Moss grows on many roofs in the South East, especially on north-facing slopes, under trees and on older clay and concrete tiles. Some owners like the look. Others worry it’s damaging the roof. The truth is somewhere in between.
This guide explains when moss is just cosmetic, when it causes problems and why cleaning it the wrong way can harm the roof.
Why moss grows on roofs
Moss likes damp, shade and a rough surface. Roofs facing north, shaded by trees or nearby buildings, or with rough, weathered tiles give it everything it needs. Concrete tiles become rougher as their surface wears, so older concrete tile roofs often have more moss. Lichen, the flat, crusty growth often seen on slate and tiles, is different from moss and usually less of a concern.
Moss also tends to come back. Removing it doesn’t change the shade, damp or rough surface that let it grow in the first place, so any cleaning is a short-term measure.
When moss is mostly cosmetic
A thin covering of moss on sound, well-fixed tiles usually does little harm. Rain still runs off, and the roof below is still dry. If the roof space shows no signs of damp and gutters drain well, light moss may simply be part of how the roof looks.
When it becomes a problem
- Blocked gutters and valleys. Moss washes off in rain and collects in gutters, outlets and valley gutters. Blocked gutters overflow and can soak walls or, with hidden gutters, overflow inside.
- Lifting tiles. Thick clumps between tiles can hold water and lift the tail of a tile, letting wind-driven rain get underneath.
- Holding water. Heavy moss keeps the roof surface damp for longer. In frosty weather, that can make some tiles flake or crack.
- Hiding defects. Heavy moss can hide cracked or slipped tiles.
See gutter and downpipe problems for more on blocked gutters.
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Why aggressive cleaning can do harm
Pressure washing is often sold as a quick fix. It can do real damage:
- it can strip the surface of concrete tiles, making them more porous
- it can force water under slates and tiles and into the roof
- it can crack or dislodge old slates and tiles
- it can wash debris into gutters and valleys.
Walking on the roof to clean it can crack tiles and slates and is dangerous. Don’t pressure-wash your roof and don’t walk on it. On garages and outbuildings with cement sheet roofs, moss removal could disturb material that may contain asbestos. See asbestos cement roofs.
What to do instead
- Keep gutters, outlets and valleys clear. That deals with most of the damage moss causes.
- Trim back overhanging branches where you can do so safely, or ask a tree surgeon.
- Check from the ground with binoculars for lifted or slipped tiles.
- Look in the loft for damp after heavy rain.
- Get the roof’s condition checked before paying for cleaning or coatings.
How a survey helps
A roof survey looks at the coverings, ridges, valleys, flashings and gutters, and the roof space where there’s safe access. It can say whether moss is hiding defects or causing problems, or whether it’s just cosmetic. A gutter survey focuses on the rainwater side. Where applicable, a drone gives close-up photos without anyone stepping on the roof.
Quick answers
Does moss damage roof tiles?
Light moss usually doesn’t. Heavy moss can hold water, lift tiles and block gutters, which is where most of the harm comes from.
Should I pressure-wash my roof?
No. It can damage tiles, force water into the roof and wash debris into gutters. Get the roof checked first.
Is moss on a north-facing roof normal?
Yes. North-facing and shaded slopes stay damper, so moss is common there. Keep an eye on gutters below them.
Related services
More advice
- Gutter and Downpipe Problems That Cause Damp
- Concrete Tile Roofs: Lifespan and Common Faults
- Winter Roof Checklist for Homeowners
- Asbestos Cement Roofs, Garages and Soffits
- All advice
This guide is general information about roofs and roof surveys. It isn’t legal, insurance or structural engineering advice; for those, speak to a suitably qualified professional.