Broadway roofs, specifically
Under Fish Hill: why Broadway roofs stay damp and turn green.
Broadway is built from the honey-coloured Cotswold limestone that its wool and cloth merchants quarried from the hills behind the village. It lies on the eastern edge of the Vale of Evesham, right at the foot of the escarpment, and its long, wide High Street runs up from The Green between grass verges, chestnut trees and 16th and 17th-century stone cottages. Most of those cottages are still roofed in traditional Cotswold stone slate or old clay. It's one of the best-known villages in England, and its roofs grow moss as readily as any we look at.
Look up from the High Street and the reason is right in front of you. Fish Hill climbs steeply behind the village to Broadway Tower, which stands at over 1,000 feet, the high point of the northern Cotswolds. On still nights cold air slides down that slope and collects in the village at the bottom, the spring-fed ground along the foot of the hill stays wet, and the escarpment keeps the morning sun off the upper end of the village for longer than on the open vale. Moss and algae want cold, wet and shade, and below Fish Hill they get all three.
The way the village is built adds to it. Broadway's cottages stand shoulder to shoulder along the street, two storeys of stone with steep gables, and long, narrow back gardens behind. Rear pitches spend much of the day in the shadow of the house next door and dry slowly after rain. That's where the heaviest growth tends to be: moss packed into the laps of the stone slate, lichen along the ridge, and the gutters below half full of what has washed off.
Stone slate itself is the third part. Each roof is hundreds of small, porous limestone slates in diminishing courses, with deep overlaps that hold water. High-pressure water would split them and push water up under the laps, and a broken stone slate can only be replaced with reclaimed stone. So on Broadway's heritage roofs the moss comes off by hand, working from a roof ladder that spreads our weight, and the biocide goes on afterwards.
On Snowshill Road, Station Road, Childswickham Road and the newer infill you'll find concrete interlocking tile instead. Those houses sit in the same cold pocket below the hill, so they grow moss too. They get a low-pressure clean or a manual scrape, depending on the tile, then the same biocide.