Metal roofing is usually aluminum or galvanized steel, although copper is sometimes used. Some metal roofing is composed of individual shingles made to resemble wood shakes, but the most common metal roofing materials are corrugated or standing seam panels. Corrugated metal panels have alternating ridges and valleys; standing seam panels have broad flat sections with vertical seams interposed for strength. Both types of panels are long and wide, typically about 2 feet wide and up to 20 feet long. Some standing seam systems require the use of hidden fasteners, clips to secure the panels, but special screws are the most common type of fastener.
Both corrugated and standing seam panels are installed with the grooves or seams running down the roof. Each type can be installed over existing shingles, on conventional wood roof decking or on special strip sheathing of 1-by-4-inch boards nailed across roof rafters or trusses. Corrugated panels are overlapped at seams and fastened with screws through both panels. Standing seam panels lock together with a tongue-and-groove connection and without screws.
Standing seam panels usually are secured at the edges with special fastening strips screwed to the roof framing with 1-inch long screws spaced 4 inches apart; the bottom or edge of the panels slide into those elements. Corrugated panels typically are screwed in place at the bottom of one roof edge, with the first vertical row of screws one corrugation valley inside the roof.
Special aluminum or galvanized steel screws with plastic or rubber washer caps are used for both corrugated and standing seam panels. Washer tops seal the screw connections from water penetration. Spacing varies with the type of roofing; different manufacturers have different recommendations. Generally, however, corrugated panels are fastened with screws every 18 inches to 2 feet, both across and up the roof.
Standing Seam Spacing
Spacing of screws on standing seam panels can vary with the type of roof decking. One manufacturer recommends 12- to 18-inch screw spacing on 1/2-inch roof decking, but it recommends 24-inch screw spacing on 3/4-inch decking, which has more depth for a screw to hold. Manufacturers of both corrugated and standing seam roofing recommend closer spacing of screws along edge trim and caps or peak caps, which are areas more subject to wind pressure.