Painting over a bitumen-coated roof is possible, but it's one of the trickier roofing jobs a homeowner can tackle — and getting it wrong can cost you far more in repairs than the paint job itself. Bitumen is an oily, tar-based product that bleeds through many standard paints, causing adhesion failure and an unsightly mess. With the right preparation and specialist products, however, you can achieve a durable, weatherproof finish.
Why Bitumen Is a Problem for Standard Paints
Bitumen (also called tar or asphalt coating) is commonly found on older NZ roofs — particularly on corrugated iron, concrete tiles, and flat membrane roofs. It was widely used as a waterproofing and rust-inhibiting treatment for decades.
The core problem is that bitumen contains oils and resins that bleed upward into any coating applied on top. Standard acrylic or oil-based paints simply cannot bond reliably to a bituminous surface, and the dark pigments will leach through, causing discolouration and peeling within months.
Step 1 — Assess the Condition of the Existing Bitumen
Before you do anything else, inspect the existing coating carefully:
- Flaking or peeling bitumen must be removed before any new coating is applied. Use a stiff-bristled brush or scraper to remove loose material.
- Cracked or cratered bitumen may indicate the underlying substrate (iron, concrete, or membrane) has deteriorated. In this case, a full restoration or replacement may be needed rather than a paint-over.
- Stable, well-adhered bitumen that is simply weathered and faded is the best candidate for overcoating.
If you're unsure about the condition of the substrate beneath, it's worth having a qualified roof painter or builder assess it first. You can find experienced local roof painters on tradedirectory.co.nz.
Step 2 — Clean the Surface Thoroughly
Proper cleaning is non-negotiable. Bitumen surfaces collect moss, lichen, dirt, and algae — especially in NZ's humid climate.
- Water blast the entire roof at low-to-medium pressure to remove loose debris, moss, and dirt.
- Apply a moss and mould treatment and allow it to dwell as per the manufacturer's instructions.
- Rinse thoroughly and allow the roof to dry completely — at least 24–48 hours of dry weather.
Never apply any coating to a damp surface.
Step 3 — Apply a Bitumen-Blocking Primer
This is the most critical step. You must use a bitumen-blocking or stain-blocking primer specifically formulated to seal in bitumen bleed. Standard primers will not work.
Look for products such as:
- Shellac-based primers — highly effective at sealing bitumen but require solvent clean-up and good ventilation.
- Specialist bitumen-blocking sealers — some roofing product suppliers stock these specifically for this application.
Check with Resene or Dulux technical staff — both have knowledgeable teams who can advise on the correct primer system for your specific situation. Wattyl also offers technical advice on specialist coatings.
Apply the primer in accordance with the manufacturer's data sheet, paying close attention to coverage rates and recoat times.
Step 4 — Choose the Right Topcoat
Once the primer is fully cured, you can apply a roof membrane or roof paint suitable for your roof type:
- For metal (iron) roofs: use a quality acrylic roof paint rated for metal substrates.
- For concrete or terracotta tiles: use a breathable, flexible roof coating.
- For flat or low-pitch membrane roofs: consider a specialist elastomeric membrane coating.
Always apply at least two topcoats for adequate protection, following the manufacturer's recommended dry times between coats.
| Roof Type | Recommended System |
|---|---|
| Corrugated iron / steel | Bitumen-blocking primer + acrylic metal roof paint |
| Concrete tiles | Bitumen-blocking primer + breathable tile coating |
| Flat membrane roof | Bitumen-blocking sealer + elastomeric membrane coating |
Safety Considerations
Working on a roof carries serious fall risks. WorkSafe NZ requires that anyone working at height takes appropriate precautions:
- Use edge protection, scaffolding, or a safety harness anchored to a suitable point.
- Never work on a wet or dew-covered roof.
- Wear appropriate footwear with good grip.
- Solvent-based primers require good ventilation and appropriate respiratory protection.
If your roof pitch is steep (generally over 25°) or the roof is high off the ground, seriously consider engaging a professional.
NZ Building Code and Consent Considerations
In most cases, repainting an existing roof is maintenance work and does not require a building consent. However, if you are significantly changing the roofing system or repairing structural elements, check with your local council. MBIE Building Performance provides guidance on what work falls under Schedule 1 exemptions — but always confirm with your council directly, as requirements can vary.
When to Call a Professional
Call a qualified roof painter or roofing contractor if:
- The bitumen is heavily cracked, flaking, or the substrate beneath is corroded or damaged.
- Your roof has a steep pitch or is difficult to access safely.
- You are unsure which products are compatible with your specific roof type.
- The roof is on a rental property — Tenancy Services Healthy Homes standards may apply, and a professional finish is important.
A licensed roof painter will have the right equipment, product knowledge, and insurance to get the job done safely and correctly. Find qualified local roof painters on tradedirectory.co.nz.
Master Painters NZ member
Licensed Building Practitioner (LBP)
Roofing Association of NZ (RANZ)
