Yes, you can use a pressure washer to clean your roof before painting — and in most cases, it's strongly recommended. A thorough wash removes lichen, moss, algae, dirt, and loose paint that would otherwise stop your new paint from bonding properly. However, using the wrong pressure or technique can damage your roofing material, void warranties, or create a safety hazard. Here's what you need to know before you start.
Why Roof Washing Matters Before Painting
New paint applied over a dirty, mossy, or chalky surface simply won't last. Lichen and algae are particularly common on NZ roofs thanks to our humid climate, and they create a barrier between the roof surface and the paint. Skipping or rushing the wash is one of the most common reasons a roof paint job fails prematurely. BRANZ research consistently highlights surface preparation as the single biggest factor in coating durability.
Choosing the Right Pressure Setting
This is where many DIYers go wrong. Too much pressure can crack tiles, lift metal flashings, strip granules from concrete tiles, or force water under laps in iron roofing.
As a general guide:
| Roof Type | Recommended Pressure | Nozzle Tip |
|---|---|---|
| Corrugated iron / steel | 1,500–2,000 psi (100–140 bar) | 25–40° fan tip |
| Concrete or terracotta tiles | 1,200–1,500 psi (80–100 bar) | 25–40° fan tip |
| Asphalt shingles | 1,000–1,200 psi (70–80 bar) | 40° fan tip |
| Butynol / membrane roofs | Avoid high pressure — soft wash only | — |
Always start at the lower end and test on a small, inconspicuous area first. Never use a zero-degree (red) nozzle on any roof surface.
Step-by-Step: How to Pressure Wash a Roof
1. Apply a biocide treatment first
For heavy lichen or moss, apply a purpose-made roof wash solution and leave it to dwell for the time specified on the label. This kills the organism at the root and makes it far easier to remove. Products are available from Bunnings and Mitre 10. Check the label to ensure it's safe for your roof type and won't harm your garden or stormwater system.
2. Work from the ridge down
Always wash from the top of the roof downward, working with the direction of the laps on iron roofing. This prevents water being forced under sheets or tiles.
3. Keep the nozzle moving
Hold the wand at a consistent angle (roughly 45°) and keep it moving — never hold it stationary on one spot. Maintain a distance of at least 300–400 mm from the surface.
4. Rinse thoroughly
Make sure all biocide residue, loose material, and detergent is fully rinsed off before you allow the roof to dry.
5. Allow adequate drying time
Most roof paints require the surface to be completely dry before application. In NZ's variable climate, allow at least 24–48 hours of dry weather. Check your paint manufacturer's data sheet — brands like Resene, Dulux, and Wattyl all publish specific surface preparation requirements for their roof coatings.
Safety on the Roof: Don't Underestimate the Risks
Working on a wet roof is genuinely dangerous. A pressure washer adds recoil force, a trailing hose, and the distraction of managing water flow — all while you're at height.
Key safety points:
- Never work on a roof alone. Have someone on the ground at all times.
- Use a properly rated roof ladder or hook-over ladder, not a standard ladder leant against the guttering.
- Wear non-slip footwear rated for roof work.
- Be aware that a wet roof surface — especially iron or tiles — becomes extremely slippery the moment water hits it.
- Check WorkSafe NZ guidelines on working at height before you start.
If your roof pitch is steep (generally over 25°), or if you're not confident working at height, this is a job best left to a professional.
Concrete and Terracotta Tiles: Extra Caution Needed
Concrete tiles are porous and can be damaged by excessive pressure, which strips the surface layer and makes them more vulnerable to future moisture ingress. Terracotta tiles can crack under high pressure. If in doubt, use a lower pressure and let the biocide do more of the work.
Iron Roofing: Watch for Rust and Loose Fasteners
Pressure washing can reveal rust spots, loose screws, or lifted laps that you'll need to address before painting. This is actually one of the benefits of a thorough wash — it exposes problems early. Any rusted areas should be treated with a suitable rust converter or primer before topcoating. Check with your paint supplier for compatible systems.
When to Call a Professional
Consider hiring a qualified roof painter if:
- Your roof pitch is steep or the roof is large and complex.
- You're not comfortable working at height.
- The roof has significant damage, widespread rust, or cracked tiles that need repair before painting.
- You're unsure whether your roof type can handle pressure washing.
- You want the job done with a warranty-backed result.
A professional roof painter will have the right equipment, insurance, and experience to prep and paint your roof safely and to a high standard. You can find qualified local roof painters on tradedirectory.co.nz.
Master Painters NZ member
Licensed Building Practitioner (LBP)
Roofing Association of NZ (RANZ)
