Stroud roofs, specifically
Why Stroud roofs go green faster than the open plain.
Stroud sits where five steep, wooded valleys meet — the Frome, the Slad, the Painswick, the Nailsworth and the Golden Valley running up towards Chalford. That geography is the whole story for a roof. The valley sides funnel and hold damp air, so on autumn and winter mornings mist pools in the bottoms for hours, humidity stays high, and the streams and woodland keep the air moving slowly. The town gets a steady, ordinary amount of rain by English standards — around 800mm a year, with October the wettest month — but it's the trapped damp, not the rainfall total, that grows moss. Properties down in the valley bottoms and on the shaded north-facing slopes barely dry out between October and April.
You see it most on the old cloth-town stock. Stroud grew rich on wool and the valleys once held the best part of 150 working mills, and the housing that came with that trade — three-storey weavers' terraces stacked up the hillsides at Rodborough and Chalford, clothiers' houses, ranks of stone cottages on the slopes above the centre — is largely Cotswold stone. Stone tile is porous and textured, which gives moss spores something to grip, so a roof that looks merely "a bit green" from the lane is often carrying a thick mat of moss and lichen in the laps by the time anyone calls us. The stone underneath is usually sound; the moss has just been holding water against it for years.
The newer stock tells the same story by a different route. The post-war and modern estates around Cainscross, Cashes Green, Paganhill and out towards Stonehouse are mostly concrete interlocking tile, and those green up heavily too because the textured surface holds moisture in this valley air. Whether it's a listed weaver's cottage at Uplands or a 1970s semi at Cainscross, the approach is the same: lift the bulk moss off by hand, then treat the cause with biocide. The same damp-valley picture carries down the A46 to Nailsworth and across to Minchinhampton and Woodchester.