A bead of sealant is one of the smallest, cheapest components on a commercial building’s exterior — and one of the most consequential when it fails.
A few metres of degraded sealant around a window surround or expansion joint can be the difference between a dry building and a recurring leak that eventually reaches structural steel or damages interior finishes. Because sealant failure usually starts small and hidden in a joint, it’s often the last thing anyone notices and the first thing that should be checked.
This article covers why exterior sealant fails on commercial and high-rise buildings, what accelerates that failure in New Zealand conditions, and what proper replacement actually involves.
Why Sealant Matters More Than Its Size Suggests
Sealant is designed as the first line of defence in a building’s weathertightness system, working alongside flashings as the second line of defence that drains any water the sealant doesn’t stop back out to the exterior. Under the New Zealand Building Code, air seals around penetrations and joints are a specific compliance requirement, not an optional finishing touch.
When a joint is designed and installed correctly, that system works quietly for years. When it fails, the consequences compound: what starts as a gap in one joint can allow moisture into the wall cavity, where it may travel well before it becomes visible.
Signs of Sealant Failure Worth Checking For
- Visible gaps where sealant has pulled away from one or both sides of a joint
- Hardening or brittleness (sealant that no longer flexes when pressed)
- Cracking or splitting along the length of a joint
- Staining or discolouration around the sealant, sometimes from plasticizer bleed on porous substrates
- Water stains, bubbling paint, or damp patches on the interior near an exterior joint
Sealant condition is one of the checks that belongs in a regular façade inspection programme rather than being assessed only once a leak has appeared.
The Core Ways Sealant Fails
Most sealant failures fall into a handful of recognisable categories, and knowing which one you’re looking at points directly to the cause.
Adhesion failure. This is when the sealant pulls cleanly away from one or both sides of the joint, leaving a visible gap along the substrate edge. It almost always traces back to surface preparation and dust, oil, moisture, or old sealant residue on the substrate at the time of installation, or the wrong (or no) primer used for that material. Adhesion failure points to a problem with how the joint was prepared, not with the sealant product itself.
Cohesive failure. This is when the sealant tears apart internally rather than losing its grip on the substrate. It held onto both sides of the joint but couldn’t handle the movement between them. This typically means the joint moved beyond the sealant’s rated movement capability, or the sealant was installed too thin or too shallow for the width of the joint.
Three-sided adhesion. A common installation defect where the sealant bonds to the back of the joint as well as both sides, usually because a backer rod wasn’t used or was sized incorrectly. A properly formed joint should only bond on two sides, allowing the sealant to flex as the joint moves. Without a backer rod, the sealant becomes rigid at the base and fails early, regardless of how well it was applied on the surface.
UV and weathering degradation. Direct sun exposure breaks down sealant chemistry over time, causing hardening, chalking, and eventually cracking. How fast this happens depends heavily on the sealant type.
Polyurethane sealants are carbon-based and degrade under direct UV exposure, which is why they’re typically specified to be painted over rather than left exposed. Silicone sealants have a silicon-oxygen backbone that resists UV breakdown far better and can be left exposed for decades, but can’t be painted and cost more up front. A polyurethane joint left unpainted and exposed to direct sun is one of the most common avoidable causes of early sealant failure on commercial buildings.
Substrate failure. Sometimes the sealant itself is fine, but the material it’s bonded to is failing underneath it — cracking render, spalling concrete, or degraded masonry. In this case, resealing the joint without addressing the substrate just delays the same failure, since the sealant can’t outperform the material it’s attached to.
What Accelerates Sealant Failure in New Zealand Conditions
Several factors specific to New Zealand’s climate and commercial building stock push sealant toward earlier failure:
- UV exposure by orientation: north- and west-facing joints take the most direct sun and typically fail years before south-facing joints on the same building
- Coastal and salt exposure: salt-laden air accelerates the breakdown of exposed sealant surfaces the same way it accelerates coating failure
- Wind-driven rain and building movement: taller buildings experience more thermal movement and wind-induced flexing across their façade, which places expansion and control joints under more cyclical stress than low-rise structures
- Incompatible repairs: sealing over old, degraded sealant with a different product is a common source of premature failure, since silicone in particular doesn’t bond reliably to old polyurethane residue, and mixing sealant chemistries on the same joint is asking for adhesion problems down the line
- Original installation quality: joints sealed without a correctly sized backer rod, at the wrong width-to-depth ratio, or over a poorly prepared substrate are set up to fail early no matter how good the sealant product is
Getting Sealant Replacement Right
Proper sealant replacement isn’t a matter of running a new bead over the old one. It generally involves fully removing existing sealant and residue back to a clean substrate, confirming compatibility between the new sealant and the surface it’s bonding to, installing a correctly sized backer rod to maintain a two-sided bond, and matching the joint’s width-to-depth ratio to the sealant manufacturer’s specification. Skipping any of these steps tends to produce a repair that looks fine on completion and fails again within a year or two.
Where sealant failure has already allowed water into a wall cavity or reached structural steel, it’s also worth checking for corrosion at the same time — our corrosion protection and speciality coatings team can assess and treat affected steelwork alongside the sealant repair.
Get Your Sealant Assessed and Replaced Properly
Connect Access carries out façade sealant replacement and joint sealing for commercial and high-rise buildings across Auckland and Hamilton, using rope access to reach every joint on a building without the cost and set-up time of scaffolding. Our technicians can assess sealant condition as part of a broader façade inspection and move directly into replacement work where it’s needed.
If you’re seeing gaps, cracking, or recurring leaks around your building’s joints, get in touch with Connect Access for an assessment, or call 027 457 1077 to talk through what you’re seeing.
