Roof Moss and Algae Treatment: Biocide Application, Zinc Strip Prevention and Pressure Washing Risks

Quick Answer: Roof moss should be treated with a biodegradable biocide (diluted solution applied by spray, then left for 6–12 weeks to die back naturally) rather than pressure washed. Pressure washing at rooftop pressures can dislodge granules from mineral felt, crack aged concrete tiles, and force water under tile laps. Zinc strip or zinc-oxide paint applied at the ridge provides long-term prevention as rain washes zinc ions down the roof.

Summary

Moss, lichen and algae are extremely common on UK roofs — particularly in wetter climates (north/west of the country) and on north-facing pitches. While moss rarely causes immediate structural damage, it retains moisture, accelerates freeze-thaw spalling, and can lift tiles on steeply pitched roofs with sufficient biomass.

The treatment debate centres on pressure washing versus biocide application. Pressure washing delivers immediate visual results (popular with customers) but carries significant risk of damage to aged tiles, felt and mortar — particularly on roofs over 20 years old. Biocide treatment is slower (takes weeks to show results) but is far safer for the roof surface.

The prevention side is underused: zinc strip, copper strip, or zinc-oxide paint applied at the ridge can suppress moss growth for 5–10 years as rainfall washes trace metal ions down the roof. This is particularly valuable for north-facing or shaded roofs that would otherwise require treatment every 3–5 years.

Key Facts

• Moss types — Acrocarpous moss (upright, cushion-forming): can be mechanically removed when dry. Pleurocarpous moss (flat-matting): harder to remove; responds better to biocide.
• Biocide chemistry — Most roof biocides use: potassium oleate soap (moss kill), benzalkonium chloride (algae kill), or dilute sodium hypochlorite (bleach-based). All must be kept off plants and away from drain gullies during application.
• Environmental requirements — Biocide runoff can kill garden plants and contaminate waterways. Most products require 6–24 hours dry weather after application. Check Environment Agency guidance — some products require WUM (Water Use Management) precautions.
• Pressure washing risk — Concrete tiles: 10–20 years old are vulnerable (granule loss on surface). Aged clay tiles: risk of hairline cracks. Weathered fibre cement: delamination. Natural slate: low risk. Mineral felt flat roof: moderate risk (granule loss). Maximum safe pressure for aged tiles: ~100 bar at distance.
• Manual removal (dry) — For heavy moss on sound tiles: use a stiff brush or scraper working down the slope (not up). Then follow with biocide. Never use metal scrapers on tile surfaces.
• Zinc strip — 150mm wide zinc strip nailed under the first tile course below the ridge. Rain dissolves trace zinc; runs down slope; inhibits moss for 7–15 years. Must be secured against wind lift. Some planning authorities in conservation areas restrict visible zinc.
• Zinc paint/spray — Liquid zinc oxide applied to cleaned roof surface. Less visually intrusive than strip. Effective for 5–7 years. Can be applied by brush, roller or spray.
• Frequency of treatment — Biocide treatment typically required every 3–7 years depending on location and slope. North/west-facing pitches may need it every 3 years.
• Ridge ventilation compatibility — Do not seal ventilated ridge systems with zinc strip applied over the ventilation slot.

Quick Reference Table

Detailed Guidance

Biocide Application Method

1. Access — Ensure safe working at height (scaffold, platform, fixed ladder system). Do not work on roof in wet conditions. See working at height.
2. Mix biocide — Follow manufacturer dilution ratio. Typical ratio: 1:4 to 1:10 biocide concentrate:water.
3. Protect gutters and downpipes — Flush through after treatment; biocide can damage plastic below if concentrated. Warn homeowner about runoff to planted beds.
4. Spray from ridge down — Use pump sprayer with low-pressure fan nozzle (not jet nozzle). Work from ridge to eaves in overlapping passes. Saturate moss/algae/lichen thoroughly.
5. Do not rinse — Leave biocide to penetrate and kill. Do not water or walk on roof for 24 hours.
6. Allow die-back — 6–12 weeks for moss to die back (turns brown/grey). Lichen may take longer (3–6 months). The dead material will gradually wash off in rain.
7. Optional mechanical removal — After 6–8 weeks, dead moss can be lightly brushed or blown off. Working down the slope (not up against tile laps).

