Pitched Roofs

Pitched roofs shed water by slope and overlap rather than by a sealed membrane. Each course of tiles or slates laps the one below, so water runs from tile to tile and off the eaves without ever needing a continuous waterproof layer. The steeper the pitch, the faster water clears and the less time it has to find a way in.

Common shapes include the simple gable (two slopes meeting at a ridge), the hipped roof (slopes on all four sides) and more complex forms with valleys where planes meet. Valleys, ridges and the junctions around chimneys are the details that need the most care, because that is where water concentrates.

A typical modern pitched roof is a layered system. A timber frame of rafters — often braced by purlins and struts, or formed as prefabricated trusses — carries the load. Over the rafters sits a breathable underlay (or older bitumen felt) as a secondary barrier, then horizontal timber battens, and finally the tiles or slates nailed or clipped to the battens.

• Rafters or trusses — the structural timbers that give the roof its shape and carry the load down to the walls.
• Underlay (felt) — a secondary barrier beneath the covering that catches any water getting past the tiles.
• Battens — timber strips the tiles hook or nail onto, setting the overlap.
• Tiles or slates — the weatherproof outer covering, chosen to suit the pitch and exposure.
• Flashing — lead or mortar detailing that seals junctions at chimneys, walls and valleys.