Failing exterior sealant is one of the most common — and most overlooked — causes of water damage in New Zealand homes. A quick visual inspection a couple of times a year can catch problems early, saving you from expensive repairs to cladding, framing, and interior linings. Here's what to look for and where to look.
Why Exterior Sealant Matters
Sealant (sometimes called caulk or mastic) fills the joints and gaps between different building elements — window frames, cladding sheets, flashings, and penetrations like pipes and cables. When it fails, water finds a way in. Under the NZ Building Code, the E2 External Moisture clause requires buildings to resist water ingress, and properly maintained sealant is a key part of meeting that standard. New Zealand's variable climate — UV exposure in summer, driving rain and wind in winter — puts sealant under significant stress year-round.
Where to Inspect First
Focus your inspection on these high-risk areas:
- Window and door frames — the joint between the frame and the surrounding cladding
- Cladding sheet joints — particularly on fibre cement products like James Hardie systems, where butt joints must be sealed
- Flashings — around roofline junctions, parapets, and where the roof meets a wall
- Penetrations — pipes, cables, vents, and air conditioning units passing through walls
- Deck and balcony junctions — where a deck meets the house wall
- Corner junctions — external corners where two cladding faces meet
- Sill trays and head flashings — above and below windows
Key Signs That Sealant Is Failing
1. Cracking or Splitting
Cracking is the most obvious sign. Sealant that has dried out or lost elasticity will crack along its length or split away from one or both sides of the joint. Even hairline cracks can allow water entry during heavy rain.
2. Shrinkage and Pulling Away
Sealant that has pulled away from the substrate on one or both sides leaves an open gap. Run your finger along the bead — if it lifts or feels loose, it has lost adhesion. This is common on older homes where low-quality or incorrect sealant was used.
3. Discolouration and Mould Growth
Black or green staining on or around a sealant bead can indicate moisture is already getting in behind it. Surface mould on the sealant itself suggests it has been consistently damp — a sign the seal is no longer watertight.
4. Bubbling or Blistering
Bubbles or blisters in the sealant bead suggest moisture has become trapped beneath it, or that the wrong product was applied over a damp surface. This compromises the bond significantly.
5. Hardness and Brittleness
Press the sealant gently with your fingernail. Good sealant should feel slightly flexible. If it feels rock hard or crumbles, it has reached the end of its service life and needs replacing.
6. Missing Sealant
Sometimes sealant is simply absent — either it was never applied, or it has completely fallen out. Check joints carefully; gaps that should be filled are a clear red flag.
7. Paint Peeling Nearby
Peeling or bubbling paint on cladding adjacent to a joint can be a secondary sign that moisture is tracking in through a failed seal, even if the sealant itself looks intact at first glance.
How to Do a Thorough Inspection
Use this simple process:
| Step | What to Do |
|---|---|
| 1 | Walk around the full perimeter of the house on a dry day |
| 2 | Use binoculars or a ladder to check upper-storey joints |
| 3 | Run your finger along all sealant beads to feel for gaps or hardness |
| 4 | Check inside for water stains on walls or ceilings near windows and penetrations |
| 5 | Inspect after heavy rain — look for damp patches or drips |
Do this inspection at least twice a year — ideally in autumn before the wet season, and in spring after winter. BRANZ recommends regular maintenance inspections as a key part of keeping your home weathertight.
Choosing the Right Replacement Sealant
Not all sealants are equal. For exterior use, look for a paintable polyurethane or silicone-modified sealant rated for exterior exposure. Check the manufacturer's data sheet to confirm it suits your cladding type. Products are available at Bunnings and Mitre 10. Always remove the old sealant fully and ensure the surface is clean and dry before applying new product — applying over failed sealant is a common DIY mistake that won't last.
When to Call a Professional
While spotting failing sealant is a job any homeowner can do, re-sealing is not always straightforward. If you find:
- Evidence of water damage behind the cladding (soft or discoloured framing, mould smell)
- Widespread sealant failure across the whole house
- Failure around flashings or at cladding-to-roof junctions
- Any signs of structural moisture damage
...you should engage a Licensed Building Practitioner (LBP) or a qualified building inspector before attempting repairs. Incorrect re-sealing can trap moisture inside the wall cavity, making things worse. You can find experienced local building inspectors and tradespeople on tradedirectory.co.nz. Check that any LBP you hire is registered at lbp.govt.nz.
Safety note: When using a ladder to inspect upper-storey areas, follow safe ladder practices as outlined by WorkSafe NZ. Never lean out beyond the ladder's side rails.
Licensed Building Practitioner (LBP)
BCITO Qualified
