25/03/2026
When looking at an ageing building, few people cast a critical eye at the state of the roof. The windows are clean, the garden tidy and the interior spotless, but the roof is almost invariably unkempt. Black moulds hide the colour, and moss slowly mats itself in the bonds, so slowly that few take notice.
The encroachment of grime and moss has its own effects: beyond the aesthetic downgrade the fabric of the building is at risk. Doing nothing is ultimately the expensive option. The risk varies with the nature of the roofing material: Thatch and wood shingles are at a more immediate risk of decay. Metals depassivate (lose their corrosion protection) under the acidic releases of lichen and moss. Lead and zinc used in soakers and rainwater disposal will thin down to eventually leak. Clay tiles need to dry between rainfalls to fulfil the promise of durability - if not premature shaling or frost damage may occur. On all material the permanent presence of a bio film is deleterious by increasing capillary creep in laps and interstices. This in turn leads to undetectable moisture ingress.
Generally materials retaining moisture are prone to contamination and combined with winter exposure, porous materials such as concrete tiles will become a good host for moulds such as A. Niger, then algae, followed by moss and lichen. As the bio film becomes established, the moistures are more permanent, and the cycle accelerates. Architects do not seem to consider this in their choice of roof material: This was of no consequence in the days of acidic rain, but cleaner air results in nature reclaiming its place in unforeseen ways.
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There is no easy answer to the return of nature: Excessive moss has to be removed by hand. Mechanical means used on the ground are ill adapted to a pitch roof. Jetting causes irremediable erosion to the surface. As a result the un-wanted host returns with a vengeance, only to hide the uneven, permanent discolorations of the roofing tiles.
After clearing the roof from the bulky growth the next and final step is to spray the area with a dedicated anti algal wash. The right product, will kill the remaining biological life at the surface and, most importantly, in laps and interstices. The dead bio film will soon loosen under diurnal cycles, helped by the wind, rain, heat and frost. Self cleansing is a lengthy affair - months in most cases with latent effects stretching over a period of two or three years. The roof will become cleaner with the change of seasons and reveal its original colour and texture.
There is no known preventative measure of any durability. Clear hydrophobic resins degrade under UV in a matter of months. The same causes - substrate porosity and sun exposure - will have the same effects, at the same rate. This however is slow and a roof treatment is to be regarded as decennial maintenance. Further maintenance will not be as heavy as the first clean, if carried out as or before the surface contamination triggers the condition for accelerated growth.
On the continent, antiseptic roof shampoo has been standard practice for decades. Roofing firms offer it as a standard way of finishing remedial work and as preparation to fitting out solar panels. In Britain the practice is expanding quickly: The process has been adapted to suit the more demanding climate of the British Isles.
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