After a roof has been cleaned, you may be offered a coating. The pitch is simple. The roof looks tired, the moss will come back, and a layer sprayed or rolled on will fix both. For some coverings in sound condition that can be a fair offer. For others, particularly porous or old coverings that need to dry out, it can seal in the very thing that causes damage.
This guide explains what coatings claim, how they work, why a film can trap moisture, what needs to be right before one goes on and which roofs we would advise against coating. It is written so you can ask a contractor the right questions, whichever way you decide. For the safe cleaning of one common covering, see our answer on cleaning concrete tiles. For the service as a whole, go to roof cleaning and moss removal.
What a roof coating promises
Coatings for pitched roofs are liquid products, usually acrylic or similar, applied over a cleaned surface. They are sold on a handful of claims, and it helps to separate them.
A better colour
This claim is straightforward. Over a faded or patchy covering a coating gives an even colour, and for a roof you see from the road that can matter. It is a cosmetic gain and it should be priced as one.
Growth kept off
The film is said to leave a smoother face with fewer pits for spores to settle in, and some products include a biocide to slow growth. A smoother face does shed a little more water. It does not change the shade, the trees or the damp that drove the moss in the first place, so regrowth on a shaded slope is slowed at best.
A longer life for the covering
This is the claim to treat with most care. A coating can lock in a surface that is starting to powder or erode, and in that sense it can protect it. A guarantee from the coating maker is a guarantee about the product, though, and it is not a statement about how long your tiles will last. Lifespans for tiles and slates depend on exposure, workmanship and maintenance, and no layer on the surface changes that arithmetic much.
Waterproofing
Some sales talk implies a coating will stop leaks. A coating does not repair a cracked tile, replace a slipped slate or renew a failed flashing (the metal strip, commonly lead, that keeps water out where the roof meets a wall or chimney). Leaks have to be found and repaired first.
How a coating works on the surface
Most pitched-roof coatings work in one of two ways, and the difference matters when you think about moisture.
Film-forming coatings
These sit on top of the tile and dry to form a continuous skin, rather as a coat of gloss paint does on a window frame. They give the most obvious colour change. They also form the strongest barrier, and a barrier works in both directions: it keeps water out, and it keeps water in.
Penetrating treatments
Other products soak into the surface and leave less of a skin. They are sold as water repellents or consolidants. They interfere less with the way the covering lets vapour escape, but they are more expensive and less common on roofs, and their results vary with the porosity of the surface.
How the product goes on
Application is usually by spray from the ground or from a platform, or by roller on a flat surface. The coating needs a dry day, a dry surface and a long enough dry spell afterwards to cure. A spell of damp weather in the days after application is when many coatings fail early.
Why a name on the tin is not enough
Product names and brochures describe the coating at its best, applied to the right surface in the right conditions. The questions we would put to any contractor are simple. What exactly goes on, what is it made of, what does the maker say about the covering it is applied to, and what happens if it has to come off?
Why a sealed face can trap moisture
Pitched roof coverings are not meant to be watertight in the way a bucket is. Clay, concrete and stone coverings are all porous. Water that soaks in during rain dries out again through the same face, by evaporation, in the days afterwards. The roof depends on that drying.
Water gets in from more than one side
A coating seals the top surface, but water also arrives from other directions. It passes up through the laps from wind-driven rain, it soaks in through cracks and chipped edges, and warm moist air from inside the house can reach the underside of the covering through gaps in the ceiling and loft. Under a sealed film that water has fewer routes out.
What follows when water cannot leave
Moisture held behind a film can blister or lift the coating, so you see peeling and flaking within a few seasons. It can also keep the body of the tile wetter for longer, and in cold weather wet material is more exposed to frost. For stone and old clay tiles that is a real risk, because frost is one of the main things that flakes their faces.
Ventilation still matters
BS 5250 is the British code on moisture in buildings, and it is why roofs have eaves ventilation and underlays chosen to suit the construction. A coating does not replace any of that, and it should never be sold as a way around it.
The reverse problem
If a coating starts to fail, it does so patchily, and the damp surface under the lifted flakes then grows moss faster than before. Taking an old coating off is slow and has to be done to the whole slope, or the roof looks mottled.
What must be true before anything goes on
When a coating works, it is because the surface underneath was ready for it. A good contractor should be able to tell you what that preparation involves, and you should see it on the quote.
The covering must be sound
Tiles that are cracked, spalled or crumbling need replacing first. A coating over a crack hides it from view and does nothing for it. The same goes for slipped tiles, failed pointing on the ridge and bad flashings. Repair comes before cosmetics, and we would expect the roof to be checked during cleaning, a point our wider service covers.
