Understanding Your Roof
A well-built roof seals out water and directs it to gutters and downspouts, which carry the water away from the house. To perform well, underlayment, flashings, and the roofing material must be installed correctly so water cannot seep under and damage the sheathing.
A roof also must provide ventilation so the attic does not overheat in the summer. Inadequate ventilation can cause moisture buildup in the attic, which will likely compromise the insulation. It can also cause ice dams, which can damage the roofing, sheathing, and framing.
Roof rafters and trusses rest on the top plates of the wall framing. If your roof has rafters, they probably meet at a ridge board (or beam), a board that runs along the peak. A roof built with trusses usually has blocking pieces placed between trusses at the ridge. Trusses or rafter tails usually extend down past the house wall and are cut plumb at their ends. A fascia board is often attached to the rafter ends, and rain gutters are installed onto the fascia. The overhanging roof edge is called the eave; it is often covered on the underside with soffit boards that fit up against the house siding.
Sheathing covers the rafters or trusses. In an older home sheathing may be made of 1X boards. In newer homes sheathing is usually made of plywood or OSB (oriented strand board). Roofing felt, often called tar paper, provides the underlayment for the roof. The felt is usually stapled down, but sometimes roofing nails or specialty nails are used. Felt is necessary to provide a vapor barrier that protects the sheathing from condensation. At the lower parts of the roof near the eaves, a self-adhesive membrane is usually applied instead of felt to provide added protection against ice dams.
Flashings are just as important as the roofing itself for sealing out rain and snow. They must be installed correctly so water cannot seep under them. Drip-edge flashing is designed for use at eaves and rakes (the ends of the roof). Usually the eave flashing is installed, then the underlayment, then the rake flashing. At a valley, wide flashing is installed over two layers of underlayment and under the roofing. Plumbing vents have their own rubber flashing, which fits under the shingles at the top half and over the shingles at the bottom half. At the chimney there is a complicated arrangement of step flashing pieces, a cricket, and counterflashings, which must be correctly installed and sealed with roofing cement to keep water out of the structure.
Finally the roofing itself covers the underlayment and most of the flashings. Whichever type of roofing you apply, it must be installed so every piece overlaps the next lower piece, allowing water to flow down. Roofing nails or staples are almost all covered by another piece of roofing above. Where they are exposed they should be protected with roofing cement.