Moss on a tile roof is usually noticed as an appearance problem. It is actually a water problem, and on an older roof it is a structural one.
What it does
It holds water. This is the main mechanism. A moss cushion holds many times its own weight in water and keeps it against the tile indefinitely, where the tile was designed to shed it in minutes. Permanently damp concrete tile degrades. Permanently damp timber battens underneath degrade faster.
It grows into the overlaps. Tiles work by shedding water down an overlap. Moss establishing in that overlap physically lifts the tile above, breaking the seal and creating a capillary path for water to travel upward between the tiles, past the lap, and onto the sarking.
This is how moss causes leaks. Not by growing through the tile, but by lifting it.
It blocks the valleys and gutters. Moss sheds constantly, and the debris washes down into the valleys and gutters. A roof with heavy moss has gutters that need clearing more often — see why gutters overflow.
It adds weight. Saturated moss across a whole roof is a meaningful load on an older structure.
It holds the tile surface open. On concrete tiles, the damp acidic environment accelerates erosion of the factory coating, which is what makes the tile porous in the first place — the condition that leads to the sealing question.
Moss, lichen or algae
They occur together and need different handling.
Moss — green, cushiony, visible structure, worst on the south-facing slope and under trees. Responds well to a biocide.
Lichen — flat, crusty, pale grey-green discs that look painted on. Anchored into the tile surface and far more stubborn — explained here.
Algae — a flat dark film, often the black streaking on a roof. Easiest of the three.
Why not to scrape or blast it off
Mechanical removal from a tile roof causes more damage than the moss.
High pressure strips the factory coating off concrete tiles, leaving a porous surface that holds moisture and grows moss faster than before. It drives water up under the laps and onto the sarking. It breaks tiles, and it dislodges ridge pointing. The full argument is in why we never pressure wash a roof.
Scraping by hand avoids the water problem and introduces a different one: walking a mossy roof is genuinely dangerous, and every step on a tile risks cracking it.
The correct method is chemical. Apply a biocide at an appropriate concentration, let it kill the organism, and let weather and gravity remove the dead material over the following weeks. Gentle low-pressure rinsing afterwards if needed.
Slower than people want, and it is the method that leaves the roof in better condition than it found it.
The things to sort out first
Disconnect any rainwater tank before treatment, and leave it disconnected through the first substantial rain afterwards. See roof cleaning on tank-fed properties.
Protect the garden. Everything that kills moss damages foliage, and roof runoff goes straight into the beds below.
Check the roof vents and make sure nothing is being sprayed into an opening — whirlybirds and roof vents.
Manage the runoff. It should not go to stormwater — the rules.
No iron-based moss killer. Effective on moss, and it stains tiles, gutters and anything below permanently orange. Keep it for lawns — see moss killers.
Keeping it off
Moss needs shade and moisture.
Cut back overhanging branches. The most effective single measure, and it reduces the gutter load at the same time.
Zinc or copper strips along the ridge release trace metal ions in rainwater that inhibit regrowth down the slope. Modestly effective, cheap, and worth doing while someone is up there — though note the caution about copper runoff onto Zincalume gutters.
Annual retail treatment on a clean roof holds the result, and that is where those products genuinely work — our assessment.
Keep the gutters clear so water leaves the roof rather than backing up onto the bottom courses.
Where it is worst in Sydney
Shaded, humid, canopied suburbs — the North Shore, bushland fringes, gully properties, and anywhere with mature trees on the southern side.
It establishes through the wet winter months and becomes obvious in spring, which is when the calls come.
For treatment, see roof moss treatment or roof cleaning.
