The short answer
Clean an old roof when the tiles or slates are sound, the fixings hold and the timbers are dry. Replace it, or at least strip and re-cover it, when tiles are flaking or slipping across whole slopes, the battens are rotten or the underlay has perished. A survey before any cleaning shows which of the two applies.
What cleaning can and cannot do
Cleaning changes the surface. It removes growth, shows you the true state of the covering, and slows the damp that keeps tiles wet. On a roof in good shape, that is useful work and it can add real years to the covering.
It cannot fix anything structural. It will not stop a rotten batten giving way, will not refix a slate whose nail has rusted through, and will not restore an underlay that has gone brittle. It will not put back the surface of a concrete tile that has already worn thin. A clean roof with a failing structure is still a failing roof, and sometimes cleaning shows the problem sooner by revealing cracks the moss was covering.
The useful test is soundness rather than dirt: is the roof sound underneath, whatever it looks like on top? If you are unsure, start from roof cleaning and moss removal and arrange a survey before you commit to anything.
Signs the roof is worth cleaning
A roof is a good candidate for cleaning when most of the following are true.
- The tiles or slates sit flat and in line, with no obvious dips along the ridge or across a slope.
- Only a few units are cracked or slipped, and they can be replaced individually.
- The loft shows dry timbers, no daylight through the covering and no staining on the underside.
- The lead at chimneys, walls and valleys is intact.
- The surface of each tile is intact, without crumbling edges or a powdery face.
On a roof like that, removing moss, treating the surface and clearing the gutters is proportionate. It is also kinder to the roof than a full replacement, which suits older houses where keeping the original covering matters, and it keeps the weathered look that a street of period roofs depends on. For natural slate, our guide to cleaning slate roofs explains the gentle methods. Where the property is listed, see cleaning listed roofs too.
Signs cleaning would be wasted money
Some roofs are past the point where cleaning makes sense. The signs are usually plain once the moss is looked at properly.
Tiles that flake in layers or crumble at the edges have lost their surface for good. Slates that slip in different places after each winter point to nail sickness, where iron nails have corroded and let go. Battens that are soft or black at the ends suggest damp has been reaching them for a long time. A sagging ridge or belly in a slope can mean the structure below has moved. And a roof covered in tingles, the small metal straps used to hold slipped slates, has had a lot of patching.
If several of these appear together, cleaning only delays a bill that is coming. Money spent on a clean first can leave a smart-looking roof that still leaks. A better use of the budget may be a strip and re-cover, using the sound slates again where possible. Our survey will say clearly which one applies, and the written quote is fixed.
How age fits in
Age is a guide, not a verdict. Typical lifespans reported by MyBuilder (2026) run from about 75 to 100 years or more for natural slate, 40 to 60 years for clay plain tiles, and 30 to 50 years for concrete interlocking tiles. Exposure, workmanship and upkeep all move these figures, and none of them is a promise. A well-fixed roof in a sheltered spot can outlast its range, and a badly fixed one can fall short.
That is why a young concrete tile roof with heavy moss is a sensible cleaning job, and a very old one with a worn face is often not. It is also why the fixings and timbers decide more than the tile does. A sound slate on rusted nails is a re-fix job, not a lost cause, since the slates can often be lifted, sorted and fixed back on fresh battens with durable nails.
Re-roofing is not automatically a Building Regulations matter, either. The Planning Portal says approval is not normally needed where less than 25 per cent of the building envelope and less than 50 per cent of the roof is affected, but a larger re-cover can bring in approval and a thermal upgrade. We deal with the paperwork where it applies.
A sensible way to decide
Begin with evidence rather than a guess. Go into the loft and look for daylight, damp and rot. Check the gutters for grit shed by crumbling tiles. Then have the roof surveyed close up, with photographs, to set beside what a sound roof looks like.
If the roof passes, clean it, treat it and watch it, using the guidance on timing a clean to pick the right season. Expect moss to return eventually, and see how long before moss comes back for what that looks like. If the roof only half passes, repairs plus a clean can make sense. If it fails, cleaning is not the answer.
Where cleaning is the answer, soft washing is usually the method we would choose for an ageing covering. We would rather tell you a roof needs replacing than clean something that will not last the winter, and a clean carried out on a failing roof tends to be money you cannot get back.
