A skylight leaking around its frame is one of the more common roofing headaches for NZ homeowners, and the good news is that the cause is usually straightforward — failed flashing, deteriorated sealant, or a poorly installed kerb. Catching it early and fixing it properly can save you from costly water damage to your ceiling, insulation, and framing.
First: Work Out Where the Water Is Actually Coming From
Before you reach for the sealant gun, take time to properly diagnose the leak. Water is sneaky — it can travel a long way from the entry point before dripping onto your ceiling. There are two main culprits:
- True water ingress — rainwater getting in through failed flashing, cracked sealant, or a damaged frame seal.
- Condensation — moisture forming on the inside of the skylight glass or frame, especially in poorly ventilated rooms like bathrooms and kitchens. This is not a leak at all, but it can look identical.
To tell them apart, check whether the dripping happens during or after rain (ingress) or on cold mornings regardless of rain (condensation). If it's condensation, improving ventilation or upgrading to a double-glazed skylight unit is the fix, not re-sealing the outside.
Common Causes of Skylight Frame Leaks
- Failed or missing flashing — the most frequent cause. Step flashing, apron flashing, and saddle flashing around the skylight kerb must be correctly lapped and sealed.
- Cracked or shrunk sealant — UV exposure and NZ's temperature swings cause sealant to harden and crack over time.
- Damaged or compressed kerb seal — the rubber gasket between the glazing unit and the kerb can perish.
- Incorrect fall or ponding — flat or near-flat roofs can allow water to pond against the skylight frame.
- Roof movement — timber framing moves seasonally, which can open up gaps around a rigid skylight frame.
Safety First Before You Go Up There
Working on a roof carries serious fall risk. WorkSafe NZ requires that anyone working at height takes appropriate precautions. For homeowners, this means:
- Only work on a dry, calm day.
- Use a properly footed ladder that extends at least 1 metre above the eave.
- Wear non-slip footwear.
- Never work alone — have someone at the base of the ladder.
- If your roof pitch is steep (over 25°) or the skylight is hard to access safely, call a professional roofer.
What You Can Do Yourself
1. Inspect the Flashing
Carefully examine the lead, aluminium, or butyl flashing around all four sides of the skylight. Look for:
- Lifted or separated edges
- Holes, cracks, or corrosion
- Missing sealant at laps and joints
- Flashing that has pulled away from the kerb
If the flashing is in generally good condition but has lifted edges or small gaps, you may be able to re-bed and re-seal it yourself.
2. Re-seal with the Right Product
Not all sealants are suitable for roofing. Use a neutral-cure silicone or a polyurethane sealant rated for exterior roofing use. Avoid acetoxy (acidic) silicones on metal flashings as they can cause corrosion. Sika offers a range of roofing-grade sealants available from most NZ trade suppliers.
Steps for re-sealing:
- Clean the area thoroughly — remove old sealant, dirt, and moss with a scraper and a suitable cleaner.
- Allow the surface to dry completely.
- Apply the new sealant in a continuous bead, pressing it firmly into the joint.
- Tool the sealant smooth and allow to cure fully before rain.
3. Check and Replace the Kerb Gasket
Many skylight units have a replaceable rubber gasket or compression seal between the glazing and the kerb. Check your skylight's brand and model — manufacturers like Velux provide replacement seals and installation guides. If the gasket is perished or flattened, replacing it is often a straightforward job.
4. Clear Debris and Check Drainage
Leaves and debris can dam water against the skylight frame. Keep the area clear, and check that any roof drainage channels around the skylight are unobstructed.
When Flashing Needs Full Replacement
If the flashing is corroded, badly torn, or was never installed correctly in the first place, a patch-and-seal approach won't last. Full flashing replacement is the right fix. This involves removing the surrounding roofing material (tiles, iron, or membrane), cutting and fitting new flashing to the correct profile, and re-laying the roof covering. This is skilled work — done incorrectly, it can make the leak worse and may affect your home's compliance with NZ Building Code clause E2 (External Moisture).
Building Consent Considerations
Simply repairing or re-sealing an existing skylight's flashing is generally maintenance work and unlikely to need a building consent. However, if you are replacing the skylight unit itself or altering the roof opening, check with your local council — some alterations fall outside the Schedule 1 exemptions. The MBIE Building Performance website has guidance on what work requires consent.
When to Call a Professional
Call a licensed roofer if:
- The flashing needs full replacement
- You can't safely access the skylight
- The leak has been ongoing and you suspect framing or insulation damage
- The roof covering around the skylight is damaged
- You've re-sealed it yourself and it's still leaking
A qualified roofer will also be able to confirm whether the installation meets the NZ Building Code and BRANZ good practice guidelines. You can find experienced local roofers on tradedirectory.co.nz.