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If you understand the structure of your roof, you can diagnose problems, initiate improvements and make repairs. With that in mind, here’s a glimpse at how roofs are built and an overview of some of the most important roofing terms.
A typical roof begins with a framework of rafters that supports a roof deck (sometimes called a subroof) consisting of sheathing and underlayment.
Sheathing, the material that provides the nailing base for the roof surface material, ranges from solid plywood to oriented strand board to open sheathing (used with wood shingles).
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 material on the roof must be able to resist wind, snow, rain, hail and sun. Many varieties of roof surface materials are available. Some may be used on nearly flat roofs, while others rely upon the slope of a pitched roof to shed water from the surface.
When dealing with a roof, you'll discover several words that may not be familiar. On pitched roofs, materials are applied in horizontal layers, called courses, which overlap one another from the eaves to the ridge. The portion of the material exposed to the weather is called the exposure, and the edge that is down-roof is called the butt.
Asphalt shingles, the most common roofing surface, are sometimes called composition or comp shingles and are divided into sections called tabs.
Typically, a roof's surface is broken by angles and protrusions, such as vent pipes, chimneys and dormers. All of these require a weathertight seal, 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.