Soft washing is the name given to cleaning a roof with a gentle flow of water and a cleaning solution, instead of a hard blast. The pressure is far below that of a jet. What does the cleaning is the solution, not the force, and the roof is left to do the rest as the dead growth loosens and rinses away. The method has become the usual alternative to jet washing because it treats the cause of the growth and leaves the covering intact. It is not a cure for every roof, though, and it is sometimes sold as if it were. Each stage of the process has a purpose, some coverings suit it while others are better served by something else, and there are clear expectations for the day itself and for the weeks that follow the wash.
What soft washing is, step by step
The term covers a short sequence, and the order matters. First, the roof is inspected from a safe position, and any loose or broken pieces are noted. Gutters, outlets and downpipes are protected or cleared, and plants and surfaces below are covered or wetted so the solution does not harm them. Heavy deposits, such as thick moss or piles of leaves, are removed by hand, because a spray cannot reach the surface underneath them.
Next comes the application. A cleaning solution, usually a biocide (a product made to kill moss, algae and lichen), is sprayed over the covering from a low-pressure pump. It is applied evenly, working across the slope so the surface is thoroughly wetted without being forced into the laps. The solution is then left to act for the time the manufacturer specifies. During that dwell time it penetrates the growth and begins to kill it.
After the dwell comes a rinse. Where a rinse is used it is low in pressure and directed in the line of the slope, and it carries away the loosened material. In many cases the rinse is light or left to the weather, and a soft brush deals with patches that have not released. The roof is finished by clearing the gutters of what has come down and taking the debris away.
The pace is different from a jet wash. A soft-washed roof often looks much the same by evening, and then brightens as the dead growth fades and falls over the next few weeks. That slow result is a sign the treatment is working through the growth and not just stripping the top of it.
Why low pressure protects the covering
A roof covering is made to shed water that falls on it from above. Soft washing respects that. The water arrives at a pressure the covering can take, and the solution does the work of loosening the growth, so there is no need to rely on force to remove it. That is why the method is described as gentle, and why it is the method we use on coverings that cannot take a jet.
Compare what happens to each part of the roof. On concrete tiles the thin coloured surface stays in place, because nothing is blasting it. On slate, the edges are not opened by a stream of water, and the nails are not shaken. On clay, the weathered edges and nibs are left alone. On a ridge, the old mortar is not blown out. And because the water is not driven up the slope, the underlay and loft stay dry. The opposite approach, and the harm it brings, has its own page: why a jet wash strips and soaks a roof.
There is a second benefit, which is the effect on regrowth. Because the solution works on the growth and its spores, and does not roughen the surface, the roof is left smoother and cleaner than a stripped one. Moss and algae find it harder to get a hold on a sound surface than on a pitted one, so the clean tends to last longer. No clean is permanent, and that is not the claim here. The fair summary is that soft washing does not make regrowth faster, and a good treatment can slow it.
A word of caution about the name. Soft washing describes a pressure and a sequence, not a standard. Some firms use the term for a quick spray with a strong chemical and no dwell time. What matters is what is used, how it is applied and what is done about the debris, and those are worth asking about.
The solution: what goes on the roof
The cleaning solution is the heart of the method, and the choice is not trivial. Biocides for roofs are made to kill moss, algae and lichen on tiles, slate and stone, and they come in different strengths and bases. Some act quickly. Others work slowly but leave a residual effect that discourages regrowth for a longer period. The right one depends on what is growing, the material of the roof and what lies below the run-off.
Several practical points apply whatever product is used. The label is the guide: the dilution, the dwell time and the conditions for use are set by the maker, and they should be followed exactly. The weather matters too. A treatment applied in a heavy downpour is diluted and washed away, and one applied in strong sun and wind dries before it can work. A calm, dry spell is best.
Run-off needs thought. The solution ends up in the gutters and then on the ground or in a drain. Plants, ponds and water features below the roof line need protection, and the route the water takes should be known before spraying begins. Metal fittings, copper and lead also deserve care, since some products can mark or attack them. A competent firm covers these points before the work starts.
We cover product choices, residual effects and how long a treatment is expected to work in our guide to roof biocide. For the purposes of this page, the point is that the solution is chosen for the roof, and not the other way round. We pick the product after the survey, when we have seen the covering and know where the water will go.
Roofs that soft washing suits
Soft washing is the right starting point for most pitched roofs that are structurally sound but covered in growth. In practice that includes the following.
