Pershore roofs, specifically
Why Pershore roofs green up faster than the higher ground around them.
Pershore sits low on the River Avon, on the flat valley floor where the river loops past the town before it heads on to meet the Severn at Tewkesbury. It's a beautiful spot — the medieval Old Bridge, the abbey, the long Georgian frontages — but the same low, damp geography that gives the town its character is exactly what greens up its roofs. The Bridge Street and Wick end of town floods often enough to carry its own Environment Agency warning zone, and even in a dry summer the valley-floor air sits heavy with moisture. Moisture is what moss, lichen and gloeocapsa algae feed on, so a roof here will carry noticeably more growth than an identical house a few miles up onto the drier ground out towards Bredon Hill or the Cotswold edge.
You see it most clearly in the historic core. Pershore is one of only 51 places the Council for British Archaeology named a "Gem Town" in 1964, and the long curve of Bridge Street, Broad Street and the High Street is lined with tall Georgian frontages standing shoulder to shoulder. They shade each other and the streets behind them for much of the day, very little direct sun reaches the lower pitches, and the original Welsh slate and hand-made clay tile holds damp far longer than modern materials. By the time someone calls us, a roof that reads as black from the street is usually a thick mat of moss sitting in the slate laps, with rust weeping off old lead and valley metal.
Pershore has another feature most Worcestershire towns don't: fruit. This is plum country — the Pershore Plum, the Yellow Egg and the Emblem all came from here, and the festival each August still celebrates them — and the gardens and orchards mean a lot of roofs sit under overhanging trees. Leaf litter and the constant drip of shade keep those pitches damp and feed the moss, so the north-facing, tree-shaded roof at the bottom of a Pershore garden is almost always the one that greens up first.
Out on the newer estates it's a different roof but the same problem. The post-war and modern housing climbing Allesborough Hill, around Three Springs Road and along Newlands is mostly concrete interlocking tile. Those tiles are textured, which gives spores something to grip, so in this humid valley they mat up heavily — we lift the bulk off by hand from a tower or roof ladder before the biocide goes on. Whether it's a Georgian town house or a 1990s semi, the cause is the same Avon-valley damp, and so is the fix.
One thing worth saying plainly: we don't pressure-blast old roofs to save time. The surface coating on decades-old concrete tile has already thinned with age, and on the heritage Welsh slate and clay of the conservation area high pressure simply cracks and strips the surface — buying you one clean season at the cost of years off the roof. Lifting the moss by hand and then treating with biocide is gentler on the tile and lasts far longer, which matters when you're looking at a roof that still has plenty of life left in it.