Moss, gutters and downpipes: why cleaning the roof means clearing the gutters

Moss gets into gutters by several routes, and most of them are quite ordinary. The first is simple weathering. A thick moss cushion on a tile is held on by threads that grip the surface, but through a winter of wet and dry spells it loosens, and gales finish the job. Pieces drop to the eaves and roll in.

The second is the grit. Tile surfaces shed small granules as they age, and moss traps and holds them. When the moss comes away it brings a good deal of that sandy debris with it. A concrete tile roof can drop a surprising amount into a gutter in a single season.

The third is us. Any clean, whether by soft wash or hand brushing, moves growth downhill. A good crew manages it as part of the job, working from the ridge to the eaves so that nothing is washed back over clean tiles, and clearing the gutter as the last stage. A poor one leaves a trail of wet moss in the gutter and calls it done. Our page on roof and gutter cleaning describes how we order the work.

Treated moss falls more slowly

Once the moss has been sprayed with a killing wash, the dead growth turns brown or grey and gradually lets go over the following weeks. That is why gutters can fill up after a roof has been treated, even though nothing seemed wrong on the day. If your roof has been treated, plan a second look at the gutters once the growth has dropped. Our guide on what a roof looks like after moss treatment sets out what to expect week by week.

A blocked gutter looks like a small thing from the lawn. It is not. When the channel is full, rain has nowhere to go and spills over the front lip, or backs up behind it, and either way it ends up where it should not be.

The first casualty is usually the fascia, the board along the bottom edge of the roof that the gutter is fixed to, and the soffit, the underside panel beneath the overhang. Both stay wet when water runs over or behind the gutter. Timber fascias can rot, and painted surfaces blister and peel. Even plastic boards stain and trap damp.

The second is the wall. Water pouring over the gutter lip runs down the brickwork or render. A strip of dark, wet wall below a gutter is a common sight, and it can bring green growth and, on a cold night, frost damage to soft brick.

Water at the roof edge

The part that gets overlooked is the roof edge itself. When the gutter is full to the brim, water can back up onto the lowest course of tiles and the felt underlay, the membrane laid beneath the battens and tiles. That is how a gutter problem turns into a leak inside the house, with a stain on the ceiling by an external wall.

Where a downpipe is blocked, the water has to go somewhere else, and a joint that was fine for years may start to drip. Streaks on the wall below a joint tell you where.

Nothing in this section needs a ladder. A short walk round the house on a wet day will show most of it.

Watch the gutters in steady rain. Water should travel along the channel and drop through the outlet into the downpipe. If it overshoots, slops over the front edge or drips from the underside of a joint, there is a problem. Look at the gutter line from a distance as well. Sagging, a visible gap in the run or plants sprouting from the top are all clear signs.

Then look at the walls. Dark vertical streaks, green patches beneath the eaves and flaking paint on the fascia all point to water that has not been taken away properly. Inside, check the corners of upstairs rooms and the ceiling beside external walls for new stains.

Here is a short list of what we look for when we survey the eaves:

• Moss and debris sitting in the channel.
• Standing water in the gutter after rain has stopped.
• Joints that drip, or brackets that have pulled away.
• Fascia boards that are soft, split or stained.
• Downpipe bottoms that are blocked or have moss at the shoe.

Gardeners will recognise one more clue: moss and small seedlings growing in the gutter itself. If something is rooting in the channel, it has been holding damp for a long time. Photographs from the ground are a good start, and we can work from them when you contact us, though a proper survey of the roof edge is still the way to be sure.

Clearing a gutter is not just scooping out what you can reach. Done well, it takes a few distinct steps, and the order matters.

The debris comes out by hand or with a small scoop, working away from the outlet so that nothing is pushed toward the downpipe. It goes into a bag, not onto the lawn or down the drain. Then the channel is flushed with water, and we watch where it goes. Water should run all the way along to the outlet and away.

The downpipe is next. We check that it runs clear, and that the water leaves the shoe at the bottom and goes into the drain or soakaway, not onto the path. A blocked downpipe is often packed with the grit and moss that came off the roof.

Falls and brackets

If water stands in a gutter that is clean, the fall is wrong. Brett Martin's installation guidance gives about 1 mm of drop for every 600 mm of gutter, and brackets no more than 900 mm apart. A long stretch sagging between brackets will hold water however often it is brushed out.

The usual cure is to re-hang the run to the right fall and add brackets where they are missing. If joints leak, they can be re-sealed or the section replaced.

When a gutter has split, rusted through or lost its shape, clearing it is only a delay. Our page on guttering, fascias and soffits explains the replacement work and what drives the price.

Whatever the repair, the route to the roof edge also matters. A gutter at first-floor height needs a safe way up. We plan the access before the visit, so you know whether it is a ladder, a tower or a scaffold.

It is tempting to book the roof clean and leave the gutters for another day. That nearly always costs more in the end, for a few reasons.

First, access. If we have a ladder or a tower up for the roof, reaching the gutter is quick. A second visit means setting up again. On a house that needs a scaffold, the scaffold might be in place for the whole job anyway. If scaffolding blocks the footway, a highway licence is needed first, which needs notice, so it makes sense to do everything under one licence.

Second, the debris. The growth you clear from the tiles lands in the gutter, so the gutter has to be the last thing cleaned. Do it earlier and the roof clean undoes it.

Third, the evidence, which matters when a repair turns out to be needed. With the roof edge exposed, we can look at the lower courses of tiles, the underlay at the eaves and the fascia in one go, and photograph anything wrong.

Fourth, the aftermath, which is easy to forget. Dead moss goes on falling for some time, so a second pass over the gutter catches what has come down since. We advise on timing at the survey, and we note in the quote which gutters are included, so there is no surprise about the follow-up.

The link between the two jobs is the reason we describe the full service on our roof cleaning and moss removal page and not as a roof-only task.

Not every gutter should be cleared. Some are better replaced, and a few need caution before anyone touches them.

Cast iron gutters, common on older houses, can look sound and be rusted through in places. They are heavy, and joint bolts that have seized can crack the channel when force is used. A cracked or corroded run is a case for repair or replacement, and no amount of cleaning will fix it. Tapping along it from a safe position on the ground, or looking for rust streaks on the wall below, gives you a first idea, and a close look at the joints and bolts from the roof edge settles it.

Plastic gutters, mostly uPVC, last a long time but can crack, sag or lose their seals. A section that has split along the back will leak behind the fascia, where nobody sees it. Check the joints, and ask your roofer to look at the brackets.

Asbestos cement rainwater goods

If your house was built or refurbished before 2000, the HSE advises assuming there may be asbestos in it. Rainwater goods made of asbestos cement are one of the places it can turn up. The HSE treats work on asbestos cement as non-licensed provided the material is carefully handled and removed without breaking it up. Scrubbing, drilling or hitting it is the way to cause trouble.

If you think your gutters, soffits or roof sheets might contain asbestos, do not clean, scrape or break them. Leave them undisturbed and ask for advice before any work starts. Our page on garage roofs covers the sheet roofs often found on outbuildings.

The last case to mention is a gutter with moss rooted deep into a failing fascia. Where the timber has gone soft, you are looking at roofline repair, and the moss is the least of it. Your written quote will cover what the survey finds, with photographs, so you can see the problem for yourself.