Technically you can paint over old flaking bitumen paint — but you really shouldn't, at least not without thorough preparation first. Applying new paint over loose, flaking material is one of the most common reasons roof paint jobs fail within a year or two. The good news is that with the right prep work, a freshly painted roof can look great and last a decade or more.
Why Flaking Bitumen Paint Is Such a Problem
Bitumen-based roof paints were widely used on New Zealand iron and steel roofs from the mid-20th century onward. Over time, UV exposure, thermal cycling (our roofs heat up and cool down dramatically between seasons), and moisture cause the film to harden, crack, and flake. When you apply a new coating over a surface that is already failing, the new paint bonds to the old flaking layer — not to the roof itself. As the old paint continues to lift, it takes your expensive new topcoat with it.
Beyond adhesion failure, flaking bitumen can also bleed through water-based topcoats, causing brown staining and discolouration. This is called bitumen bleed, and it's a well-known headache for roof painters.
Assessing the Condition of Your Roof
Before you buy a single tin of paint, get up there (safely — see the safety note below) and assess what you're dealing with:
- Lightly weathered/chalky bitumen — surface is dull and chalky but still largely intact. This is the best-case scenario and requires less prep.
- Moderate flaking — paint is lifting in patches. Significant prep required before repainting.
- Heavy flaking or peeling — large sections are loose or missing. Consider whether full removal is more practical than spot-treatment.
- Rust underneath — if the steel or iron substrate is rusting where paint has lifted, this must be treated before any new coating goes on.
Also check for any damaged or missing roofing sheets, loose screws, or failed flashings. It's far better to fix these now than after you've painted.
Surface Preparation: The Most Important Step
Professional roof painters will tell you that preparation is 80% of the job. For a bitumen-painted roof, this typically involves:
1. High-Pressure Washing
A water blaster (pressure washer) is your first tool. Wash the entire roof to remove dirt, moss, lichen, and loose paint. Use a fan tip rather than a pinpoint jet to avoid denting the iron. Allow the roof to dry completely — at least 24–48 hours in fine weather.
2. Mechanical Removal of Loose Paint
Any paint that is still flaking after washing must come off. Use a wire brush, scraper, or angle grinder with a wire wheel to remove loose material back to a firm edge or bare metal. Don't skip this step — it's the difference between a job that lasts and one that fails.
3. Treat Rust
Bare metal exposed by flaking paint will rust quickly. Treat any rust spots with a rust converter or sand back to bright metal and apply a zinc-based primer suitable for steel roofing. Products from Resene and Dulux both offer rust-inhibiting primers designed for metal roofs — check their technical data sheets for compatibility with bitumen surfaces.
4. Apply a Bitumen-Compatible Primer or Sealer
This is the critical step that most DIYers miss. Because bitumen can bleed through standard water-based paints, you need either:
- A bitumen-compatible sealer/primer that locks down the old surface and prevents bleed-through, or
- A solvent-based topcoat that is inherently compatible with bitumen.
Always check the manufacturer's technical data sheet (TDS) to confirm compatibility. Wattyl and Resene both publish detailed TDS documents online. If in doubt, call their technical helplines — they're genuinely helpful.
Choosing the Right Topcoat
For New Zealand conditions — high UV, coastal salt air in many regions, and significant temperature swings — you want a high-build, elastomeric roof membrane or a quality acrylic roof paint rated for metal roofing. Key things to look for:
| Feature | Why It Matters |
|---|---|
| UV resistance | NZ has high UV levels year-round |
| Flexibility/elastomeric | Allows for thermal expansion of metal |
| Bitumen compatibility | Prevents bleed-through and adhesion failure |
| Mould/lichen inhibitors | Common issue in NZ's damp climates |
Light colours reflect heat and can reduce summer cooling loads — worth considering for Energywise reasons if your home runs hot in summer.
Safety First
Working on a roof carries a serious risk of falling. WorkSafe NZ requires that fall risks be managed — even for homeowners doing their own work. At minimum:
- Use a roof ladder or chicken ladder hooked over the ridge
- Never work on a wet or damp roof
- Consider hiring scaffolding for anything more than a quick inspection
- Work with a buddy who can call for help if needed
For steep or high roofs, this is genuinely a job best left to professionals.
When to Call a Professional
Consider hiring a qualified roof painter if:
- Your roof is steeply pitched or more than one storey high
- There is extensive rust or structural damage to the roofing iron
- You're unsure which products are compatible with your existing coating
- The roof has complex flashings around chimneys, skylights, or penetrations that need careful sealing
A professional roof painter will have the right equipment, access to trade-grade products, and the experience to spot problems you might miss. You can find qualified local roof painters on tradedirectory.co.nz.
Note: Roof painting is generally not restricted building work under the NZ Building Code, but if your roof work involves structural repairs or alterations, check with your local council whether a building consent is required. The MBIE Building Performance website has useful guidance on consent requirements.
Master Painters NZ member
Licensed Building Practitioner (LBP)
Roofing Association of NZ (RANZ)