All growth must be gone
Moss, lichen and algae have to be taken off and treated, or the coating sits on living material and lifts as soon as it does. Hand scraping or low-pressure treatment is the right start. High-pressure washing can roughen concrete tiles and drive water into the laps, and a coating on top does not make that good.
The roof must be properly dry
A roof that was wet when it was coated holds that water inside. After cleaning, a sensible gap is needed before any coating, and dry weather lasting until the product has cured. A coating rushed onto a damp roof on the same day is the pattern we would be most suspicious of.
Gutters and the roofline
The roofline needs to be clear before coating begins, since debris dropped on wet coating sticks to it. See moss and gutters for why we clear them at the same visit as a clean.
Roofs where we would not advise a coating
Some roofs should be left uncoated, whoever is offering to do it.
Stone slates
Cotswold stone slates are porous limestone, laid by hand and hung from pegs. They are meant to take in water and give it up. A film over the face would hold moisture in stone that is already vulnerable to frost, and it would change the look of a material valued for its natural surface. The English Heritage policy set out in the 2000 council guidance asks for like to replace like on historic buildings, and a modern film is the opposite of that approach.
Old clay tiles
Handmade clay tiles have uneven, absorbent faces. They are best cleaned gently and left to weather.
Listed buildings and conservation areas
The council's own guidance gives every grade of listed building complete protection. Changing the colour or surface of a historic roof can affect its character, and listed building consent may be needed, so ask the council's conservation team first. The district has 41 conservation areas, and our guide to cleaning listed roofs covers the gentler methods that suit protected buildings.
Roofs with damage or damp
Any roof with cracked or flaking tiles, active leaks, persistent damp or poor ventilation should be repaired before a coating is even discussed.
Roofs that need a new covering soon
Where a covering has little life left, money spent on a coating is better put towards the replacement, and we would say so at the survey.
Where a coating is not advised, the survey records the reason in writing, with photographs of your own roof.
Concrete tiles: the case where coatings are argued over
Concrete tiles are offered a coating more often than any other covering, so they get their own section.
Why they are offered
Concrete tiles lose their surface slurry, the thin coloured top layer, over many years of weathering. That leaves a rougher, paler face that holds moss. A coating promises to renew the colour and the smooth face.
The argument for
On a sound concrete tile, cleaned thoroughly and dry, a good coating can even out the colour and make a smoother face. Some owners value that, particularly before a sale.
The argument against
The tile is already porous, and a coating adds a barrier to its drying. If the cleaning was done by pressure washing, the surface may have been damaged already, and the coating then hides the damage. Several years on, a failed coating looks worse than weathering would have done.
Our position
We lean towards cleaning and, where growth is heavy, a biocide treatment, then letting the tiles weather. If an owner still wants a coating, we want to see the tiles dry and sound and to know the product's data sheet for concrete. How to clean concrete tiles without harming the face is covered in the sibling answer on cleaning concrete tiles.
Cost
A coating is priced on survey, since the product, the area and the preparation all vary. The cleaning that has to come first has MyBuilder's 2026 ranges of about £5 to £10 per m² for hand scraping and £15 to £20 per m² for soft wash, and a coating is an extra line on top of them.
What to ask, and what to do instead
If a contractor suggests a coating, a few questions sort the sound advice from the sales pitch.
Questions to put
What covering do I have, and does the maker say the product suits it? What happens to moisture already in the tile? How long must the roof dry beforehand, and what weather is needed afterwards? What does the guarantee cover, and who stands behind it if the contractor is not around? How is the coating removed if it fails? A clear answer to each is a good sign. A vague answer to the last one is not.
Alternatives that cost less
Regular light cleaning does more for most roofs than a one-off coating. Cutting back overhanging branches reduces the shade that feeds moss. Clearing gutters and valleys of leaf litter each autumn takes away the seedbed. A biocide treatment applied after cleaning slows regrowth without sealing the face. And repairing a tired roof does more for its life than a coat of paint on it.
Our approach
We clean by hand or with a low-pressure treatment, check the roof as we go, clear the gutters, remove the waste and let the surface dry and weather. Our work carries a 10-year workmanship guarantee in writing. Where a coating would help, you hear which roof and why. Where it would not, you hear that as well.
Offered a coating and unsure? Ring 01453 933613, or send the quote with photographs of the roof on WhatsApp or through the form, and you will get our opinion before anything goes on.