Moss on the roof: why it matters more than it looks
Moss looks harmless and sometimes rather pretty. It is neither, given long enough.
Why moss matters more than it looks
Moss holds water. A cushion of it on a tile keeps that tile wet long after the rain has stopped, and in winter the water freezes, expands and cracks the surface. On a concrete tile it strips the face; on a slate it works into the nail holes.
It also moves. Every shower washes a little more of it into the gutter, where it packs down into something close to compost. Most blocked gutters on a house with a mossy roof are blocked with the roof.
Why your roof and not next door’s
Aspect
A north-facing slope gets little sun, stays damp, and grows moss first. The south slope of the same roof is often nearly clear.
Shade
Trees overhanging or close to the roof keep it damp and drop the debris moss grows in. Cutting them back slows the moss as well as the leaves.
The tile
Rough concrete tiles hold water and grow moss readily. Smooth clay and slate shed it. Older concrete tiles that have lost their surface are worst of all.
How it should come off
By hand, with a scraper and a brush, working down the slope. Not with a pressure washer: high-pressure water forces its way under the tiles and into the felt, strips the protective surface off concrete tiles so they grow moss faster afterwards, and can shift tiles and pointing. Once the moss is off, the gutters are cleared of what has already fallen into them, and a roof that is cleared properly stays clear for years.
More guides
Why gutters overflow, and what it does to a house
The four causes of water over the edge, and why the wall underneath is the real cost.
02How often should gutters be cleaned?
Twice a year for most houses. Which houses need more, which can go longer, and how to tell.
03Signs of a roof leak, inside and out
What to look for on the ceiling, in the loft and from the garden, and why the leak is rarely where the stain is.
04After a storm: what to check from the ground
A ten-minute walk round the house that finds most storm damage before the next rain does.
05What a roof and gutter inspection covers
The six things that get looked at, and what each one tells you.