Applying a waterproof coating to a small flat roof area is something many NZ homeowners can tackle themselves, provided the existing roof surface is structurally sound and the area is genuinely small — think a carport, lean-to, or small deck roof rather than a full dwelling roof. Get the preparation right, choose the correct product for your substrate, and you'll get a durable, weather-tight result that stands up to New Zealand's variable climate.
Understand the Limitations First
Waterproof coatings are a maintenance and remediation solution, not a structural fix. If your flat roof has sagging, rotting timber, delaminating plywood, or persistent ponding water, a coating alone won't solve the problem. New Zealand's NZ Building Code requires roofs to shed water effectively — if your roof has inadequate fall (less than 1:50 for most membrane systems), you may need more than a coating.
Also check whether your work requires a building consent. Minor maintenance and like-for-like repairs are generally exempt under Schedule 1 of the Building Act, but always confirm with your local council before starting, as exemptions depend on the scope of work.
Tools and Materials You'll Need
- Stiff-bristle broom and/or pressure washer
- Degreaser or roof cleaner
- Putty knife and flexible polyurethane sealant (e.g. Sika Sikaflex)
- Fibreglass reinforcing mesh (for cracks and joints)
- Primer suited to your substrate
- Waterproof roof coating (see product selection below)
- Short-nap roller, brush, and extension pole
- Safety footwear with good grip
- Safety harness if working at height (required under WorkSafe NZ guidelines for work at or above 3 metres)
Choose the Right Coating for Your Substrate
Different flat roof surfaces need different products. Using the wrong coating is one of the most common DIY mistakes.
| Substrate | Recommended Coating Type |
|---|---|
| Concrete or masonry | Acrylic or polyurethane membrane coating |
| Existing butyl or EPDM rubber | Rubber-compatible liquid membrane |
| Corrugated iron / steel | Rust-inhibiting primer + elastomeric roof paint |
| Fibreglass (GRP) | GRP-compatible topcoat or polyurethane |
| Plywood (with membrane) | Polyurethane liquid membrane with reinforcing fleece |
Products from Sika (such as their Sikalastic range) and Ardex are well-regarded in the NZ trade for liquid-applied membranes. For painted finishes over iron or steel, Resene and Wattyl both offer roof and metal coatings suited to NZ conditions. Always read the product data sheet and follow the manufacturer's instructions — coverage rates, recoat times, and temperature requirements vary.
Step-by-Step Application
1. Clean the Surface Thoroughly
This is the most critical step. Remove all dirt, moss, lichen, loose paint, and debris. Use a stiff broom first, then a pressure washer if available. Apply a moss and mould treatment if needed and allow to dry completely — in NZ's humid conditions, this can take 24–48 hours or more.
2. Repair Cracks, Joints, and Flashings
Use a flexible sealant to fill any cracks, open joints, or gaps around penetrations (pipes, vents, skylights). For larger cracks or movement joints, bed fibreglass reinforcing mesh into the first coat of coating while it's still wet. Pay particular attention to flashings — these are a leading cause of flat roof leaks in NZ homes.
3. Apply Primer
Most quality liquid membrane systems require a primer coat to ensure adhesion. Apply evenly with a roller or brush and allow to cure fully as per the product instructions before proceeding.
4. Apply the Waterproof Coating
Apply the first coat with a short-nap roller, working from the furthest point back towards your exit. Maintain an even, consistent thickness — thin coats applied in multiple layers are far more effective than one thick coat. Allow each coat to cure before applying the next. Most systems require two to three coats for full waterproofing performance.
5. Check Coverage and Cure Time
Once the final coat is applied, allow the full cure time before exposing the surface to foot traffic or rain. Check the weather forecast — most coatings need at least 4–6 hours of dry weather after application, though this varies by product.
Safety on the Roof
Flat roofs can be deceptively slippery, especially when wet or covered in algae. Wear non-slip footwear at all times. If the roof edge is unguarded and you're working at height, WorkSafe NZ requires fall protection — a safety harness anchored to a suitable point, or edge barriers. Don't work alone if possible.
When to Call a Professional
Call a qualified roofer if:
- The roof area is large, steeply pitched, or difficult to access safely
- There is structural damage, rot, or significant ponding
- Flashings need replacing or re-fixing (this can involve associated carpentry or plumbing work)
- You're unsure which product or system is appropriate
- The roof is on a habitable dwelling and you want the work to meet NZ Building Code compliance
For any work involving roof drainage connections to stormwater systems, a licensed drainlayer may be required — check with the Plumbers Gasfitters & Drainlayers Board. You can find experienced, local roofing professionals on tradedirectory.co.nz who can assess your roof and recommend the best solution.