Products: Wet & Forget, Algon, Thompson's Roof Seal and Mossgone (biocide). All are available in trade quantities.

Pressure Washing: When It Is Safe

Pressure washing is only appropriate where:

• Tiles are confirmed to be in sound condition (no hairline cracks, no granule loss, no delamination)
• The roof has been visually inspected beforehand
• Maximum 80–100 bar at 300–400mm distance; wide fan nozzle (25–40°) not jet nozzle
• Never use on fibre cement over 15 years old, aged concrete tiles, or any mineral felt

Even on sound concrete interlocking tiles, pressure washing can wash out mortar from ridge and hip pointing. Re-point all exposed mortar after cleaning.

Note: Pressure washing provides no long-term prevention. Moss regrowth after pressure washing (without follow-up biocide or zinc treatment) typically begins within 12–18 months.

Zinc Strip Installation

1. Expose the first tile course below the ridge (carefully lift tiles or remove ridge tiles)
2. Cut 150mm zinc strip to required length (standard roll 6m)
3. Lay zinc strip with upper edge tucked under the ridge tiles or final tile course by 25–50mm
4. Secure to batten or tile with galvanised roofing nails at 300–400mm centres
5. Ensure lower edge of strip is exposed to rainfall (not covered by tile lap)
6. Re-seat ridge tiles; repoint with mortar or use dry-fix ridge system

Copper strip is an alternative — more effective zinc equivalent and potentially longer-lasting. Price premium applies.

Lichen Removal

Lichen is more adherent than moss and often requires a stronger approach:

• Apply concentrated biocide; allow 3–6 months for full die-back
• Use a soft brass wire brush (never steel) to mechanically remove dead lichen
• Repeat treatment if any live lichen remains
• Lichen removal often reveals surface staining that requires bleach-based treatment

Frequently Asked Questions

Can I pressure wash moss off an old felt flat roof?

No — mineral-surfaced felt flat roofs must not be pressure washed. The granule surface protects the bitumen from UV. Removing granules accelerates deterioration. Apply a dedicated roof biocide at low pressure (garden sprayer) and allow to work. If the felt is in poor condition, treat the moss as a secondary issue and focus on flat roof replacement.

My customer wants immediate visual results — can I brush moss off and then treat?

Yes, with caution. For sound concrete or clay tiles: dry-brush the moss (working down the slope — never up) to remove the bulk, then apply biocide to kill remaining spores, algae and roots. This gives immediate visual improvement while the biocide handles the biological residue. Wet moss is heavier and harder to remove; work on dry days.

Is zinc strip harmful to the environment?

Zinc is a regulated substance. In the concentrations washed off zinc strip, the impact is minimal in normal outdoor environments. However, it should not be installed where runoff enters a sensitive watercourse (e.g., chalk stream) in quantities from large roofs. If in doubt, check the EA's position on zinc runoff for the specific site.

Regulations & Standards

• COSHH Regulations 2002 — Biocide handling: PPE (gloves, eye protection), vapour precautions for concentrated product

• Biocidal Products Regulation (BPR) No. 528/2012 (UK retained) — All roof biocide products must be BPR-registered (PT10 category)

• Work at Height Regulations 2005 — All roof work requires risk assessment and appropriate edge protection

• Environmental Protection Act 1990 — Biocide runoff into surface water: duty of care

• HSE — Working at height on roofs

• Environment Agency — Biocide guidance

• National Federation of Roofing Contractors (NFRC)

• pitched roof repairs — Pitched roof repair techniques

• roof tile types — Tile types and their vulnerabilities

• working at height — Safe access for roof work

• chimney repairs — Repointing after cleaning