A biocide is a liquid applied to a roof to kill the living growth on it: moss, algae and lichen. It is the part of a roof clean that does the lasting work, because scraping or washing growth off a tile removes what you can see but leaves spores and roots behind. People searching for a roof biocide usually want two answers. How does it kill the growth, and how long before it comes back? The first has a clear answer. The second does not have a single number, and anyone who quotes one without looking at your roof is guessing. This guide explains the mechanism, what you will see in the weeks afterwards, the conditions that decide how long a treatment holds, and how it fits alongside soft washing on a Bath roof. We have kept to what can be said with confidence and marked the rest as depending on the roof.
What a biocide does to moss, algae and lichen
Moss, algae and lichen are living things, and a roof biocide is a chemical made to kill them. The active ingredient is absorbed by the growth and disrupts its cells. Moss and algae take up the liquid through their surface, since neither has a true root system or a waxy skin to keep it out. Once the cells are damaged, the growth stops spreading, dries out and dies back over the following weeks.
The important point is that the treatment works on contact and by soaking in. That is why a roofer wets the whole surface rather than spotting the visible patches. Growth you can see on a tile is only the top of it. Fine fragments sit in the pores of the surface and in the gaps between overlapping tiles, and a biocide that reaches them is what slows regrowth.
Biocide is not a cleaner. It does not lift dirt or remove the dead material itself. It leaves the dead growth in place to fall away as the weather works on it, or to be rinsed off in a follow-up wash. A second thing to know is that products differ. Some are made for a single application and are left to do their work. Others are applied after a soft wash. Ask the roofer for the product name and read its data sheet and label, because that, not a general description, tells you the contact time, the safe use and any precautions for plants, pets and water runoff.
Why the roof looks unchanged for weeks afterwards
The most common worry after a treatment is that nothing has happened. The moss is still there, brown or black and looking worse if anything. That is normal. A biocide kills the growth in place, and it takes time for dead moss to dry, shrink and lose its grip.
In the first few weeks you may see moss turn brown, then grey, then crumble. Rain and wind carry much of it down the roof and into the gutters. Algae streaks fade more slowly, and lichen is slower again because it is built to cling tightly. A roofer who tells you the roof will look new the same afternoon is describing a wash, not a biocide.
What this means for the schedule
Because the effect builds over time, the sensible order on a heavily covered roof is to remove the thick moss first, by hand or with a soft wash, and apply the biocide over the cleaned surface. That lets the liquid reach the tile rather than sitting on a thick mat of growth. On a light covering, a biocide alone can be enough, with the dead growth left to weather off.
We would rather describe what to expect than promise a finish. On a roof with years of growth, the full result is something you judge after a season of weather, not on the day. If a ridge or valley holds thick moss that is lifting tiles or blocking the run of water, that is removed by hand regardless of the biocide, and the survey notes it.
How long a biocide treatment lasts
No published, universal figure exists for how long a roof biocide holds, so we leave the figure out. The duration is set by the roof and its surroundings far more than by the liquid itself. Two identical treatments on two roofs can give very different results.
The factors that matter most are these.
- Shade and orientation. A slope that faces north, or sits under a taller building, stays damp longer and regrows sooner.
- Trees. Overhanging branches drop leaves, seed and moisture, and shade the slope. Our page on trees and moss on roofs covers this in detail.
- The covering. Rough, porous surfaces, such as old clay or concrete tiles and stone, hold moisture and give growth a grip. Smooth slate sheds water faster.
- Pitch. A shallow slope holds water longer than a steep one.
- Debris. Dead leaves and moss in gutters and valleys keep the roof damp and feed new growth.
Regrowth also varies by part of the roof. It often returns first on the shaded slope or along a damp valley while a sunny slope stays clear. That is why two retreatment dates on one roof are sometimes sensible. A roofer who looks at your roof can tell you which slopes are likely to need attention first, and that goes in the survey notes instead of a blanket number.
