A mossy roof and a blocked gutter are usually the same problem seen from two ends. Moss grows in the damp parts of the tile covering. Rain, wind and the occasional scrape of a branch bring bits of it down. It lands in the gutter, mixes with grit from the tiles, leaves and seeds, and forms a dense mat across the outlet. The gutter can no longer carry water away, and what happens next is the real cost: overflowing water, damp walls, rotting fascias and ice in winter. This guide follows the path from the roof to the drain and explains why cleaning one without the other seldom lasts.
How growth on the tiles becomes debris in the gutter
Moss is a living cushion that holds water and sheds it slowly. When it dries in summer, it shrinks and crumbles. When it is soaked, it swells and the weight of the water pulls it away from the tile. Heavy rain and strong wind take it down the slope. Birds pull at it for nesting material, and squirrels and crows disturb it while foraging. Every time that happens, a part of the roof's moss ends up in the gutter.
The tiles themselves contribute. Concrete tiles shed fine grit from their surface over time, and this is carried into the gutter by water. A roof that has moss on it often has a surface that is wearing, so the quantity of grit is greater. Mixed with moss, that grit makes a heavy, claggy sludge. In autumn, leaves and seed pods go in on top, and in spring blossom and catkins add to the mat.
The result is a gutter that fills from the bottom with a compacted layer of organic matter, which retains water, holds more growth and eventually grows its own plants. Gutters that have been left for a few seasons can sprout grass, small shrubs and even saplings.
What a blocked gutter does to the house
The gutter has one job: take the water from the roof edge to the downpipe and the drain, away from the walls. When it cannot, the water goes elsewhere.
Overflow down the wall
Water spills over the front of the gutter or behind it, and runs down the outside wall. Brick and render soak it up, and in time you see dark streaks, green algae, damp patches inside, peeling paint around windows and spalling brick in frost. Our page on leaking gutter joints deals with a related cause of wet walls.
Rot in the fascia and soffit
The fascia board behind the gutter and the soffit under the eaves are usually timber or uPVC over timber. Constant overflow wets the board from the back. Timber fascias soften and rot, and the gutter brackets lose their hold. That can be a larger repair than the gutter ever was, and the cost of replacing a fascia and a gutter run together includes access equipment that could have been avoided.
Water behind the felt
If the gutter overflows at the eaves and the roof felt is tucked into the gutter, water can back up under the lowest course of tiles. The first signs are damp patches on an upstairs ceiling just inside an outside wall. This is more common than people expect on older roofs where the eaves detail was not built for blockage.
Ice and weight
A gutter full of wet moss and debris is heavy. Add ice, and the brackets bend or the whole run sags. Water then sits in the low point, and the cycle repeats. A sagging gutter is a clue that it has been carrying more than it should.
Ground and foundations
When water overflows near the wall, it saturates the ground beside the foundation and the path. On clay soils, which are common in parts of north Hampshire and Berkshire, repeated wetting and drying near a foundation is something to avoid.
The signs to look for
You do not need a ladder to spot the early signs. Watch the gutters in heavy rain from a window or the garden. Water pouring over the front lip or dripping from the joints is the clearest. Look for vertical streaks of green or black down the wall below the gutter. Look for plants growing at the gutter edge, which you can often see from the ground. Check the downpipe outlet at the base for a lack of flow, or for a puddle that never clears. Inside, a damp mark on the ceiling beside an external wall in an upstairs room may trace back to the eaves. Our guide to lichen, algae and black streaks helps you tell wall staining from roof growth.
| Sign at the house | Probable cause | First step |
|---|---|---|
| Water pouring over the gutter lip in rain | Blocked gutter or outlet | Clear the gutter and outlet |
| Plants growing at the gutter edge | Long-term debris build-up | Clear, and treat the roof above |
| Green or black streaks on the wall below | Repeated overflow | Fix the source, then clean the wall |
| Sagging gutter run | Weight of debris or failed brackets | Clear, then check the brackets |
| Soft or flaking timber fascia | Water behind the gutter | Survey, repair or replace the board |
Why cleaning the roof helps the gutters
Clearing a gutter without treating the roof is like emptying a bath with the tap still running. The moss above will keep supplying the mat, and the gutter fills again, sometimes in a single season. A roof clean removes the thick growth that sheds into the drainage and, with a biocide treatment, slows the regrowth that follows. A treated roof also drops less grit and fewer fragments for some time.
The reverse is also true. A roof clean sends a surge of dead moss and debris into the gutters, and unless they are cleared at the end of the job they block within days. That is why we treat the gutter clear as part of a roof clean and not an optional extra, and why we clear the downpipes and check the outlets for flow before we leave. Gutter clearing on its own is outside the scope of what a roof cleaning visit covers, as the main aim is the roof. Our page on whether roof cleaning makes a mess of the garden explains how we handle the debris.
Some growth is best left. Thin lichen and a light film of algae do not send material into the gutters in any quantity, and our page on whether moss on roof tiles is a problem helps you judge when moss counts and when it does not.
How often gutters need clearing
There is no single interval. A gutter under a mature tree, or below a mossy roof, may need attention twice a year, once in late autumn after the leaves have come down and again in spring. A gutter on an open, clean roof may manage for longer. The right approach is to look, not to follow a calendar. Check after the first big autumn storm, after any gale that strips a tree, and when you see water behaving oddly in rain.
Whoever does the clearing needs safe access. Reaching a gutter from a ladder means leaning out sideways, holding a scoop and a bucket, and trusting a gutter that may be full of wet debris to bear your weight. Our page on roof cleaning safety sets out why we use towers and scaffold on higher houses. For a low bungalow or garage, a gutter vacuum on a pole is a sensible option. For two storeys, the better method is a platform.
What happens in winter
Cold weather turns a blocked gutter from a nuisance into a hazard. Water sitting in a full gutter freezes, expands and pushes the joints apart, and ice builds along the lip and down the downpipe. When it thaws, the water finds the new gaps. A downpipe packed with moss and ice can split along its seam. On a path below, an overflowing gutter that freezes leaves a sheet of ice by the door. We often get calls in January about leaks that began as a blockage in October.
The practical lesson is to clear before the cold, and to look again after the first hard frost. It is also the reason we prefer to treat a roof in autumn or spring, so the winter has less growth to feed the gutters.
Fascias, brackets and falls
When we clear a gutter, we also look at it. The run should have a gentle fall towards the outlet, so water flows rather than sits. Brackets should be firm and evenly spaced. Joints should be sealed. The fascia behind it should be sound. A gutter that sags between brackets holds a pond, and a pond grows moss. Problems we find are noted and photographed, so you see them. A new set of brackets or a re-aligned run is a small job compared with a rotten fascia. If the gutter and fascia need more than a re-fix, our guttering, soffits and fascias service covers the repair or replacement.
Guards and mesh are sometimes suggested. They keep large leaves out, but fine grit, seeds and moss still gather on them, and some guards trap debris on top and make the clearing harder. They are worth considering under overhanging trees, not as a cure for a mossy roof.
What we do at the survey
On a roof clean, we start from the top. The free survey looks at the tiles, the growth, the ridge and verges, and then the gutters, downpipes and fascias, with photographs of each. The report shows what is blocked, what is damaged and what is simply dirty, and a fixed written quote follows for the work that fits. If a gutter is the main problem and the roof is clean enough, we say that.
We clear the site when we finish, and our ten-year guarantee covers our workmanship in writing. To learn what a full clean involves, see roof cleaning in Basingstoke. If you have a gutter that is overflowing today, ring 01256 213956 and tell us what you can see from the garden.
