The short answer
The work on site is usually a matter of a day or so for a typical house, though the roof's size, access, weather and the amount of growth all move that. The visible result takes longer. A biocide needs time to kill the moss and algae, and the dead growth then loosens over the following weeks, so a roof looks its best some time after the visit.
Two clocks run on a roof clean: the time the crew spends on the roof, and the weeks the treatment takes to show.
What sets the time on site
There is no single figure that fits every house, and anyone quoting one without seeing the roof is guessing. The time comes from a handful of things we can see at the survey, and it is the reason a survey comes before any time estimate. The wider process is described on our roof cleaning and moss removal page.
Why does one roof take longer than another?
Area comes first: more slope means more to treat. Access is next. A terrace with a rear slope reached only through the neighbouring houses, or a roof behind a conservatory, takes longer to set up than a detached house with a clear run around it. Height matters, because a three-storey frontage needs scaffold or a tower before anyone steps on the tiles, and that has to go up and come down.
The thickness of the growth counts too. Light algae staining treats quickly, while deep moss mats may need lifting by hand first. Gutters and downpipes that need clearing add to the day. Roofs with many valleys, dormers or chimneys have more edges to work along, and each one slows the pace. Our page on scaffolding for roof cleaning explains when access equipment is needed.
Weather and drying
Roofing is outdoor work, and weather decides whether a booked day goes ahead. Heavy rain washes treatment off before it can act. High winds make working at height unsafe and carry spray where it should not go. A hard frost makes a slope slippery. In Somerset's wetter months a visit can slip by a few days to find a suitable window.
Does the roof need to be dry first?
It depends on the product, which is why the label or the maker's instructions come first. Some treatments are applied to a damp roof, others need a dry spell afterwards, so the applicator should confirm it before starting. What we can say is that rain too soon after application is the usual reason a second visit is needed.
After the treatment goes on, there is nothing more for the crew to do on that day. The roof is left to dry and the weather takes over. Tiles that were covered in moss will often look no better than before for a while, because the growth is dead but still clinging. Patience here is part of the method, not a sign that anything has gone wrong.
Dwell time: why the result arrives later
Dwell time is the period a biocide stays on the surface to do its work. A product's maker states how long it needs, and the person applying it must follow those instructions, because the label is the legal standard for how a biocide is applied. A treatment rinsed or rained off early has not had its dwell time.
When will the roof look clean?
Most of the change comes in the weeks after the visit. Moss turns brown and dry, algae streaks fade, and wind and rain carry the loosened material down into the gutters. How fast depends on the season and the weather. A wet, mild spell tends to clear a roof faster than a dry, cold one. Because the result is gradual, we suggest judging the roof a good while after the work.
Not every roof needs the same wait. Light algae can fade quickly, while thick moss takes longer. If you want the roof presentable for a particular date, such as a sale or an event, tell us at the survey. Our page on cleaning a roof before selling goes through the timing for a sale.
Planning the visit around your day
On the day, the crew sets up access, protects what needs protecting, treats the roof, clears the gutters if that is part of the job and leaves the site tidy. Each morning you hear what is planned for that day, so nothing catches you out on the drive or in the garden. If the weather turns, the plan is reset and you are told.
Do I need to be at home?
It helps on the first morning, so we can agree where cars, plants and washing lines need moving, and what to keep closed. Pets and children are better kept away from the working area, because ladders, tools and treated water are about. Once the work is under way you can usually carry on with your day. Our page on protecting gardens during roof cleaning covers the preparation.
The time estimate comes with the survey, alongside a single total in writing. Send an address and a street photo of the roof, or book the visit, and the survey covers the area, the access and the growth, so you learn how long your own roof will need.
More on timing a roof clean
Can roof cleaning be done in winter?
It can, when the weather allows. Frost, ice and strong wind make roofs unsafe, and some products need a minimum temperature, so check the label. Cold, damp months also slow the clearing, because treated growth wears off more gradually. A mild, dry spell is easier, but the survey can be done in any season.
Why does my roof still look mossy a week after cleaning?
Dead moss does not drop off at once. It browns, dries and loosens, and rain and wind remove it over weeks. Brushing it off by hand risks damaging the tiles, so it is better left to the weather. If a patch is still thick after a few months, tell us and we will take a look.
The full picture: Roof cleaning and moss removal in Taunton ยท More short answers