Droitwich Spa roofs, specifically
Why Droitwich Spa roofs green up faster than the villages around it.
Droitwich Spa sits low in the valley of the River Salwarpe, which runs straight through the middle of the town and out through Vines Park. It's the same low ground that makes the town flood — in July 2007 the High Street went under several feet of water and dozens of homes and businesses were wrecked — and it's the same low ground that keeps roofs damp. Even in a dry summer that flat, wet valley floor holds moisture in the air, and moisture is exactly what moss, lichen and gloeocapsa algae feed on. Add the restored Barge and Junction canals threading through the town and meeting the river at Vines Park, and you've got a microclimate that greens a roof noticeably faster than an identical house up on the drier ridges towards Dodderhill, Hanbury or the open ground out past Westwood.
What makes Droitwich unusual is how split the housing stock is. On one side you've got the Tudor and Georgian core — the timber-framed black-and-white frontages on Friar Street, the old shops on the High Street, the streets around St Andrew's and Queen Street — much of it sitting on ground that has subsided over centuries of brine and salt extraction. The famous higgledy-piggledy lean of the old High Street buildings, and the demolition of the upper stage of St Andrew's church tower back in the 1920s, are both down to that subsidence. Roofs on those buildings are old clay, stone and slate, often running at odd angles into shared valleys, and they cannot take pressure or heavy-handed treatment.
On the other side you've got the new estates. The town has grown hard in the last decade or two — phase after phase of new housing at Copcut on the south-west edge, and the Yew Tree Hill development to the south, consented for up to 765 homes — alongside the established residential districts of Westacre, Chawson, Witton and Boycott. These are almost entirely modern interlocking concrete tile: tougher than the old town's clay, but textured, and in this damp valley they mat up with moss and streak with black algae far sooner than people expect. Owners of houses that went on barely ten or fifteen years ago are often surprised the roof is already greening. In this valley it's normal.
So Droitwich gives us two very different jobs under one postcode. The Tudor centre needs the lightest possible touch — hand-scraping, biocide kept off old mortar and lead, extra care on anything that has shifted with the subsidence. The estates need the bulk moss lifted off by hand and then a biocide treatment to stop it coming straight back. What ties the two together is the cause: the same low, damp Salwarpe-valley air greens both. And on both we won't blast a roof with high pressure to save time — on fragile old clay it cracks tiles and forces water in, and on a fifteen-year-old concrete tile it strips the surface coating and takes years off the roof to buy you a single clean season.