Note: Do not rely on this information. It is very old.
House-leek (Sempevvivum tectorum), a plant belonging to the Crassulaceae, native to the Alps, but commonly seen on the roofs of out-houses in England and elsewhere, being supposed to be a protection against lightning, whence its German name "Donnerkraut." It has a rosette of fleshy spinously-pointed radical leaves, and multiplies itself by closely-grouped offsets. Its flower-stalk is pink and fleshy, and bears a flat reflexed cyme of pink flowers, each with about a dozen petals. The leaves contain malic acid, and are a popular remedy for bruises, swellings, stings, burns, corns, freckles, etc