Roof Moss Removal and Cleaning: What Is Safe and What Is Not

On a sound, well fixed roof, a covering of moss is largely cosmetic. Tiles and slates are designed to shed water, and a layer of green on the surface does not stop them doing it.

The genuine problems are secondary. Moss holds moisture against the surface, which accelerates frost damage on already-porous concrete tiles. It sheds into gutters and valleys where it forms dams. And thick growth can lift the edges of slates slightly, letting wind driven rain track underneath.

So it is worth dealing with, particularly on a north facing slope under trees, but it is not an emergency and anybody telling you your roof is at immediate risk from moss is overselling.

A jet wash removes moss beautifully and it also removes the surface coating from concrete tiles, exposing the porous material beneath. A pressure-washed concrete roof grows moss back faster and weathers quicker than it did before.

On slate, high pressure water can lift and crack individual slates, break the tail seal between courses, and drive water up under the laps into the roof space. We have attended plenty of leaks that began the week the roof was cleaned.

It also blasts debris into the gutters and, on older roofs, dislodges the bedding mortar at the ridge. If you have a pressure wash done, at minimum insist it is done at low pressure, from the correct direction down the slope, and by somebody who will refix anything that moves.

Manual removal first. Moss is scraped and brushed off carefully by hand from a scaffold or tower, working down the slope so nothing is driven under the laps. It is slower than jetting and it does not damage anything.

Then the gutters, valleys and outlets are cleared of the debris that comes down, because leaving it there simply moves the problem into the drainage.

Finally a biocide treatment is applied. This kills the remaining spores and slows regrowth for two to three years. Applied properly it works quietly over several weeks rather than producing an instant transformation.

Cut back overhanging branches. Shade and constant leaf drop are the two biggest drivers of moss growth, and trimming a tree does more good than any treatment.

Keep gutters clear so water leaves the roof properly. Standing water and damp edges feed growth at the eaves where it does most harm.

Copper or zinc strips fitted near the ridge release trace metal salts when it rains, which suppresses growth down the slope below them. It is inexpensive and reasonably effective on roofs that are otherwise sound.

If the roof is near the end of its life, cleaning it is money that would be better kept toward the replacement. A clean tired roof is still a tired roof.

If the tiles are already spalling or delaminating, cleaning accelerates the loss. Deteriorated concrete tiles shed material every time they are touched.

And if you are being offered a clean plus a sealant coating, be cautious. Roof coatings applied over old concrete tiles are heavily marketed, expensive, can trap moisture, and can void the tile manufacturer's position. We do not recommend them.

A typical domestic roof clean with manual moss removal, gutter clearance and biocide treatment runs from around £300 to £800 depending on roof size, pitch and access.

It is usually a one day job. Expect a substantial amount of debris, and expect the roof to keep shedding small amounts for a few weeks afterwards as treated growth dries out and falls.

It is a good moment to have the roof inspected properly, because somebody is up there anyway. We photograph anything we find while cleaning and tell you about it whether or not it leads to more work.