Moss on your roof: should you remove it?

A thin patch of moss on a roof is not an emergency. Thick, spreading moss is worth dealing with, because it holds water against the tiles, can lift and displace them as it grows, and breaks off into gutters and downpipes. The right way to remove it is gently. A pressure washer can do more harm than the moss ever would.

Here is how to tell whether your roof needs attention, and what a proper roof clean involves.

Why moss grows on roofs

Moss likes shade, moisture and a rough surface to grip. Concrete tiles, older clay tiles and north-facing roof slopes give it all three. Overhanging trees add shade and drop leaves that hold even more moisture.

That is why we see heavy moss most often in Filton, Frampton Cotterell and the leafier parts of north Bristol, where tree cover keeps roofs damp for much of the year. Houses on open streets with south-facing roofs tend to stay clearer.

What moss does to a roof

Holds water against the tiles

Moss acts like a sponge. It keeps the tile surface wet long after rain has stopped. In winter, water that sits in and around the tiles can freeze and expand, which speeds up wear on the roof surface.

Lifts and displaces tiles

As clumps thicken, they can grow into the gaps between tiles and push them apart. That creates routes for wind-driven rain to get underneath.

Blocks gutters and downpipes

Loose moss washes off in heavy rain and lands in the gutters. Over time it builds up, blocks outlets and causes overflows that can lead to damp patches on walls. Moss and lichen also creep onto solar panel frames on the same roof, which our post on whether solar panels need cleaning covers.

Signs your roof needs a clean

• Moss is thick enough to see clearly from the street
• Clumps have started appearing on the path, drive or patio
• Gutters overflow during heavy rain
• Tiles look lifted or out of line
• Green or black growth is spreading across more of the roof each year

Why not just pressure wash it?

Pressure washing is the quick option, and it is the one we advise against. A high-pressure jet can strip the surface off concrete tiles, which leaves them more porous and more likely to grow moss again. It can also force water up and under the tiles, and it throws debris everywhere.

We use a low-pressure soft wash instead, which removes moss, lichen and algae without blasting the tiles. A biocide treatment then kills off what remains, and results keep improving over the following weeks.

Should you remove moss yourself?

Please do not climb onto your roof. Wet tiles and moss are slippery. HSE advice is that all roofs should be treated as fragile until a competent person has confirmed otherwise.

If you can see moss you can reach safely, such as on a low porch or garden building, a plastic scraper and a brush will lift it. Avoid metal tools, which scratch tiles, and avoid scrubbing hard on a brittle roof.

When is the best time to remove moss?

Moss can be removed at any time of year in dry, mild weather. Many homeowners book in autumn so the gutters are clear before winter, or in spring once the worst of the wet weather is over. Freezing conditions and heavy rain are the only real limits.

Before starting, we look at the condition of the roof and will tell you honestly if soft washing is not suitable, for example where tiles are cracked or brittle.

Roof cleaning and moss removal

Our roof cleaning service covers moss removal, a biocide treatment and clearing the moss and debris that comes down. Most domestic roofs take between half a day and a full day. If your windows also need doing after the work, our window cleaning service can follow on. We cover Keynsham, Bristol, Bath and north Bristol. Send us a photo of your roof or call Tom on 0117 279 0045 for a free quote.