Painswick roofs, specifically
A steep, wooded valley side: why Painswick roofs hold so much moss.
Painswick is often called the Queen of the Cotswolds. It's a cloth village, built by clothiers and weavers from the local honey-coloured limestone, and more than 380 of its buildings are listed — 17th and 18th-century cottages and clothiers' houses, nearly all under traditional Cotswold stone slate. Few villages in England have kept that much of their original stone, and that's the starting point for how we clean it.
The setting does the rest. Painswick is built on the steep western side of the Cotswolds above the valley of the Painswick stream, one of the Five Valleys that run down toward Stroud, with Painswick Beacon rising to over 280 metres behind. Lanes like Tibbiwell drop sharply downhill, houses are stacked one above another on the slope, and many back onto gardens that fall away toward the stream. Damp air rises out of the valley, and a roof on the shady side of a steep street can go most of the winter without properly drying.
Trees add to it. Painswick is a green village — the clipped yews in St Mary's churchyard, gardens full of mature trees, and woodland toward Cranham and the Beacon. Overhanging branches keep roofs in shade, drip on them long after the rain has stopped and drop leaf into valleys and gutters. On a stone-slate roof under trees we usually find thick moss in the laps, lichen on the ridge and gutters part-filled with leaf and moss.
Stone slate is porous and laid in graded courses with deep laps, so it holds water and can't be pressure washed: high pressure splits it and forces water underneath, and a broken slate can only be replaced with reclaimed stone. On Painswick's heritage roofs the moss is removed by hand from a roof ladder, then the biocide goes on. Nothing else is applied.
Newer houses on the edges of the village and along the approach roads tend to have concrete interlocking tile, and the hamlets have a mix of everything. They grow moss on this hillside too. Concrete tile gets a low-pressure clean or a hand scrape, whichever suits it, followed by the same biocide.