Where this starts
A roof dries by sun and wind. Where either is missing — a north-facing slope, a slope shaded by trees or a taller building, a sheltered valley — the covering stays damp for longer after every shower, and that is all moss needs. Add a porous covering that holds water in its surface and the growth is inevitable rather than a sign of neglect.
SO THE FIRST QUESTION IS WHETHER IT MATTERS HERE, and often it does not. A thin growth on a sound covering, away from valleys and gutters, is doing very little. What does matter is moss in the DRAINAGE path: growth filling a valley, blocking the gaps between tiles where water is meant to run, or breaking off in quantity and blocking gutters and downpipes, which is how a roof fault becomes a rainwater fault.
THE SECOND THING THAT MATTERS is mechanical. Thick moss growing into the head laps of a slate or a plain tile can lift the unit slightly, which opens the lap it depends on. And in a hard frost, moss holding water against a porous covering makes freeze-thaw damage worse where it grows.
WHY PRESSURE WASHING IS THE WRONG ANSWER, and it is worth being blunt: a pressure washer drives water UP under the laps, where the covering was never designed to resist it, and floods the underlay and the roof space. It strips the surface off concrete tiles, taking the factory coating and leaving a rougher, more absorbent surface that grows moss faster afterwards. And it dislodges bedding, mortar and units, on a roof somebody is standing on. Roofs are cleaned by hand, from above, downward, with scrapers and brushes.
STAINING IS NOT ALWAYS MOSS. Black streaking is often algae, which is cosmetic and does nothing mechanical. A white or pale bloom is usually lime leaching out of concrete tiles as they age, which is also cosmetic. Distinguishing them saves paying for a cure for something that is not a problem.
What it could be, and what tells them apart
These are candidates to check against your own building, in the order they most often turn out to be the answer. None of them is a diagnosis of your situation.
A slope that never dries
Most commonGrowth on the north or shaded slope only, with the sunlit slope clear, on a roof otherwise in the same condition throughout.
How to check: Which slope it is on. Growth following the shade is the drying conditions rather than a defect.
Overhanging trees
Most commonHeavy growth under or near a tree, with leaf litter in the valleys and gutters and the growth heaviest where the canopy is closest.
How to check: What is above the worst area. Shade and falling debris together are the strongest combination for growth.
A porous or aged covering
CommonGrowth over the whole roof rather than one slope, on concrete tiles of some age whose surface has weathered rough, sometimes with a chalky feel.
How to check: How rough the surface feels. A weathered concrete tile holds water in its surface and grows moss over its whole area.
Blocked drainage paths rather than growth itself
CommonMoss filling a valley or a gutter, water tracking where it should not, and sometimes a damp patch inside below the affected area.
How to check: Whether the growth is in a drainage path. That is the case where the moss is a cause rather than a symptom.
Algae or lime bloom rather than moss
CommonBlack streaking running down the slope, or a white chalky bloom, rather than a green cushion — no thickness and nothing lifting.
How to check: Whether it has depth. Streaking and bloom are cosmetic; moss with thickness is the one that does anything mechanical.
A previous cleaning that made it worse
Less commonFast regrowth after a pressure wash, a rougher tile surface than the sheltered areas, and sometimes damp in the roof dating from the cleaning.
How to check: Whether the roof has been pressure washed. Stripping the surface off a tile makes it more absorbent and it grows back faster.
What to check, in order
The order matters — each step is here because of what it rules out for the ones after it.
- 1
Growth following the shade or the trees is a drying-conditions story and largely cosmetic; growth over a whole roof regardless of aspect points at the covering's surface.
- 2
Moss in a valley, a gutter or the channels between tiles is the case where it causes something rather than merely indicating something.
- 3
Only moss has thickness and only thickness lifts laps or blocks drainage. Paying to remove streaking is paying to change the colour.
- 4
The commonest real harm from a mossy roof is not on the roof: it is the blockage downstream when the growth sheds in a storm.
- 5
It both explains rapid regrowth and raises a question about water driven under the laps at the time, which can show up as damp years later.
The calculations behind this
Several of the causes above are a quantity that was specified or assumed. These compute them, with the formula and the standard behind each.
When to stop and get somebody in
- For any work on the roof itself, since access is the dangerous part and a pitched roof is not a place to learn.
- Where moss is filling a valley or a gutter, because the drainage consequence is the real problem and clearing it properly means getting to it.
- Where there is damp inside below a heavily mossed area, which suggests water is tracking where the covering cannot handle it.
- Where somebody is proposing to PRESSURE WASH the roof — that drives water under the laps, strips the surface off concrete tiles and dislodges bedding, and it is worth declining rather than supervising.
- Where moss is growing into the laps of slates or plain tiles and lifting them, since the lap is what keeps the roof watertight.
This page lists candidates to check, not a diagnosis — nobody has seen your building. If the symptom involves anything structural, read this first.