Evesham roofs, specifically
Inside a loop of the Avon: why Evesham roofs hold on to moss.
Most of old Evesham sits inside a tight horseshoe bend of the Avon, with the river wrapped round three sides of the town centre. Low land beside a river floods — July 2007 brought the town's worst flooding on record after the heaviest rain in 200 years, and in May 1998 the river rose nearly six metres within hours — but even in ordinary years the meadows and the water keep the air around the town moist. Moss, lichen and gloeocapsa algae need exactly that, so roofs here tend to grow more than those on the drier slopes towards Bredon Hill.
Then there's the Vale. Evesham lends its name to the Vale of Evesham, the fertile low ground long given over to market gardens, orchards and asparagus. Soil and air that grow plums and asparagus so well suit moss just as much. North-facing tile stays damp for most of the year, and the villages round about — Badsey, Bretforton, Wickhamford, Offenham — share the same conditions.
In the town centre, the streets running down towards the river — the High Street, Bridge Street, Vine Street and the lanes around Merstow Green and the Bell Tower — have old clay, stone and slate roofs close together. Each roof shades the next, and the lower slopes rarely see the sun. Old tile holds water much longer than new, so a riverside roof that looks dark from the street is usually carrying a dense layer of moss in the laps, and its gutters are already filling with what washes off.
The estates have concrete tile instead. Bengeworth on the east bank, Great and Little Hampton across the river, and the newer homes at Greenhill and Four Pools are largely post-war and modern interlocking concrete. Its rough face holds onto spores, and in the Vale's damp the moss thickens quickly, so we clear the bulk by hand from a tower or roof ladder before applying biocide.
We don't pressure-blast older concrete tile. After a few decades the surface coating has worn thin, and high pressure strips it further: a clean that lasts a season at the cost of years of the tile's life. Removing the moss by hand and treating with biocide is kinder to the roof and lasts much longer.