Most houses have a sloping (or 'pitched') roof, made from a triangular-shaped wooden frame that's covered with tiles or slates. The weight of the tiles is supported by pairs of sloping rafters, whose heads meet at a central ridge board and whose lower ends are fixed to timber wall plates on the external walls of the house.
Tiles are made from baked clay or concrete, and can either be flat or moulded into various profiles. Slates are smooth flat pieces that are cut from rock (natural slate) or man-made. Pitched roofs can be built in a number of different ways, depending on the size and design of the house.
The simplest pitched roof (generally known as a 'close-couple single' roof) has a horizontal joist (or 'tie'). This is fixed to the lower ends of each pair of rafters and to the wall plates to stop the weight of the roof pushing the external walls out. The ceiling plaster for the room below is usually attached to these joists.
A roof that spans a large area may have horizontal beams (called 'purlins') fixed half-way down the rafters, in addition to the joists or ties across their lower ends. This type of roof is called a 'double' or 'purlin' roof. A very large roof can have several purlins fitted at intervals down the rafters.
Diagonal struts give extra support to the purlins. They're fixed at one end to the purlin and at the other to the wall plate of a central, load-bearing partition wall. Horizontal timbers (called 'binders') and vertical ones (called 'hangers') can also be fitted to support the joists.
This type of roof is built from individual prefabricated trusses. Each of these is a triangle made from a pair of rafters and a joist braced with struts. As these trusses are linked by horizontal and diagonal bracing timbers, very few trussed roofs can be converted into living space - and you shouldn't cut into one without getting professional advice first.
The trusses are usually fixed to the walls with metal straps. Because trussed rafters are relatively light, they allow for a fairly wide span and don't need a ridge board or load-bearing partition wall. Older houses with this type of roof might also have purlins that add support to the rafters.
Rear extensions and outbuildings often have a timber-framed flat roof covered with roofing felt. But in fact, no roof can be completely flat - as rainwater would just settle and cause it to collapse. So timbers in the roof frame (called 'firring strips') taper slightly to let the roof drain. Roofing boards are laid on these strips, with three layers of roofing felt bonded together with bitumen then placed on top of the boards.
The joint between a flat roof and an adjacent wall is sealed with metal flashing. Vents at the eaves are essential to let air circulate and moisture escape, and there may also others through the roof alongside the junction with the house wall. Insulation is vital where the roof is over a living space, and is provided either by blocks of insulating material between the firrings or sheet insulation laid on the ceiling.
Covering a pitched roof
Before the tiles or slates are fixed in place, a roof is lined with 'sarking' - which can be made from bituminous roofing felt or another breathable material. To make sure it's waterproof, the sarking is laid horizontally in strips with each new section overlapping the one below it, working from the bottom of the slope up to the ridge. Sawn softwood battens are nailed to the rafters over the sarking. Tiles and slates are then laid across the roof in rows or courses, with the bottom of each course overlapping the one below. As with sarking, they're laid starting at the eaves and working up the slope to the ridge.
Tiles have a nib that hooks over the battens and holds them in place, although some are fixed with nails or clips as well. Slates are held in place with nails alone or nails and copper rivets. The size and type of tiles and slates and the pitch of the roof all determine the fixings that are employed. Curved tiles are used for capping the ridge (or the hips of a hipped-end roof) to keep the junction of the two slopes weatherproof. Where a tile or slate meets a wall or chimney, a strip of lead, zinc or mortar (known as 'flashing') covers the join to make it weatherproof. Flashing, or specially formed valley tiles, are fitted along the valley where two roofs meet.
Gable and hipped-end roofs are the two most common pitched roof shapes. With a gable roof, the end walls of the house are built right up to the ridge - following the pitch of the roof and forming a triangular gable end. Hipped-end roofs slope down at the end as well as the sides, making theirs a more elaborate construction. They need additional timbers known as hip rafters, jack rafters, crown rafters and cripple rafters. A large house could well have both types of roof, and the junctions where they meet form 'valleys'.
The eaves are the bottom edges of the roof where the lower ends of the rafters join the external walls of the house. The ends of the rafters might be cut level with the walls, in which case they're called 'flush eaves'. Fascia boards are nailed across the ends of the rafters to protect them from the elements, and the guttering is fixed to the fascia boards.
Alternatively, the ends of the rafters might project beyond the walls, with the guttering fixed to them. These are known as 'open eaves'. Projecting rafters are sometimes boxed in by a fascia board and a soffit board, in which case they're called 'closed eaves'.
The sloping edges of a gable roof are called 'verges' and, as with the eaves, they can either be level with the exterior walls or extend beyond them. If the verges are level with the walls, the last rafters will be on the inside of the walls and the gap between the roof covering and the brickwork will be sealed with mortar. Where the verges extend beyond the walls, an outer rafter is fitted beyond each wall to cover the ends of the battens and wall plate. Long timbers (called 'barge boards') are fitted to the ends of these rafters, and the underside might be enclosed with soffit boards.
It's essential that air can circulate through a roof space so condensation can't build up and cause the timbers to rot. There may be vents at the eaves and ridge, as well as breathable roofing felt and ventilation tiles at the upper edge of the roof slope.