Lichen on concrete roof tiles is a common headache for NZ homeowners, especially in wetter regions like the West Coast, Northland, and Wellington. The good news is you don't need a pressure washer to tackle it — in fact, high-pressure water can damage the surface of concrete tiles and force moisture under them, potentially causing bigger problems. A chemical treatment combined with careful manual scrubbing is often the safest and most effective approach.
Why Avoid Pressure Washing on Concrete Tiles?
Concrete roof tiles have a textured, slightly porous surface that can be eroded by high-pressure water. Pressure washing can:
- Strip the protective surface coating or paint from tiles
- Drive water under tiles and into the roof structure
- Dislodge or crack older, more brittle tiles
- Spread lichen spores to other parts of the roof
For these reasons, most roofing professionals recommend a low-pressure or no-pressure chemical approach for lichen removal.
Understanding Lichen on Roof Tiles
Lichen is a combined organism — part fungus, part algae — that anchors itself firmly to porous surfaces. Unlike moss, which sits on top, lichen actually bonds to the tile surface, which is why it can leave staining even after the organism is dead. NZ's humid climate makes roofs particularly susceptible, and south-facing or shaded roof sections are usually worst affected.
What You'll Need
- A biocidal roof treatment or lichen/moss killer (available at Bunnings or Mitre 10)
- A garden sprayer or pump sprayer
- A soft-bristle brush or long-handled scrubbing brush
- Garden hose with a gentle spray setting
- Safety gear: non-slip footwear, gloves, safety glasses, and a safety harness if working at height
- A ladder in good condition
Step-by-Step: Removing Lichen Without a Pressure Washer
Step 1 – Choose the Right Chemical Treatment
Look for products specifically labelled for roof lichen and moss removal. Common active ingredients include sodium hypochlorite (bleach-based), benzalkonium chloride, or proprietary biocides. Always read the label carefully — some products need diluting, others are ready to use.
Avoid using undiluted household bleach directly on tiles without checking the manufacturer's guidance, as it can affect surrounding vegetation and may not be suitable for all tile coatings. If your tiles have been painted, check with the paint manufacturer (such as Resene or Wattyl) before applying any chemical treatment.
Step 2 – Protect Surrounding Areas
Wet down garden beds and plants below the roofline before you start, and rinse them again after treatment. Cover any sensitive plants with plastic sheeting if possible. Chemical runoff can harm vegetation.
Step 3 – Apply the Treatment
Using your pump sprayer, apply the biocidal solution evenly across the affected tiles. Work from the ridge down toward the gutters. Make sure the lichen is thoroughly saturated. Most products recommend leaving the solution to dwell for 15–30 minutes (or as directed on the label) to penetrate and kill the lichen.
Step 4 – Scrub Gently
Using a soft-bristle brush, gently scrub the treated areas in a downward motion (ridge to eave). Avoid scrubbing across the tile or using wire brushes, which can scratch the surface. For stubborn patches, reapply the solution and allow more dwell time before scrubbing again.
Step 5 – Rinse with Low Pressure
Rinse the tiles using a garden hose on a gentle setting — not a jet nozzle. Work from the top down. You may find that some staining remains even after the lichen is dead; this can fade naturally over several weeks of rainfall, or you can repeat the treatment.
Step 6 – Apply a Preventative Treatment
Once the roof is clean, consider applying a long-term biocidal sealer or roof treatment to slow future lichen growth. Some products are designed to be left on the roof and work over time as rain activates them. This is especially worthwhile in high-rainfall or shaded areas.
Safety First — Working at Height
Roof work carries serious fall risks. WorkSafe NZ considers working at height one of the leading causes of serious injury in New Zealand. If your roof pitch is steep, the tiles are slippery (especially when wet), or you're not confident working at height, do not attempt this yourself. Always use a properly rated safety harness anchored to a secure point, and have someone on the ground with you.
How Long Does It Take to Work?
| Stage | Timeframe |
|---|---|
| Chemical dwell time | 15–60 minutes (check label) |
| Visible lichen death | 1–7 days |
| Stain fading (natural rain) | 4–12 weeks |
| Preventative treatment duration | 1–5 years (product dependent) |
When to Call a Professional
Consider hiring a qualified roof painter or roof cleaning specialist if:
- Your roof is steep, high, or difficult to access safely
- The lichen coverage is extensive across the whole roof
- Tiles appear cracked, loose, or damaged — these need assessment before any cleaning
- You're unsure whether your tiles have a painted or sealed coating that could be affected by chemicals
- You want a professional-grade treatment with a workmanship guarantee
A professional roof painter can also advise whether your concrete tiles are due for repainting after cleaning, which adds another layer of protection against future biological growth. You can find qualified local roof painters on tradedirectory.co.nz.
Master Painters NZ member
Licensed Building Practitioner (LBP)
Roofing Association of NZ (RANZ)
