Can Moss Cause roof leaks?
TL;DR: Moss rarely causes a leak on a sound tiled roof, because the underlay beneath the tiles catches any water that gets through. It can lead to one by lifting tiles, blocking valleys and gutters, or pushing water onto old, failing underlay. Pressure washing moss off a roof is a common cause of new leaks, and damp on an upstairs wall is often an overflowing gutter rather than the roof.
Why moss doesn't usually cause a roof to leak
A pitched tiled roof in the UK has two layers of protection. The tiles help the bulk of the rain water to run away, and underneath them tiles is an underlay, either traditional bitumen felt or a modern breathable membrane, which catches any water that gets past the tiles and carries it down to the gutter. On a roof in reasonable condition, a certain amount of water getting under the tiles is expected, and the underlay deals with it. That is why so many roofs carry a thick layer of moss for years without a drop coming through the ceiling.
Felt underlay perishes with age, and it can tear, sag or become brittle, particularly near the eaves where it is most exposed. Some older roofs have no underlay at all. On a roof like that, any water that finds its way past the tiles has a clear route into the loft and anything that pushes more water under the tiles makes a leak much more likely.
How moss can lead to a leak
Heavy moss causes leaks in a handful of specific ways, all are preventable if moss is removed early enough.
Lifting tiles
As moss builds up along the bottom edges and sides of the tiles, it can push them up slightly and open a gap. Wind-driven rain can then blow underneath, putting far more water onto the underlay than it would normally have to deal with.
Holding water against the laps
Tiles overlap so that water runs over each joint and down to the next tile. Thick moss sitting across those overlaps holds water in place and can draw it back up under the tile above instead of letting it run down.
Cracked tiles
Moss keeps tiles wet, and in cold weather that trapped water can freeze and crack them. A cracked tile gives water a far more direct route in than a mossy one.
Blocking valleys
Where two roof slopes meet, the valley carries a large volume of water in a narrow channel. Moss washing down from both slopes collects there along with leaves and silt, damming the valley so that water spills sideways under the tiles.
Blocking the gutters
Moss sheds constantly and on north-facing roofs in particular it collects in the gutter and compacts into a dense silt. When the gutter fills, water backs up at the eaves, where the underlay is often in its worst condition, a good reason to clean your gutters.
the common (unexpected) way moss causes leaks
One of the most common routes to a new leak on a mossy roof is the attempt to clean the roof. Checkatrade warns that a pressure washer can force water underneath the roof covering and create a leak that was not there before. A jet aimed up the roof drives water under the laps and straight onto the underlay, and on an older roof that can be enough to reach the loft.
Foot traffic can cause the same kind of damage. Concrete and clay tiles are not designed to be walked on, and slate cracks very easily underfoot. A roof that was watertight before it was cleaned can have cracked tiles afterwards, which is why we survey every roof before starting and carry out the work from access equipment rather than standing on the tiles. Moss should be brushed or scraped off gently from the ridge downwards, then treated with a biocide, as we explain in how to remove moss on a roof.
Signs that moss may be letting water in
Tiles along the lower courses that look lifted, uneven or out of line with those around them
Thick moss packed into valleys or along the bottom edge of the roof
Gutters that overflow in heavy rain or have plants growing out of them
Damp patches or staining in the loft, especially near the eaves
Damp high on an upstairs wall that gets worse during rain
Cracked or slipped tiles visible from the ground
What to do if you suspect a leak
Start in the loft. On a dry day, look at the underside of the roof with a torch for staining, damp timber or torn and sagging underlay, then check again during or just after heavy rain. That tells you whether water is coming through the roof or whether the problem lies somewhere else.
Then look at the gutters. If they are full of moss and silt, clearing them may solve the problem on its own, especially if the damp is showing on a wall rather than a ceiling.
If the loft shows water coming through, you need a roofer to find and repair the entry point, whether that is a cracked tile, a lifted tile or failing underlay. Moss removal alone will not fix a damaged roof. Once the roof has been repaired, gentle moss removal followed by a biocide treatment will stop the growth from lifting tiles or blocking the drainage again.