Concrete and clay tiles in sound condition
These are the coverings that gain most from the method. The surface is preserved, the growth is treated and the laps are not flooded. Clay tiles that are well into their life still need care, but low pressure is the safest way to clean them.
Natural slate
Slate calls for a gentle approach, hand work on the thick growth and a soft treatment on the rest. Soft washing fits that profile well, as long as the slate is not already failing.
Stone tiles
Old stone tiles can be soft washed with great care and a mild product. The solution must not be left to pool, and the rinse must be very light. For these roofs we also lean on hand work. See our page on cleaning stone roofs.
Flat roofs
Flat roofs in felt, EPDM or GRP gather algae and debris, and a mild solution with a soft brush deals with both. Here the aim is to keep the surface and the outlets free so water drains, without scouring a membrane that is already thin.
Light growth and algae streaking
Black streaks and a green film respond quickly to a soft wash, and little hand work is needed. This is the easiest case, and it is the one where the method is most likely to give a result that lasts.
When soft washing is not the answer
A gentle method has limits, and knowing them saves money. Soft washing cleans a roof. It does not repair one. Several situations call for something else, or for something first.
If the covering is breaking down, cleaning will not rescue it. Tiles that are crumbling, slates that have lost their nails, a sagging ridge or an underlay that has perished are repair or replacement problems. Washing a failing roof may spend money on a surface that is about to be stripped anyway. How we weigh that choice is explained under clean or replace.
Thick moss needs to come off first. If a deep mat is sprayed without being removed, the solution cannot reach what is under it, and the roof stays damp. Hand scraping is part of the plan on a heavily overgrown roof, not an extra.
Lichen is slower. The crusty patches grip firmly and can need a stronger or repeated treatment, plus patience. A soft wash may tackle it, but we do not promise a spotless result in one visit.
Access can rule it out. A roof that cannot be reached safely cannot be washed safely, and a tower or scaffold has to be arranged before work begins. A roof with serious defects may also be unsafe to walk on.
Finally, some situations call for consent or caution. On a listed building, a method that alters the surface of old fabric is worth raising with the B&NES conservation team beforehand. Asking costs nothing.
What to expect on the day and in the weeks after
On the day, expect a sequence you can follow from the ground. The area below the roof is covered or wetted, the roof is accessed with a ladder, tower or scaffold, and the spraying begins. It is usually not a loud job, because the pump is gentle. The solution is applied, a pause follows, and then comes a rinse or brush and clearing of the gutters. A small roof can be done in a few hours and a large or awkward one takes longer.
Do not expect a perfect result before the firm leaves. The roof may look streaky or barely lighter. Over the following weeks the growth goes pale, dries and releases, helped by rain and wind. At that point the roof looks its best. A short follow-up check is sensible, to brush off stubborn patches and clear any fresh debris from the gutters.
It also helps to know what is normal. Some streaking as the moss dies back is common. A little runoff staining on a path or wall can occur where the solution was not contained, which is why protection beforehand matters. Fading of an already tired coloured tile can happen on any cleaning method, and an honest firm tells you about that risk in advance.
The cost is covered on our page about what a roof clean typically costs. MyBuilder's roof cleaning cost guide (2026) reports a typical UK rate of £15 to £20 per square metre for soft washing, against £10 to £15 for pressure washing. The higher rate reflects the dwell time, the products and the extra steps. Your fixed quote follows the survey.
Is soft washing right for your roof?
Whether a soft wash suits your roof comes down to a few questions that a survey answers. What is the covering and what condition is it in? How thick is the growth, and is it moss, algae, lichen or a mixture? What is below the roof that the run-off might reach? How is the roof reached? Are there repairs that should be done first?
If the answers point to a sound roof with a surface layer of growth, soft washing is usually the right method and it is what we recommend. If they point to a fragile or failing covering, we tell you and suggest a different route. That might be a repair, a limited hand clean, or a conversation about the roof's remaining life.
A survey means looking at the covering close up, checking the ridge, the lead and the gutters, and photographing the roof as it stands. The pictures go out with the quote, which names the method, the products, the access and any repair items on separate lines. One fixed price is set down on paper, and ten years of workmanship cover is written in with it.
If you want to talk it through first, the wider service sits under roof cleaning, and you can ring 01225 285181 or send a WhatsApp message or a form enquiry. Tell us the covering and roughly when the house was built, and we can say whether a soft wash is likely to be the answer before anyone climbs a ladder.