The short answer
Moss on a felt shed roof should be treated with a biocide and brushed off gently, never pressure washed, because water forced under the laps lifts the felt. Tiled sheds can be treated like a small house roof. If the felt is blistered, torn or soft underfoot, cleaning is the wrong answer, and a new covering will cost less than repeated repairs.
Why sheds grow moss so quickly
Most garden buildings sit in the corner of a plot, beside a fence or hedge, with trees and shrubs above. That means shade, dripping branches and still air, which are the three things moss likes. The roof is low and often shallow in pitch, so rain leaves slowly, and leaf litter settles in the dips and along the eaves.
The roof covering is usually mineral felt, felt shingles, corrugated bitumen sheet, or on a better building, small clay or concrete tiles. Felt is soft and slightly rough, so spores root in the surface grit. As the felt ages and the mineral coating wears, the surface gets rougher and moss takes hold faster.
Moss on a shed is more than untidy. It keeps a film of damp on a covering barely thicker than a coin, and the roof boards under it are often thin sheet timber. Water that gets through shows up as damp, mouldy tools and a soft ceiling long before it appears on the house. Our page on overhanging trees covers the cause that most often sits above a shed.
Cleaning a felt or felt shingle roof
The gentle route is the right one. Apply a moss and algae biocide with a garden sprayer, following the label on the product you choose, and leave it to work. After the moss has died back, brush it off with a soft broom, working downhill with the laps and not against them. Do not scrape with a trowel or wire brush. Both tear the mineral surface and cut the felt.
Pressure washing is the main thing to avoid. A jet forced under the overlap of a felt sheet or a felt shingle lifts the edge, drives water into the joint and strips the protective grit. We cover why in more detail on our pressure washing roofs page.
Walking on felt is risky too. A shed roof will not take an adult's weight in the way a house roof does, and the boards underneath may be thin or rotten. Reach from a ladder placed against the building, or with a long-handled brush, and keep your weight off the covering.
Cleaning a tiled shed roof
A tiled shed, such as a summerhouse or a garden office with clay or concrete tiles, behaves much like a small house roof, and the same methods apply. A low-pressure biocide wash is the usual choice, with the dead moss loosened and rinsed away afterwards. If the tiles are old or the battens are light, the frame deserves a look before the moss does: tile weight on a framed building can be considerable and a sagging ridge tells you the structure is tired.
Look at the ridge and the bargeboards at the same time. Pointing that has cracked and tiles that have slipped let water in as readily as moss does. Our page on checking a roof during cleaning lists what a close look includes.
Slope matters on a tiled shed as much as on a house. A shallow pitch holds water longer, so the clean is only the start; letting in more light by lifting the crown of the tree above it does more than any wash.
On any shed, keep the biocide and run-off off plants and out of ponds. Cover or soak the border before you start, and read the label for how long children and pets should stay off the treated ground.
When the roof needs replacing, not cleaning
Moss often arrives once a covering has already begun to fail. Torch-on felt is typically quoted at 10 to 20 years, according to the flat roof guide published by Less, which adds that a good felt job can last 20 years while a badly laid one can give out sooner. Those are national figures, and a shed under trees can fare worse.
Look for signs that cleaning will not help:
- blisters or bubbles in the felt that crackle when pressed;
- splits along the laps or at the edges;
- bare patches where the mineral grit has gone;
- soft or spongy boards under the covering;
- stains inside the shed beneath the moss.
If you see several of these, a new roof will outlast any amount of cleaning. Check the boards before the new covering goes on, and replace any that have gone soft. New felt on damp or rotten timber fails early.
Our advice follows the roof: a sound felt roof gets a clean, and a failed one gets a recover.
Keeping moss off the shed in future
Cut back overhanging branches so the roof gets light and stops being dripped on. Keep a gap between the shed and the hedge behind it so air can move. Clear leaves from the roof and gutter at least after leaf fall, using a soft brush and not standing on the felt.
A shed gutter, if there is one, blocks quickly because the run is short and the downpipe is narrow. Water butts help, but only if the inlet is not choked with debris.
If the shed sits next to your house, glance at its roof when you check the main one. The how often to clean a roof page gives a sensible rhythm for the house, and the shed can follow it. Our main roof cleaning and moss removal page explains what we do on larger buildings. For a shed, send a photo through WhatsApp or the enquiry form, or ring 01793 626096, and we will say whether the roof needs a clean, a repair or a new covering.