My roof deck has boards, is plywood sheathing necessary?
The short answer; yes. A boarded roof deck is often very aged and in poor condition. They have been subject to many years of weathering, expansion and contraction, extreme temperatures, and often poor ventilation. They split when nailing into them, affecting the integrity of your roof and any warranties involved. Years of warping can make them bumpy and uneven with deep cracks and holes.
A fresh coat of plywood ensures all the nails have the backing they need to be fully secured to your roof deck. It also provides a clean and flat working surface which helps with the installation process and, most importantly, the overall look of your roof.
Also, many contractors won’t warranty a roof without plywood sheathing since a boarded surface is unreliable in the long-term and in extreme weather conditions. – Army Roofing, www.armyroofing.com