Metal Roofing
In the U.S., metal roofs generally date from the mid-19th century, when galvanized zinc and tin-plated iron became widely available. The most common type is probably the standing-seam roof, composed of strips of sheet metal crimped together in a thin, vertical seam that projects about 1" from the roof. Metal shingles have been available since the 1870s, although historic patterns now are scarce. (See Resources, p. 87.)
Terne, galvanized steel, and copper are all used for standing seam and other metal roofs. While copper is the most expensive of the three, it requires no painting or maintenance. Both galvanized steel and terne must be kept painted or they will eventually rust. That said, even a heavily rusted or weathered metal roof can be restored with protective coating systems and elastomeric membranes like those made by Acrymax (see Resources, p. 87). A more recent option similar in appearance to galvanized steel but offering a longer lifespan is Galvalume, steel with a coating of aluminum, zinc, and silicon. Many roofing metals are now offered with coatings ranging from factory-applied paint in a choice of dozens of colors, to stone coatings and textured finishes intended for the new home market.
Before beginning, remove all old roofing and nails. Traditionally, metal roofs were installed over skip sheathing (see p. 42), but closed decking or solid plywood decking works just as well. Roofing felt, tar paper, or other underlayments are not necessary and could cause trouble: any material in contact with the metal that can leach acidity may damage the roof.
As with most roofing, it’s important to install edging on the perimeter of the roof. With galvanized steel or terne, use a drip edge; with copper, a copper locking strip. Both types of edging have one side that juts up along the edge of the roof and another side that extends several inches onto the decking, where it will be covered by the metal roofing.
The edging supports the roof overhang (usually 1" or less on a standing-seam roof) and helps prevent water from infiltrating underneath the metal. The edging should be nailed along the bottom of the roof, and at gable ends and edges.
Standing-seam roofs are composed of metal “pans” with edges bent up 1" on one side, and 1 ½" on the other. As installation progresses, the pans are crimped together to make a watertight seam. The first and last pans—known as edge pans—have one side that’s bent down 1". When one of these edge pans meets a wall along its side, it can be bent up about 6" to form integral flashing. It probably goes without saying, but use copper nails on copper roofs and galvanized steel nails for steel and terne. Galvalume specifications vary from stainless steel to aluminum–zinc alloy and other types of nails, depending on the anticipated exposure.