Can Moss Cause a Roof Leak?

Moss grows as a thick, spongy mat, and a sponge keeps water where you do not want it. After rain, a mossy roof stays wet far longer than a clean one, because the growth soaks up moisture and releases it slowly. Tiles beneath stay damp for days, which is the first step towards trouble.

Capillary action adds to it. Water is drawn through very narrow gaps against gravity, and the overlap between one tile and the next is exactly that sort of gap. Moss packed into the lap holds water there, and in a wet spell the moisture can be drawn sideways and upwards under the tile, past the point where it would normally run off. That is how a mossy lap can become a leak in an otherwise sound roof.

Growing moss can also push up the edges of tiles slightly, as the mat thickens under them, opening the gap further. And its weight, when saturated, is not nothing on an old roof with tired battens. None of this happens overnight. It builds over seasons, which is why a stain that appears after heavy rain may be a symptom of years of growth. Our answer on why moss grows on the north side of a roof explains where it starts.

Moss does not stay on the tiles. Clumps work loose, wash down and collect in the gutters. A gutter full of moss and grit cannot carry water away, so rain spills over the front or the back. Water that spills back overtops the edge of the roof and soaks the fascia, soffit and the top of the wall.

Valleys are the channels where two roof slopes meet, carrying water from both. They are a pinch point. If moss and debris clog a valley, water backs up, rises over its edges and gets under the tiles on either side. For that reason a roofer examining a leak below a valley looks at the growth as much as at the lining beneath it. Our answer on moss in gutters covers that side of the problem.

Mortar valleys, where the tiles are bedded in cement rather than lined with metal, add a further weakness. Moss rooting in cracked mortar widens the cracks, as covered in mortar valley cracking. Clearing the growth and the debris is part of a leak repair as well as part of a clean, and it should be checked before you accept that something deeper has failed.

Tiles that stay wet are more exposed to frost. Porous tiles absorb water, and hard frost turns that water to ice, which swells enough to crack or flake the surface. Moss makes that worse, since a mat that stays damp keeps feeding moisture to the tile beneath it. Over several winters the surface weakens, cracks open, and what began as a green patch becomes a broken tile.

The same freeze can push moss away from the tile at the edge of the mat and leave a pocket for water. Thaw after thaw, the cycle repeats, and eventually water finds a route to the underlay. If that sheet beneath is torn or sagging, the water reaches the battens and then the ceiling.

That does not mean every mossy roof leaks. A thin film of moss on a steep, sound roof with intact underlay may sit there for years without letting water through. The question is whether it is thick, whether it sits at the laps, valleys or eaves, and whether anything beneath is already weakened. A look from the ground, followed by a close check on the roof itself, answers that. The hub for the wider job is roof cleaning and moss removal.

If moss is thick, or sits in laps, valleys or gutters, the answer is generally yes, because removing it takes away the sponge and lets you see what is under it. Removal by hand, with a stiff brush working downwards so the tile laps are not lifted, is gentler than a jet wash, which Marley does not recommend because it can damage tile and slate surfaces. A treatment applied afterwards slows regrowth.

Do not scrape moss upwards against the laps or walk on the roof to do it. Tiles break underfoot, and moss on the surface is slippery. Our answer on removing moss from a roof yourself covers the risks. Clear the gutters from the ground or from safe access only.

If you already have a leak, clearing moss alone will not cure it. The roofer needs to find where the water entered and mend that, with the moss removed so the roof can be seen. Send us photos of the moss, the gutters and any stain on the ceiling, and we survey the roof, show you what is causing the trouble and set a single written price against the work.