Stow-on-the-Wold roofs, specifically
800 feet up on an open wold: why Stow roofs rarely dry out.
There's an old couplet about this town: "Stow-on-the-Wold, where the wind blows cold." It's etched on the wall of St Edward's Hall in the Market Square, and it tells you everything you need to know about why roofs up here struggle. Stow is the highest town in the Cotswolds, standing 800 feet up on the top of an open hill where eight roads meet, with the Roman Fosse Way running through. Down in the sheltered valleys of Bourton or the Swells the weather passes over; up on the wold it sits. Wind, cloud and damp linger across the rooftops far longer than they do lower down, and that constant moisture is the single biggest reason Stow roofs carry such heavy growth.
The layout of the town keeps that damp in. Stow is built around a large market square with tall stone buildings on every side, and the narrow passages locals call "tures" — made narrow and winding to funnel sheep into the market — run off it between high walls. Rear pitches along Sheep Street and Digbeth Street and the shaded side of the Square see little direct sun in winter. On those roofs we typically find moss packed into the stone, lichen spread across the slates and gutters full of what has washed down.
Wind is the other half of it. On an open hilltop there's nothing to break the weather, and rain gets driven up under the laps and into the joints of the slate. The town is built from oolitic limestone, with Cotswold stone slate on many of the older roofs from its wool-trade days, and that porous stone takes on whatever water the wind pushes into it.
That's why method matters. Pressure would split the slates and force more water under them, and a broken stone slate has to be replaced with reclaimed stone. So in Stow the moss is taken off by hand from a roof ladder that spreads the load, and the roof is then treated with biocide.
Further out, toward the A429 and on the newer housing at the edges of town, roofs are mostly concrete interlocking tile, with some Welsh slate and plain tile on later extensions. They're just as exposed. Depending on the tile they get a low-pressure clean or a hand scrape, then the biocide.