THE MOST DIFFICULT KINDS of roof to walk on are those on buildings put up in the past hundred years. These roofs are usually tiled, either with peg tiles or pantiles, and they slope steeply. It is as if architects like Lutyens and Mackintosh and Voysey only realized in the twentieth century that it rains a lot here and steep slopes on roofs provide better drainage. The best kinds are shallow and made of slates, preferably with a stone coping or a low wall at the edge, designed to hide from view the fact that the roof slopes at all. The more ornamentation on a roof the better, the more gables, belvederes, single chimney stacks and mansarding, the easier it is to climb. Single detached houses are useless to the serious climber. Standing alone as they do, no matter how shallow their roofs, no matter how many footholds the tops of their dormer windows, their pediments and parapet rails provide, they remain islands. The open air, the gap between them and the house next door, which may be several or many feet, is the sea that divides them from the continent. Climbers need terraces, each house joined to the one next door and preferably not divided from it at roof level by a stack that is a barrier to their progress, a high wall spanning the breadth of the roof and carrying a dozen closely set chimney pots or cowls.

The experienced climber despises television antennas and dishes as aids to balance keeping. He treads fast and light-footed on tile and coping and window ledge. He understands that the first big mistake the climber makes is to dislodge a slate and set it clattering down to ricochet off the coping and crash on the ground. He holds on only to that which is firm and steady, avoiding hundred-year-old chimney pots, drainpipes, and flimsy plaster moldings. The best climbers are light in weight and supple.

Most roof sounds pass unheard by the householder who knows it’s impossible anyone is walking in the sky up above her head. What she hears must be the wind, the rustling and rasping of tree branches. Or a cat may be up there. She has seen a cat on these roofs. At the veterinary practice in St. John’s Wood they tell her that most of the cats they treat are brought in with broken legs. The cat on the roof or balcony sees a butterfly and leaps in pursuit of it into the shining void.