Green, black or orange growth on roof tiles is one of the most commonly reported maintenance concerns we hear from homeowners across Brighton and Hove. It is also one of the most misunderstood — many people either ignore it entirely or attack it with a pressure washer, both of which can lead to problems. This guide explains the different types of growth, whether they actually cause damage, and the correct approach to treating and preventing them.
Moss, Lichen and Algae: What Is the Difference?
The three main types of biological growth you will encounter on roof tiles are distinct and behave differently. Moss is the most visible — it forms thick, spongy green or brown cushions, most commonly in shaded areas and roof valleys where moisture sits. It holds water against the tile surface and can lift tile edges as it expands. Lichen is harder to spot and harder to remove — it forms flat, crusty patches in grey, orange or pale green and bonds chemically to the tile surface rather than simply sitting on it. Algae is the dark grey or black streaking often mistaken for general dirt, and it spreads rapidly in damp, humid conditions. On coastal properties like many across Brighton and Hove, the combination of maritime humidity, prevailing south-westerly rain, and shaded north-facing slopes means all three types are extremely common.
Does Growth on Roof Tiles Actually Cause Damage?
Yes, though the timeline varies. Algae staining is primarily aesthetic in the short term, but the biofilm it forms can accelerate weathering of the tile surface over years. Moss causes more immediate problems: it retains moisture against the tile, which accelerates frost damage in winter as water expands and contracts within the tile surface. Dense moss growth in valleys and at eaves can also obstruct water flow, causing it to back up under tiles or overflow gutters. Lichen is the most structurally damaging — its root-like structures (rhizines) penetrate the surface of softer tiles and the mortar of ridge and hip beds, and removing it without the correct treatment can pull material from the tile face.
- ✓Moss holds moisture against tiles and can physically lift tile edges as growth expands.
- ✓Ice formation within saturated moss accelerates surface spalling on concrete and clay tiles.
- ✓Moss and debris accumulation in valleys restricts water flow and increases the risk of pooling.
- ✓Lichen bonds to tile surfaces and mortar — removal without biocide treatment can cause surface damage.
- ✓Algae biofilm accelerates UV and frost weathering of the tile surface over time.
The Wrong Way to Deal With It
Pressure washing is the most common mistake. While it produces a visually dramatic result, it removes the protective surface layer from concrete tiles and drives water into the batten and underlay space in volumes that normal rainfall never would. It also dislodges any loose ridge mortar, lifts tile nibs, and can crack fragile older slates or handmade clay tiles. Dry scraping from the ground using a long-handled brush is safer than pressure washing but is indiscriminate — it removes live growth but leaves the spore base intact and does nothing to treat the lichen that has bonded to the surface. Neither approach addresses the underlying cause.
Pressure washing a roof voids the manufacturer's warranty on most modern concrete tiles and can damage the granular coating on older plain tiles. If you are considering this approach, it is worth understanding that the result — while temporarily clean — typically leads to faster regrowth and accelerated surface weathering.
The Correct Approach
- ✓Apply a biocide treatment — a properly formulated moss and algae killer applied by spray kills the biological growth at root level. Most products require a dwell period of four to eight weeks before visible results.
- ✓Allow the dead material to weather off naturally — this avoids the surface damage caused by mechanical removal and means the treatment has fully penetrated the growth.
- ✓Clear gutters and valleys of accumulated debris once growth has died back — this restores proper drainage and removes the organic matter that supports regrowth.
- ✓Consider a follow-up inhibitor treatment — some biocide products include a residual inhibitor that slows regrowth for two to five years.
- ✓Address the underlying cause where possible — persistent heavy moss growth on a particular slope is often partly caused by overhanging trees or poor drainage; removing the cause reduces the rate of regrowth.
When Is Professional Treatment Needed?
For most domestic roofs, a professional biocide treatment is the safest and most effective route. Safe access to apply the treatment evenly across the full roof slope requires the right equipment and experience — uneven application leaves untreated patches that recolonise quickly. A professional visit also allows for a condition check of the ridge mortar, flashings, and tile fixings at the same time, which often reveals small issues worth addressing before they become larger ones.
Our property maintenance service includes roof biocide treatment, gutter clearance, and condition surveys across Brighton, Hove and the wider Sussex area — all carried out from safe access with no pressure washing.