Moss is one of the most common roof problems in the UK. But does it actually cause damage, and what is the best way to deal with it? Here is the full picture.
Drive through almost any residential street in the East of England and you will see roofs with patches of green or orange moss growing between the tiles. It is extremely common — but does it actually matter?
The short answer is yes, it does. Left untreated, moss can significantly shorten the life of your roof. Here is everything you need to know.
Why Does Moss Grow on Roofs?
Moss thrives in damp, shaded conditions. North-facing roof slopes, roofs shaded by trees, and properties in areas with high rainfall are all particularly susceptible. The porous surface of concrete and clay tiles provides an ideal substrate for moss spores to take hold.
Once established, moss spreads quickly. A small patch in spring can cover a significant area of roof by autumn.
Does Moss Actually Damage Roofs?
Yes — in several ways:
Moisture retention — Moss acts like a sponge, holding moisture against the tile surface long after rain has stopped. This prolonged contact accelerates the deterioration of the tile surface and the mortar beneath.
Freeze-thaw damage — Moss-retained moisture in the tile surface freezes in cold weather, expanding and causing the tile to crack or spall. Over several winters, this can cause significant damage to individual tiles.
Root penetration — As moss matures, it develops root-like structures (rhizoids) that penetrate the surface of porous tiles, physically breaking them down from within.
Blocked drainage — Thick moss growth can block the gaps between tiles, preventing water from draining freely and causing it to back up under the tiles.
Gutter blockages — As moss dies and breaks down, it washes into gutters and downpipes, contributing to blockages.
What About Lichen and Algae?
Lichen (the crusty, grey-green growth often seen on older roofs) and algae (which causes black or green staining) are different organisms but cause similar problems. Lichen is particularly tenacious — it bonds strongly to tile surfaces and is harder to remove than moss.
Black algae staining is primarily a cosmetic issue, but it can indicate that the tile surface is becoming more porous and therefore more vulnerable to moss and lichen colonisation.
How to Remove Moss Safely
What Not to Do
Pressure washing — This is one of the most common mistakes homeowners make. High-pressure water can dislodge granules from the tile surface, damage mortar joints, force water under tiles, and dislodge ridge caps. It can also be dangerous when working at height. We strongly advise against pressure washing your roof.
Dry brushing — Vigorous brushing can dislodge tiles and damage mortar. It also spreads moss spores across the roof surface.
The Professional Approach
The safest and most effective approach is a two-stage treatment:
Stage 1: Biocide treatment — A professional-grade biocide (moss killer) is applied to the roof surface. This kills the moss, lichen, and algae without damaging the tiles or mortar. The dead growth then gradually washes away over the following weeks and months with normal rainfall.
Stage 2: Preventative treatment — Once the roof is clean, a long-lasting preventative treatment is applied to inhibit future growth. Quality treatments can remain effective for 3–5 years.
For heavy growth, a gentle soft-wash (low-pressure water with a biocide solution) may be used to speed up the removal process.
How to Prevent Moss Coming Back
- Zinc or copper strips — Fitted along the ridge, these release ions that inhibit moss growth as rainwater washes over them. An effective long-term preventative measure.
- Regular treatment — A biocide treatment every 3–5 years keeps growth under control.
- Tree management — Reducing shade by trimming overhanging branches reduces the damp conditions that moss favours.
Is It Worth Treating an Old Roof?
If your roof is approaching the end of its life, it may not be worth investing in moss treatment. However, for a roof that is otherwise in good condition, treatment can add years to its life and improve the appearance of your property significantly.
Get a Free Assessment
Our team covers Peterborough, Cambridgeshire, Lincolnshire, and Norfolk. If you are concerned about moss or lichen on your roof, call us on 07878 827 758 for a free assessment and quote.
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Same Day Roof Repair Team
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