A roof protects a house from damage by the elements, especially water. Roofs are designed to shed water; the parts comprising a roof combine to direct water off the roof and away from the house.

Anatomy of a roof

A typical roof (see illustration below) begins with a framework of rafters which supports a roof deck (sometimes called a subroof) consisting of sheathing and underlayment. The roof deck, in turn, provides a nailing base for the roof surface material.

The roof deck. Though the type of roof deck used can vary depending on the roof surface material, most decks have both sheathing and underlayment.

Sheathing, the material that provides the nailing base for the roof surface material, ranges from solid plywood to fiberboard to open sheathing (used with wood shakes).

Sandwiched between the sheathing and the surface material is the underlayment, usually roofing felt. A heavy, fibrous black paper saturated with asphalt, roofing felt is thick enough to resist water penetration from outside, yet thin enough to allow moisture from inside the attic to escape.

The roof surface. The material on the roof must be able to withstand wind, rain, snow, hail, and sun. A wide variety of roof surface materials is available—the different types are discussed below and on pages 31-33.

The surface of the roof is often broken by angles and protrusions, all of which require weatherproofing— usually provided by the flashing. Made from malleable metal or plastic, flashing appears as the drip edge along the eaves and rakes of a roof, the collars around ventilation and plumbing pipes, the valleys between two roof planes, and the "steps" along a chimney or dormer. Less obvious flashing also protects other breaks in the roof, such as around some solar panels and skylights. At the roof edges, metal, wood, or vinyl gutters catch water runoff and channel it to the ground via the downspouts, which direct water away from the house and into the soil.

Types of roofing materials

Roofing varies widely in size, shape, and material. Traditional sloping roofs are usually covered with overlapping layers of asphalt shingles, wood shingles or shakes, or tile, though you can find such roofs covered with slate, aluminum, or galvanized steel.

Flat or low-sloping roofs are most often surfaced with alternating layers of roofing felt and asphalt, with a layer of gravel on top. These are known as built-up, or tar-and-gravel, roofs. Some flat roofs are covered with insulating Polyurethane foam.