Resealing roof flashing is one of those jobs many NZ homeowners can tackle themselves — provided the flashing itself is still structurally sound and the repair is minor. If the flashing is lifting, cracked, or corroded, or if the leak is significant, you're better off calling a qualified roofer. But for small gaps, dried-out sealant, or a bit of butyl tape that's given up the ghost, a careful DIY reseal is absolutely achievable.
Understanding Roof Flashing and Why It Fails
Roof flashing is the metal (usually aluminium, copper, or galvanised steel) or flexible membrane material fitted around penetrations — chimneys, skylights, pipes, valleys, and wall junctions — to direct water away from vulnerable joins. In New Zealand's variable climate, flashing sealants are put under constant stress: UV exposure, thermal expansion and contraction, and heavy rainfall all cause sealants to dry out, crack, or delaminate over time.
The NZ Building Code clause E2 (External Moisture) sets the standard for weathertightness, and flashing details are a critical part of meeting it. A failed flashing seal is one of the most common causes of roof leaks in NZ homes.
Silicone vs Butyl Tape: Which Should You Use?
Both products have their place, but they suit different situations.
| Product | Best For | Pros | Watch Out For |
|---|---|---|---|
| Silicone sealant | Gaps, joints, around pipe penetrations | Flexible, waterproof, UV-resistant | Must be applied to a clean, dry surface; some types don't bond well to certain metals |
| Butyl tape | Overlapping flashing joins, under cappings | Self-adhesive, no curing time, good for metal-to-metal | Can dry out and shrink over many years; surface prep still essential |
| Polyurethane sealant | High-movement joints | Excellent adhesion and flexibility | Slower to cure; paintable |
For most DIY flashing reseals, a neutral-cure silicone (not acetic-cure, which can corrode metal) or a quality butyl tape are your go-to options. Look for products rated for roofing and outdoor use — brands like Sika offer roofing-specific sealants widely available in NZ. You'll find a range of suitable products at Bunnings or Mitre 10.
Step-by-Step: How to Reseal Flashing
1. Safety First
Working at height carries real risk. Always use a stable ladder, wear non-slip footwear, and never work on a wet or frosty roof. If your roof pitch is steep (generally above 25°) or you're not confident working at height, please don't attempt this yourself — WorkSafe NZ has clear guidance on working at height safely.
2. Inspect the Flashing
Before reaching for the sealant, check whether the flashing itself is:
- Still firmly fixed and not lifting away from the surface
- Free from significant rust, corrosion, or physical damage
- Correctly lapped and positioned
If the flashing is damaged or incorrectly installed, resealing over the top is only a temporary fix.
3. Remove Old Sealant
Use a utility knife or scraper to remove as much of the old, failed sealant as possible. Residual sealant prevents good adhesion. Clean the area with a rag and a suitable solvent (methylated spirits works well on most metals — check the sealant manufacturer's instructions).
4. Allow the Surface to Dry Completely
This is critical. Silicone and butyl tape will not bond properly to damp surfaces. Choose a dry, settled day — not uncommon advice in NZ, but worth waiting for.
5. Apply Your Sealant or Tape
- Silicone: Apply a continuous bead along the joint, then tool it smooth with a wet finger or caulking tool to ensure full contact. Don't leave gaps.
- Butyl tape: Cut to length, peel the backing, and press firmly into place. Run over it with a roller or the heel of your hand to ensure full adhesion.
6. Check Your Work
Once cured (allow 24 hours for silicone), inspect the seal visually. A garden hose test — running water over the area — can help confirm the repair before the next downpour.
Does This Work Require a Building Consent?
Minor maintenance and repair of existing flashing — like resealing a joint — is generally exempt from building consent under Schedule 1 of the Building Act 2004. However, if you're replacing or significantly altering flashing, the rules may differ. Always confirm with your local council before starting any work you're unsure about. You can find guidance at MBIE Building Performance.
When to Call a Professional
Call a qualified roofer if:
- The leak is ongoing or significant — water staining on ceilings, mould, or wet insulation suggests more than a surface seal issue
- The flashing is lifting, corroded, or damaged — resealing won't fix a structural problem
- You're working near roof penetrations tied to plumbing or gas — any work on pipes, gas flues, or drainage connections may involve restricted work under the Plumbers, Gasfitters and Drainlayers Board (PGDB) rules, which must be carried out by a licensed tradesperson
- Your roof is steep, fragile (e.g. old terracotta tiles), or difficult to access safely
- You're a landlord — under the Healthy Homes Standards, rental properties must be weathertight, and a professional repair with documentation is advisable
A licensed roofer can also identify underlying issues you might miss. You can find qualified local roofers on tradedirectory.co.nz.