Lichen is one of the toughest roof invaders New Zealand homeowners face — its root-like rhizines grip deep into concrete tile surfaces, making it far more stubborn than moss or algae. The good news is that with the right products, a methodical approach, and sensible safety precautions, most homeowners can tackle a lichen problem themselves. However, if your roof is steep, high, or heavily colonised, calling in a professional is the smarter and safer choice.
Why Lichen Is So Hard to Remove
Unlike moss, which sits loosely on the surface, lichen is a symbiotic organism (part fungus, part algae) that physically bonds to the concrete. Simply pressure-washing it off can damage the tile surface and force moisture into the substrate. The key is to kill it first, then remove it — not the other way around.
New Zealand's humid climate, particularly in the North Island and coastal regions, creates ideal conditions for lichen growth. Left untreated, lichen can degrade the surface of concrete tiles, reduce their water-shedding ability, and eventually lead to leaks — so it's worth dealing with promptly.
Safety First — Before You Go Near the Roof
Working at height is the leading cause of serious injury and death in DIY situations in New Zealand. Before you start:
- Never work on a wet or damp roof — lichen-covered tiles are extremely slippery.
- Use a roof ladder or crawl board to spread your weight and avoid cracking tiles.
- Wear non-slip footwear, a safety harness if available, and have a second person present.
- Check the weather forecast — work only on dry, calm days.
- Review WorkSafe NZ's guidance on working at height before starting.
What You'll Need
| Item | Purpose |
|---|---|
| Biocide / lichen treatment spray | Kills lichen before removal |
| Garden sprayer or pump sprayer | Applying treatment evenly |
| Stiff-bristle brush or long-handled scrubbing brush | Loosening dead lichen |
| Low-pressure hose or soft-wash setup | Rinsing without damaging tiles |
| Safety harness, roof ladder, non-slip boots | Working safely at height |
| Protective gloves, goggles, old clothing | Chemical protection |
Step 1 — Choose the Right Biocide Treatment
Do not start by pressure washing. Instead, apply a purpose-made biocide or roof treatment product first. Look for products containing active ingredients such as benzalkonium chloride or similar biocidal agents, which are formulated to kill lichen, moss, and algae.
Well-known options available at NZ hardware stores include roof wash and moss/lichen treatments from brands stocked at Bunnings and Mitre 10. Always read the product label carefully and follow the manufacturer's dilution and dwell-time instructions.
Tip: Some products are designed as a spray-and-leave treatment — the lichen dies off over several weeks and gradually washes away with rain. This is the gentlest method for your tiles.
Step 2 — Apply the Treatment
- Mix the biocide to the manufacturer's recommended concentration in your pump sprayer.
- Apply evenly across all affected areas on a dry day, working from the ridge down.
- Avoid spraying on windy days to prevent drift onto garden plants — many biocides can harm vegetation. Wet down surrounding plants beforehand as a precaution.
- Allow the product to dwell for the time specified on the label (this can range from 30 minutes to several hours, or even a few weeks for spray-and-leave products).
Step 3 — Scrub and Rinse
Once the lichen has been killed (it will typically turn orange, brown, or white):
- Use a stiff-bristle brush with a long handle to gently scrub the dead lichen loose. Work carefully to avoid cracking or dislodging tiles.
- Rinse with a low-pressure hose — a standard garden hose is usually sufficient. If you use a pressure washer, keep it on the lowest setting (under 1500 PSI) and hold the nozzle at a low angle to avoid forcing water under tiles or stripping the tile surface.
- Work from the ridge down so loosened debris washes clear of already-cleaned areas.
Step 4 — Prevent Regrowth
Once the roof is clean, consider these preventive measures:
When to Call a Professional
Consider hiring a qualified roof painter or roof cleaning specialist if:
- Your roof pitch is steep (generally over 25°) or the roof is high — fall risk increases significantly.
- The lichen infestation covers a large area or has been present for many years.
- You notice cracked, broken, or slipping tiles — these need assessment before any cleaning work.
- You're unsure whether your concrete tiles can withstand scrubbing or chemical treatment.
A professional roof cleaner or roof painter will have the right equipment, safety gear, and product knowledge to get the job done efficiently. You can find qualified local roof painters on tradedirectory.co.nz.
For guidance on roof maintenance and what constitutes building consent work, MBIE Building Performance is a useful reference — though routine cleaning and painting of an existing roof generally does not require consent (always confirm with your local council).
Master Painters NZ member
Licensed Building Practitioner (LBP)
Roofing Association of NZ (RANZ)
