Surface Selection: The preparation of the surface onto which copper gutter applications will be applied depends in part on the substrate selected and the copper gutter applications. A number of considerations, however, must always be taken into account.
In selecting the substrate, a key consideration is the method of attachment of the copper. All applications that rely on nails or screws to attach the copper or cleats to the underlying structure require a nailable deck, nailing strips within the deck, or wood blocking at specific locations. Such applications include standing seam roofs; batten seam roofs, flat seam roofs, continuous edge strips and cleats, and flashings around roof penetrations.
Regardless of the attachment method used, the structural integrity of the substrate should not be compromised. It must be able hold the roof under sustained design wind conditions, as well as to conform to all other required codes and standards.
The most common substrate for copper is wood, usually 1/2" to 3/4" plywood. Lumber should be kiln-dried and laid with all joints true and even to provide a smooth surface. It is recommended that wood be allowed to weather for a few days after installation. During this period it should be protected from rain, allowing it to conform to atmospheric temperature and moisture level, while settling into place.
There have been many recent developments in fire retardant treated (FRT) plywood and lumber. Most of these products use wood or plywood that is pressure-impregnated with chemical salts in water solution to inhibit combustion. Many of these salts are corrosive to copper, as well as other metals and materials. If leaching of theses salts brings them into contact with the copper, corrosion will occur. This is particularly likely in areas with high humidity, if condensation occurs, or if water is introduced during construction or at a later time. Any areas where salt laden moisture can collect then evaporate, thereby increasing the concentrations of salts, will accelerate the corrosion process. For a complete and updated report on Fire Rated Plywood and Corrosion contact CDA.
Other materials used as substrates for copper include: concrete, brick, masonry units, terra cotta, and stucco. The guidelines discussed above apply to these materials as well. Smooth, dry surfaces, compatibility with copper, and provision for fasteners are all required for an acceptable substrate.