure, and substrate compatibility, and tooled to a firm, weather-shedding profile.
- Weather-Line Reinstatement & Check
- Each joint is inspected for continuity, adhesion, and profile so the reinstated line forms an unbroken weather seal across the elevation. Where required, repaired joints can be water-tested to confirm performance.
Key Considerations
- Movement Class Is Everything – A sealant that can't accommodate the joint's actual movement will tear or debond. Specifying the right movement capability is the core of the job.
- Backing Rod Is Not Optional – Without a backing rod the sealant bonds on three sides and tears under movement. Correct joint geometry is what makes the seal last.
- Cut Out, Never Overband – Sealing over perished sealant hides the failure rather than fixing it. Full removal back to sound substrate is the only durable approach.
Value to the Client
- Restores the façade's weather line and stops joint-driven leaks.
- Prevents hidden damage — render debonding, corrosion, internal damp — fed by failed joints.
- Delivers watertightness across a whole elevation at low cost relative to the damage avoided.
- Extends the life of surrounding render, coatings, and structure.
- Closes a commonly overlooked gap in the building's maintenance plan.
Frequently Asked Questions
How do I know if our building's expansion joints have failed?
The clearest signs are sealant that has gone hard and cracked, pulled away from one or both joint faces, split along its length, or dropped out of the joint entirely. Internally, unexplained damp patches or staining that track vertically down a wall often line up with a failed joint on the outside. Because perished joints look like harmless thin lines from the ground, they are frequently missed until the resulting water damage appears — which is why joints should be inspected specifically as part of any façade condition survey.
Why can't we just seal over the old joint sealant?
Because overbanding traps the failure underneath. The new sealant bonds to old material that has already lost adhesion or gone brittle, so the repair fails at the buried interface — usually within a year or two — while looking fine from the surface. The only durable method is to cut the perished sealant out completely, take the joint back to clean sound substrate on both faces, install a fresh backing rod, and reseal. It costs a little more upfront and lasts many times longer.
What is a backing rod and why does it matter?
A backing rod is a flexible foam rod pushed into the joint behind the sealant. It does two critical things: it sets the correct depth of sealant relative to the joint width, and it acts as a bond breaker so the sealant only adheres to the two joint faces, not the back. This two-sided adhesion is what lets the sealant stretch and compress as the joint moves. Without a backing rod, sealant bonds on three sides and tears itself apart under the first movement cycles.
How long should a properly renewed expansion joint last?
A correctly specified and installed movement-rated sealant joint typically lasts 10 to 20 years, depending on exposure, joint movement, and the sealant chosen. The biggest factors in longevity are correct movement-class selection, proper joint preparation and priming, and correct backing-rod geometry — the three things most commonly skipped in a cheap reseal. We specify and install to the sealant manufacturer's requirements so the joint achieves its full rated service life.
Is expansion joint renewal a capital works item for our strata scheme?
Façade-wide joint renewal on common property is generally treated as capital works expenditure under the NSW Strata Schemes Management Act 2015, and is well suited to inclusion in the 10-year capital works plan. Because it protects the building envelope at relatively low cost, it is one of the more efficient preventative items an owners corporation can programme. We can provide scope and pricing documentation formatted for committee review and capital works planning.
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