Roof demossing means getting the moss, lichen and algae off a roof and keeping it off for a few years. It is not a pressure wash. On a tiled roof most of the work is done by a treatment that kills the growth at the root. The rest is low pressure rinsing and clearing the dead material out of the valleys and gutters.
The calls come mostly in late winter. Someone backs out of the driveway, looks up, and the south side of the roof has gone green over three months of damp weather.
What moss actually does to a roof
Moss holds water. That is the whole problem with it. A clean tile sheds a downpour and dries out in an hour of sun. A tile carrying a mat of moss stays damp for days, and anything under it stays damp too.
It also grows where you cannot see it. Moss creeps into the laps where one tile sits over the next, and as it thickens it props the tile up a fraction. Wind gets under a lifted tile. Rain gets pushed sideways up the lap instead of running down it. On an older roof with no sarking underneath, that water has nowhere to go except into the roof space.
Lichen is the tougher one. The grey and orange crusts on old terracotta root into the surface of the tile rather than sitting on top of it, so they do not brush off and they come back fastest. Moss in the mortar along the ridge capping matters too, because damp mortar breaks down and loose capping is a far bigger repair than a clean.
The shaded side goes first
Look at almost any Melbourne roof and the growth is on one pitch. Usually the south face, or whatever side sits in the shadow of a gum tree, a neighbouring two storey house or a tall fence line. That side gets less sun, dries slower, and collects the leaf litter and bark that moss feeds on.
Homes under established gums are the worst of it. The litter drops all year, not just in autumn, and it packs into the valleys where it stays wet permanently. The moss starts there and spreads outwards across the tiles.
How a demoss is done properly
The job starts with a look at the roof, not a hose. What the tiles are made of, how brittle they have gone, whether the capping is sound, and where the growth is worst.
Then the treatment goes on at low pressure and is left to work. The growth dies back and weathers off over the following weeks rather than all at once. Where a homeowner wants the bulk of it gone straight away, a low pressure rinse lifts the dead moss once the treatment has taken, and the debris is cleared from the valleys and flushed through the downpipes before we leave.
What does not happen is high pressure. A pressure wand will strip moss off a concrete tile and take the glazed surface with it, which leaves a rougher, more porous tile that grows moss back faster than it did before. It also drives water up under the laps and into the roof. We soft wash roofs at low pressure for the same reason we soft wash render and weatherboard: the chemistry does the cleaning, the water only rinses. Colorbond is the same story. It gets less moss than tile, but it gets algae streaks and lichen spots, and its paint finish does not want a wand held against it.
Nobody should be walking a wet tiled roof
A mossy tile with water on it is as slippery as ice. Old terracotta is brittle and cracks under a boot, and the crack often does not show up until it rains. Our crews are work at heights certified and we carry public liability insurance, which matters more on a roof than anywhere else on a property.
If you have been thinking about doing it yourself with a ladder and a stiff broom, this is the one exterior job we would talk you out of. Ladder falls put people in hospital every year, and no roof is worth a broken hip.
All that moss ends up in your gutters
Dead moss does not vanish. It washes down the pitch in the next decent rain and packs into the gutters and downpipe outlets, where it holds water the same way it held it on the tiles. Silt and moss together set into something closer to mud than leaf litter, and it will not flush out on its own.
So the gutters get cleared and the downpipes flushed as part of the same visit. If your gutters are already full, it makes sense to book roof and gutter cleaning together and have the whole lot done in one go rather than paying for access twice.
How often does a roof need demossing?
There is no single answer, and anyone giving you one has not looked at your roof. Tree cover, which way the pitch faces and what the roof is made of all change it.
A north facing Colorbond roof on an open block might go many years without needing anything beyond a gutter clean. A tiled roof in the shade under gums can be green again within two or three years of a treatment. The sensible approach for most homes is a look each year when the gutters are done, and a demoss when the growth is established rather than waiting until the valleys are full.
When to book it
Green or black patches spreading down one side of the roof, crusty grey lichen on the tiles, moss in the gutters, or plants sprouting out of the valleys. Any of those and the roof is due. If the tiles are lifting or the ridge capping is cracked and loose, get the roof looked at before it is cleaned. A clean will not fix a mortar problem.
Demossing sits alongside the rest of the exterior work. If the walls have gone green as well, the same low pressure method covers those, and roof soft washing is usually quoted with the house wash when both are needed.
If your roof has gone green on one side, send us a couple of photos from the ground and we will tell you whether it needs treating now or whether it can wait until the next gutter clean. Quotes are free and we work seven days. Call 03 8682 8767 or start with roof and gutter cleaning if the gutters are the more urgent job.