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Moss on the roof: when it matters

A bit of moss on a north facing slope is normal in the South East, especially under trees, and on its own it does not mean the roof is failing. It becomes a problem when it is thick enough to hold water against the tiles, lift their edges or wash down and block the gutters.

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Signs to look for

  • Thick cushions of moss along the tile edges or the ridge
  • Clumps of moss in the gutters or on the ground below
  • Overflowing gutters in heavy rain
  • Tiles that look lifted where moss has grown underneath
  • Green staining running down walls below the gutter line

What causes it

Moss needs shade and moisture. North facing slopes, roofs under trees and older textured concrete tiles give it everything it wants. Smoother tiles and slate hold less of it, which is one reason moss is a bigger issue on interwar and postwar concrete roofs.

The trouble it causes is secondary. Moss holds water against the tile, which speeds up frost damage on older tiles, and when thick growth gets under the edge of a tile it can lift it enough to let wind-driven rain in. When it dies back in dry weather it washes into the gutters and blocks the outlets.

What we do

We remove moss by hand from the roof, working from proper access rather than walking on the tiles, then clear the gutters and downpipes. Where it helps, we apply a biocide treatment that slows regrowth. While we are up there we check for cracked, slipped or lifted tiles and tell you about anything we find.

We do not pressure wash roofs. It strips the surface from concrete tiles, forces water under the laps and shortens the life of the roof it is meant to be cleaning.

What it usually costs

The cost depends on the size and pitch of the roof, how we reach it safely, and whether gutter clearing and a treatment are included. It is usually a fraction of the price of a repair it can prevent. We quote for free after a look, and the figure is fixed in writing.

Questions

Does moss damage roof tiles?

Not directly, but it holds water against them and can lift their edges, which leads to frost damage and leaks over time. Light moss is cosmetic; thick moss is worth removing.

How often should moss be removed?

On a shaded roof every few years is typical. A treatment after removal slows regrowth, and keeping overhanging branches cut back helps more than anything.

Is it safe to clear moss myself?

Working on a roof without proper access is how people get hurt, and walking on old tiles breaks them. Clearing the gutters from a secure ladder is fine; the roof itself is a job for us.

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