In replacing a roof, make certain that all three layers have at least minimum standard materials applied and installed correctly on a sound structural system of rafters and trusses. The numerator of a span number represents allowable spans for roofs. Sheathing should be the same material as the rest of the roof and, in all cases, the same thickness of material when repairing only an area of roof.
Licensed builders are preferred for roof repairs.
Adequate materials may be in short supply at a given moment. Substitution should be made only after checking with the nearest code inspector, architect, or engineer. Rather than accepting less than minimum standards of material, it would be better to tolerate patching until minimum materials and procedures are available. A good repair job is required now to keep from having trouble at a future date and to avoid depreciating the value of your property.