A leaking skylight curb is a frustrating but fairly common problem in NZ homes, especially after the kind of wind-driven rain we get in many parts of the country. The good news is that applying a liquid waterproofing membrane to a skylight curb is a task many capable homeowners can tackle themselves — provided the leak is straightforward, the substrate is sound, and you follow the correct process. If the flashing is badly corroded, the curb is structurally compromised, or the leak is inside the roof space rather than at the curb joint, you'll want a professional waterproofer or roofer involved.
Understanding the Skylight Curb and Why It Leaks
A skylight curb is the raised timber or metal frame that the skylight dome or glazing unit sits on. Water typically gets in at the junction between the curb and the surrounding roofing material — whether that's iron, membrane, tiles, or torch-on. Common causes include:
- Failed or cracked original sealant or flashing tape
- Timber curb movement causing gaps
- UV-degraded original membrane
- Poor original installation with inadequate upstand height
Before you reach for a tube of sealant, identify exactly where the water is entering. Trace the leak on a dry day — water can travel a long way from the entry point before it drips inside.
Is This Restricted Building Work?
Applying liquid membrane to a skylight curb is generally not restricted building work under the NZ Building Act, and in most cases does not require a building consent (it's considered maintenance and repair). However, if you're replacing the skylight unit itself, altering the curb structure, or the work affects the weathertightness of a new or significantly altered building, check with your local council first. MBIE Building Performance has useful guidance on what counts as exempt work under Schedule 1.
There is no electrical or plumbing work involved in sealing a curb, so no licensed trades are required for this specific task.
Choosing the Right Liquid Membrane Product
Not all liquid membranes are suitable for roofing or UV-exposed applications. For a skylight curb, look for a product specifically rated for roofing and external waterproofing. Good options available in NZ include:
- Sika products — Sika NZ offers liquid-applied waterproofing systems suitable for roofing details
- Ardex waterproofing systems — Ardex NZ has membrane products used in roofing and wet-area applications
- Mapei — Mapei NZ also supplies liquid membranes suited to external use
Always read the product's technical data sheet. Check that it's compatible with your substrate (timber, metal, existing membrane), that it can handle ponding water if relevant, and that it's UV-stable if it won't be covered.
Step-by-Step: Applying Liquid Membrane to a Skylight Curb
1. Safety First
Work on a dry, calm day. Use appropriate fall protection if your roof pitch or height creates a fall risk — WorkSafe NZ has clear guidance on working at height safely. Non-slip footwear and a roof ladder or crawl board are essential.
2. Clean and Prepare the Surface
This is the most critical step. Remove all loose sealant, dirt, moss, and old flashing tape. A stiff brush, scraper, and a solvent wipe (as recommended by your membrane manufacturer) will help. The surface must be clean, dry, and dust-free.
3. Repair Any Gaps or Cracks First
Fill significant gaps or cracks with a compatible polyurethane sealant before applying membrane. Allow to fully cure per the manufacturer's instructions.
4. Apply Reinforcing Fabric at Joints
At the curb-to-roof junction, embed a polyester reinforcing mesh or fleece into the first coat of liquid membrane. This bridges movement and prevents cracking — skipping this step is one of the most common DIY mistakes.
5. Apply Membrane in Coats
Apply the liquid membrane by brush or roller in the number of coats specified on the data sheet (typically two to three). Maintain the minimum wet film thickness specified. Allow each coat to cure before applying the next.
6. Check the Upstand Height
The membrane should extend at least 150 mm up the curb above the finished roof surface — this is a general requirement under NZ Building Code clause E2 (External Moisture). If your curb is lower than this, water can back up under the skylight in heavy rain.
7. Inspect and Test
Once fully cured, run a hose over the area for several minutes and check inside for any sign of moisture.
Common Mistakes to Avoid
| Mistake | Why It Matters |
|---|---|
| Applying over damp or dirty surfaces | Membrane won't bond; will peel or blister |
| Skipping reinforcing mesh at joints | Membrane cracks with movement |
| Insufficient upstand height | Water backs up in heavy rain |
| Using interior-grade membrane | Will degrade rapidly outdoors |
| Single thin coat | Won't achieve required film thickness |
When to Call a Professional
Call a qualified waterproofer or roofer if:
- The curb timber is rotten or structurally unsound — this needs repair before any membrane work
- The leak is extensive or has caused internal damage (mould, wet insulation, ceiling staining)
- Your roof is steep or high and safe access is not possible without scaffolding
- The skylight is on a torch-on or butyl membrane roof where specialist knowledge is needed to tie in correctly
- You're unsure whether the work requires a building consent
A professional waterproofer will also be able to provide a warranty on their work, which matters for insurance and future property sales. You can find qualified local waterproofers and roofers on tradedirectory.co.nz.
Site Safe
SiteWise
Licensed Building Practitioner (LBP)
