Problem: Slipping shingles. Common causes include using too few fasteners, overdriving and underdriving the fasteners, and driving fasteners (especially staples) at odd angles.
Solution: Follow the shingle manufacturer's fastening instructions as printed on the bundle wrappers. Don't trade accuracy for speed.
Problem: Excessive shingle overhang. Many installers believe that increasing the overhang on the first course of shingles helps direct water into the gutter and is a substitute for a metal drip edge. Eventually, however, the shingles crease under their own weight and break off, exposing the edge of the sheathing beneath.
Solution: Shingles should overhang ½ inch to ¾ inch beyond the metal drip, according to recommendations from the Asphalt Roof Manufacturers Association.
Problem: Felt underlayment incorrectly lapped under (instead of over) the metal drip edge.
Solution: Lap the felt underlayment over the drip edge. Any water that may get under the shingles — for instance, from wind-driven rain — will be carried by the felt underlayment over the top of the drip edge.
Problem: No drip edge. If asphalt shingles are installed without a drip edge, it's only a matter of time before the plywood sheathing and fascia begin to rot. Water dripping off the edges of the roofing is drawn by capillary action to the exposed edges of the plywood sheathing.
Solution: Install a metal drip edge at rakes and eaves. The main purpose of a metal drip edge is to interrupt the wicking of water to the sheathing edges.
This article originally appeared in the October 1999 issue of our sister publication, THE JOURNAL OF LIGHT CONSTRUCTION.