Your roof protects your home, and the people inside, from the elements, especially water and sun.  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 begins with a framework of rafters that supports a roof deck consisting of sheathing and underlayment.  The roof deck, in turn, provides a nailing base for the roof surface material.

The roof deck.  Through 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.  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 waterproof enough to resist water penetration from outside, yet porous 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 and protect the house from these elements.  A wide variety of roof surface materials is available.  The surface of the roof is often broken by angles and protrusions, all of which require weatherproofing, usually provided by flashing.  Made from malleable metal or plastic, flashing appears as the drip edge along the eaves of a roof, the collars around ventilation and plumbing pipes, the valleys between two roof planes, and the steps along a chimney.  Less obvious flashing also protects other breaks in the roof, such as skylights.  At the roof edges, 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 or tile,  though you can also use slate, aluminum, or galvanized steel.  Flat or low-sloping roofs in the past were  surfaced with alternating layers of roofing felt and asphalt or tar, with a layer of gravel on top.  These are know as builtup or tar and gravel roofs.  Flat roofs are more commonly covered with insulating polyurethane foam or a rubberlike modified bitimen membrane know as EPDM or TPO.