The type and condition of an old roof covering may make its removal desirable, but many types of roofing can be applied directly over old wood shingles or other coverings. One of the advantages of this method is that it provides insulation, avoids removing the old material, and reduces the amount of cleaning necessary after completion of the work. In case of rain during the period of reroofing, the interior of the house is also protected.
The deterioration of nails used for roofing purposes is the principal cause of many roof failures. Nails often rust and break off, allowing the roof covering to become loose long before the material itself shows signs of wear. The use of rust-resistant roofing nails is, therefore, an important factor in preserving roof surfaces. Large-headed zinc-coated roofing nails are recommended for asphalt-prepared shingles and roll roofing.
Very few nails are used for build-up roofing, but the few that are necessary should be zinc-coated. Copper or copper-clad nails should be used for slate and file roofs, and galvanized five-penny or six-penny nails are used for fastening asbestos-cement shingles.
Nails used to fasten metal roofs should be of the same or similar metal as the roofing material; that is, copper nails for copper roofs and large-headed zinc-coated nails for galvanized roofs and terneplate. For wood shingles, blued-steel nails may be used, but hot-dipped zinc-coated nails are preferable. Roofing nails come in different lengths and should be selected according to the thickness to be penetrated.