Roof cleaning and moss removal in Bath

Moss needs very little. It has no roots in the usual sense, only fine threads that anchor it, and it lives off rain and the dust that settles on the tiles. It settles first where a roof stays damp the longest: the north-facing slope, the shaded side of a tree, the lower courses near the eaves, the joints between tiles where grit collects. Concrete tiles, which are porous and rough, give it a good grip. The 1960s concrete-tiled house in the drawing above is the textbook case, with a thick green mat running down the shaded slope and thinning towards the ridge.

A little moss is cosmetic. A thick mat is another matter, because it behaves like a sponge. It soaks up rain and holds it against the tile for days after the weather has cleared. In winter that trapped water freezes and thaws, and a surface that is already soft or crazed can flake. On interlocking tiles, which have shaped channels meant to carry water down the slope, a mat of moss blocks the channel and pushes rain sideways, towards the joint and the batten beneath. Battens are the sawn timber strips the tiles hang from.

Weight, and what falls off

A saturated moss mat is heavier than it looks, though never enough to threaten the roof structure on its own. The more practical worry is where it goes. Moss dies back in dry spells, crumbles, and washes into the gutter with the next shower. That is how a roof that looks only a little green ends up with gutters packed solid, and why we take the moss out by hand instead of letting rain move it for us.

What we look at before cleaning

Moss can hide trouble. A tile cracked across its width, a ridge piece that has lost its bedding, or a small gap where a slate has slipped can all sit under a green mat for years. If we scrape the moss away and find loose or broken pieces, we stop and show you before going further, because a roof that needs repair is better repaired than cleaned over. We also check how well the roof sheds water at the lowest courses, since the moss there will be thickest.

Not every roof near Bath is concrete. Welsh slate roofs on older terraces grow moss differently, in the laps between slates, and clay tiles on older houses collect it in the rough face of the tile. The method changes with the covering, which is why the survey comes first and why we do not quote a flat rate for a clean before we have seen your roof.

Where overhanging branches drip onto the slope, expect the moss to come back in that spot first. We tell you on the day, so nothing comes as a surprise, and we point out which trees are the cause.

For the full method, our guide to moss removal by scraping, brushing and biocide goes slope by slope. If you are weighing up a clean against doing nothing, the answer on whether moss damages a roof sets out when it matters and when it does not.

Look in the gutter of a mossy roof and you will usually find the roof's moss lying in it. Rain and wind loosen growth from the slope and carry it down. Some of it packs into the gutter, some into the outlet where the gutter meets the downpipe, and some into the hopper, the funnel-shaped box at the head of a downpipe. Add leaves, silt from the tile surface and sometimes a handful of seedlings growing from the wet, and a gutter can be blocked without its owner ever seeing it from the ground.

A blocked gutter does its harm quietly. Water spills over the lip and runs down the wall, staining the face of the stone and soaking the foot of the wall. Or it stands in the gutter, and the standing water finds the back of the fascia board, the board fixed along the eaves, and softens the timber. In winter that water freezes and can split a joint or pull a cast-iron section off its brackets.

What we do about it

Because we clean the roof from the top down, the gutter is emptied last, once the loosening is done. A trowel or scoop takes out the bulk by hand into a bucket or bag. The gutter is then flushed with water to show up low spots, joints that leak and any section that holds water instead of shedding it. We check that each outlet is clear before the downpipe is dealt with.

If we find the gutter itself failing, with a sagging run, cracked joints or a rusted section, we tell you and show you. Gutter repair or replacement is its own job and is quoted separately, so the clean stays a clean.

Why the order of work matters

Cleaning the gutter first and the roof second is a false economy: the roof then fills it again before the day is out. Cleaning the roof and leaving the gutter is worse, because you have just moved the moss from one place to a place where it does more harm. So we carry out roof, gutter and downpipe in one visit, and we catch the moss as it comes down the slope so it does not wash through.

The practical benefit is easy to check afterwards. On the next wet day, a clear gutter carries water to the downpipe and away. Water that still falls over the edge in a steady sheet is a sign the run is out of level or the outlet is undersized, and that is something worth a closer look.

Nesting birds sometimes use a gutter or the eaves, and a nest in use is protected. If we find one we stop and leave that section until the birds have finished.

Our page on moss, gutters and blocked outlets covers the knock-on effects in more detail. If the gutters themselves need work, see gutters, soffits and fascias for repair and replacement.

Once the bulk of the growth is off, the surface still carries spores, thin films of algae and fragments of moss lodged in the texture of the tile. A biocide is a treatment made to kill those. It is sprayed on at low pressure and then left to work. Over the following days and weeks, what remains dies off and is gradually rinsed away by the rain, so the roof often looks cleaner a month later than it does on the day.

The benefit is a slower return. Treated surfaces take longer to green over because the spores that would have started the next crop have been killed. How much longer depends on conditions we cannot control: how shaded the slope is, how close the trees stand, how damp the site is and what the tile is made of. We never promise a number of years. A roof under a large tree will green over sooner than one in an open garden, whatever is put on it.

Choosing and applying a biocide

We apply the treatment in dry, still weather and keep it off windows, paths, planting and cars. Rain soon after application can wash it off before it has done its work, so we pick the day with the forecast in mind. The mix suits the covering: some products stain stone or mark a metal flashing, a strip of lead or similar metal fixed where the roof meets a wall. We cover or avoid those details.

If you have pets, a pond or edible plants close to the house, tell us at the survey. We will take the care the situation needs, and our guide to whether roof biocide is safe for pets and plants sets out the sensible precautions.

Roof coatings, assessed fairly

Some firms offer to paint or spray a coating over the clean roof. Honestly, the case for it is mixed. A coating can freshen a faded concrete tile and seal the surface, and in some cases it makes the tile slower to take growth. Against that, it adds a layer that must be kept up, it can flake in time, and it can change how the tile weathers and how it dries. It can also raise questions at a sale or with a lender, since it hides the surface a surveyor wants to see. We do not recommend a coating as a routine add-on. If you want one, we talk through the trade-offs first.

For most roofs, a hand clean and a biocide is enough, repeated when growth returns.

See how biocide roof treatments work and how long to expect them to last. Our assessment of roof coatings covers the benefits and drawbacks in full, and the answer on coatings and mortgage valuations is worth reading before you commit.