Biocide with soft washing, and why the order matters
A biocide is rarely used alone on a roof that already carries a lot of growth. The usual approach is a soft wash followed by a treatment, and understanding the difference between the two helps you read a quote.
A soft wash is a gentle rinse with a cleaning solution, which loosens growth and dirt while keeping water from being forced beneath the tiles. It is the method we describe on our page on soft washing roofs. For soft washing, MyBuilder's roof cleaning cost guide (2026) gives a typical UK range of £15 to £20 per m², against £10 to £15 per m² for pressure washing, and the guide itself notes that a pressure washer "may damage delicate tiles". A fixed quote follows once we have surveyed the roof.
The cleaning removes the bulk. The biocide then treats what remains and the surface itself. Done in that order, the liquid has the best chance of reaching the places regrowth starts. Done the other way round, with a wash straight after the treatment, you risk rinsing off the active ingredient before it has had time to work.
Roofers use different products and methods, and some treat as part of the wash itself. Neither approach is wrong. What matters is that you know which was used, what contact time it needs and what it leaves behind. When you compare quotes, ask each roofer to say if the treatment is included, because a lower price can mean a wash with no biocide at all, which leaves the roof ready to regrow.
Winter, weather and when to apply
Timing affects how well a biocide works. The liquid needs to stay on the surface long enough to soak in, so rain soon after application can weaken the effect. A dry spell of a day or two is the usual aim, and a roofer will watch the forecast rather than book blind. Very hard frost is another reason to wait, since a frozen surface does not take up the liquid properly.
People often ask whether winter is a bad time. The weather is only part of it. Moss is actively growing and holding water in the cooler, wetter months, so a treatment then can stop it in its tracks, but the conditions must allow the work. We cover that question on our page about roof cleaning in winter, including when a delay is the better choice.
Working at height in poor weather is also a safety question for the crew. A roof that is wet, icy or windy is a slip risk and we do not push a job through for the sake of a date. Ladders, roof ladders and edge protection are set up before anyone steps onto the slope, and on a tall Bath terrace that often means scaffold or a tower rather than a ladder alone.
Gutters need a look at the same time. Dead moss washed down by the first rain lands in them, and a blocked gutter keeps the eaves damp, which feeds the next round of growth. We clear them as part of the job, and we take the waste away when we leave. On a house with a parapet gutter, a lead-lined gutter hidden behind the edge wall, the outlet is checked as well, since a blocked outlet there sends water into the wall.
The practical answer is that the best moment is a dry, mild window with a forecast that holds, and we will tell you when one is coming for your roof.
What a biocide cannot do, and what to ask before you book
Biocide is one part of our roof cleaning work, and it controls growth. It does not repair a roof. It will not refix a slipped slate, replace a cracked tile, renew failed pointing or reline a leaking valley. If the roof has real faults, treating the surface first can hide them, which is why we look at the condition before we clean. The survey photographs show you the tiles as we found them, so you can judge the condition yourself.
It also does not make a roof permanently clean. Treatment slows regrowth, and the roof will need attention again at some point, and why moss comes back has a guide of its own. Anyone offering permanent protection from a spray is overstating it.
Before you book a treatment, these are fair questions to ask any roofer.
- Which product is being used, and can you see its label or data sheet?
- Is the biocide applied after the wash, and how long does it need before rain?
- What happens to the runoff, and how are plants, ponds and neighbours' gardens protected?
- Is it safe for pets and children once dry? The answer is on our biocide and pets page.
- How will the dead growth be removed, and who clears the gutters afterwards?
On older Bath roofs, with natural slate, stone tiles or lime-pointed ridges, add one more question about the method, since harsh treatment can mark or damage a delicate surface. Leave the chemistry to the data sheet and use the survey to settle the plan for your roof. Book the treatment for a dry, still day, as rain within a few hours can wash the product off before it has worked, and wind carries spray onto nearby plants.