Last autumn I looked at a 1968 Monier-type concrete tile roof in Spreydon that the owner had treated with a spray-on moss killer twelve months earlier. The product had done its job — the moss was dead and brown. The problem was that nobody had swept or washed it off. The tile surface under the mat was dark with moisture and showing early lichen recolonisation at the edges. Killing it is the easy part. Removing it is where most residential treatments fall short.
Why lichen and moss behave differently
Moss is a plant. Its rhizoids grip the tile surface but do not penetrate the concrete matrix. Kill it and the body dries, shrinks, and can be removed without much damage to the tile. Lichen is a fungus-alga symbiosis, and the fungal component embeds into porous concrete using acids. On older tiles where the surface hardener has broken down, lichen removal always damages the surface to some degree — you are pulling out something that grew into the material, not onto it. The pits left behind are the first places moss re-establishes the following season.
Copper and zinc strips: what they actually do
Copper and zinc ridge strips leach metal ions in rain runoff. Those ions are toxic to moss and algae in the first metre or two below the strip, so ridge tiles under copper stay cleaner than tiles at mid-slope. The effect diminishes with distance. On a typical 4-metre roof slope the bottom third gets almost no protection.
On a roof that already has established moss or lichen, a ridge strip does nothing. It is a preventive measure for a clean surface. Fitting one while existing growth holds moisture into the tiles is spending NZ$300 to NZ$600 on something that may show results in three to five years while the damage continues. The strip belongs as the last step after a full clean and treatment.
Chemical treatment: biocides, alkaline wash and pH runoff
Most spray-on moss treatments use a sodium hypochlorite base, a quaternary ammonium compound, or a proprietary formulation with surfactants. All of them kill surface growth. The differences are in residual action and what they do to runoff. Bleach-based treatments brown off growth within two to four weeks but produce alkaline runoff that affects plantings below the downpipe. BRANZ research on roof surface treatments consistently finds that products applied at manufacturer-recommended dilutions and rinsed with a low-pressure wash within 48 hours minimise the impact, while concentrated applications left to weather off over weeks produce persistent alkaline runoff into soil and stormwater.
Quaternary ammonium products are slower — allow four to eight weeks — but less alkaline and less likely to bleach overspray onto dark-painted fascia. They also tend to have longer residual action on porous concrete, suppressing regrowth for several months after application.
High-pressure washing and when it damages tiles
A pressure washer at 2,000 psi is effective for removing dead moss after treatment. On a tile where the surface hardener is intact, that pressure removes biological debris without damaging the concrete. On a tile over 25 years old with an exposed matrix, the same pressure strips cement paste, opens the surface further, and guarantees faster recolonisation. The tell: if runoff from pressure washing is cement-grey rather than brown-green, the tile surface is being abraded, not cleaned.
Low-pressure soft-wash at 400 to 600 psi with biocide already applied is safer on aged tiles and still shifts dead material after three to four weeks drying. A professional soft-wash on a 120 square metre Spreydon bungalow runs NZ$500 to NZ$850. High-pressure washing with a 200-bar unit should not be used on anything pre-1990 without a tile condition check first.
Timing treatment relative to a reroofing decision
The standard advice is that a roof with over 40 percent moss coverage needs replacing, not treating. That advice is often premature. Coverage alone does not tell you whether the tiles are structurally sound. What matters is whether tiles are cracked, whether hip and ridge mortar is failing, and what the absorption rate of the concrete is — questions for a licensed building practitioner (roofing) who will tap-test tiles and check batten condition, not a roof cleaner’s quote.
If reroofing is set for the next twelve months, spend NZ$150 on biocide and a sprayer to reduce the slipping hazard for the crew. If the timeline is three to five years out, a proper clean, treatment and penetrating sealer earns its return. If your property sits under heavy tree cover, the moisture problem runs deeper than open Canterbury sky — see what happens to roofs under the Waitākere bush canopy.
Frequently asked questions
Will a spray-on moss killer damage my concrete tiles? Applied at the correct dilution and rinsed within 48 hours, most biocide products do not damage concrete tile. The risk is leaving concentrated bleach-based product to weather off over weeks, which can lighten tile colouring and raise runoff pH enough to affect garden beds below the downpipes.
How long does moss take to come back after treatment? On a tile that has been properly cleaned, treated and sealed, recolonisation typically takes three to five years. On a tile that was treated but not cleaned or sealed, moss re-establishes within one to two seasons because dead organic matter left on the surface provides an ideal growing medium for new spores.
What does a full moss and lichen treatment cost in Christchurch? A biocide application on a 120 square metre roof runs NZ$400 to NZ$700. Adding a soft-wash removal of dead material four to six weeks later brings the total to NZ$800 to NZ$1,400. A penetrating concrete sealer after that adds NZ$400 to NZ$650 and significantly extends the treatment interval.