Sealing gaps around exterior cladding joins is one of the most important weathertightness tasks a homeowner can tackle. Done correctly, it keeps moisture out of your wall system, protects the structure of your home, and helps meet the intent of the NZ Building Code's E2 clause on external moisture. With the right products and a bit of patience, most straightforward joins are well within DIY reach.
Why Sealing Cladding Joins Matters
NZ's climate — particularly in wetter regions like Auckland, Wellington, and the West Coast — puts exterior cladding under constant pressure from wind-driven rain. Gaps at joins, corners, window surrounds, and penetrations are the most common entry points for moisture. Left unsealed, water can track behind cladding, causing rot, mould, and costly structural damage. BRANZ research consistently identifies poor sealing as a leading cause of weathertightness failures in NZ homes.
What You'll Need
- Exterior-grade polyurethane or silicone sealant (paintable polyurethane is usually preferred for timber and fibre cement)
- Backer rod (foam rod to fill deep gaps before sealing)
- Caulking gun
- Masking tape
- Isopropyl alcohol or a suitable cleaner
- Putty knife or scraper
- Wet finger or caulking tool for smoothing
- Safety glasses and gloves
Choosing the Right Sealant
Not all sealants are equal, and using the wrong one is a common DIY mistake.
| Sealant Type | Best For | Paintable? |
|---|---|---|
| Polyurethane (e.g. Sika Flex) | Timber, fibre cement, most claddings | Yes |
| Silicone | Around windows, flashings, metal | Usually no |
| Acrylic/Latex | Interior use only | Yes |
For most exterior cladding joins — including fibre cement (such as James Hardie products) and timber weatherboard — a paintable polyurethane sealant is the standard trade choice. Sika and similar brands offer exterior-grade options widely available at Bunnings and Mitre 10. Always check the manufacturer's datasheet to confirm compatibility with your specific cladding material.
Important: Check your cladding manufacturer's installation guide before choosing a sealant. James Hardie, for example, specifies approved sealant types for their products, and using the wrong one can void warranties.
Step-by-Step: Sealing Cladding Joins
1. Inspect and Prepare the Gap
Remove any old, cracked, or peeling sealant with a scraper or utility knife. The gap needs to be clean, dry, and free of dust, oil, and loose material. Wipe surfaces down with isopropyl alcohol and allow to dry fully — don't attempt this job on a wet or humid day.
2. Size Up the Gap
For gaps deeper than 6–8 mm, insert a backer rod first. This foam rod gives the sealant a backing surface and ensures the correct sealant depth (roughly half the width of the gap). Without it, sealant in a deep gap can crack as it cures.
3. Mask Up
Apply masking tape either side of the join for a clean, straight finish. This is worth the extra few minutes — it makes a big difference to the final result.
4. Apply the Sealant
Cut the nozzle at a 45° angle slightly smaller than the gap width. Apply steady, even pressure on the caulking gun and move at a consistent pace along the join. Aim to slightly overfill the gap so you have material to tool back.
5. Tool and Smooth
Within a few minutes of application (before a skin forms), smooth the sealant with a wet finger or caulking tool, pressing it firmly into the join. Remove the masking tape immediately while the sealant is still wet, pulling it back at a 45° angle.
6. Cure Time
Allow the sealant to cure fully before painting over it — check the product datasheet, but most polyurethane sealants need at least 24–48 hours. Painting too soon can cause adhesion failure.
Common Mistakes to Avoid
- Sealing over damp or dirty surfaces — sealant won't bond properly
- Skipping backer rod in deep gaps — leads to premature cracking
- Using interior-grade acrylic outdoors — it will fail quickly
- Painting silicone sealant — paint won't adhere to most silicones
- Sealing over existing failed sealant without removing it first
Does This Work Need a Building Consent?
Routine maintenance sealing of existing joins is generally exempt from building consent under Schedule 1 of the Building Act. However, if you're replacing or significantly altering cladding, or if your home has had weathertightness issues, check with your local council before starting. Visit MBIE Building Performance for guidance on what work requires consent.
When to Call a Professional
Some situations go beyond straightforward DIY sealing:
- Widespread sealant failure across the whole house — this may indicate a systemic weathertightness problem
- Visible rot, mould, or soft framing behind cladding — stop and get a builder to assess
- Flashings that need replacing — incorrect flashing is a leading cause of leaks and should be done by a qualified builder
- Monolithic cladding systems (e.g. plaster/stucco) — these require specialist knowledge
- Any work that may trigger consent requirements
A Licensed Building Practitioner (LBP) is the right person for more complex cladding and weathertightness work. You can find qualified local builders on tradedirectory.co.nz.
Safety Note
Wear safety glasses and gloves when working with sealants. Work in a well-ventilated area, and keep sealants away from children. If working at height, use a stable ladder and follow safe working practices as outlined by WorkSafe NZ.