Acceptable Roof Decks according to GAF-Elk is defined as using a smooth surface decking, flat even decks are required. If you use decking that is buckled or pitted it can telegraph through a roof system. Sufficient nail holding capacity is needed and is a very important aspect of a deck in its ability to securely hold the roofing nails in place. Roof decks should be dimensionally stable; these approved decks can withstand weather expansion and contraction. Roof decks must be dry. Wood and plywood stored outdoors prior to installing will no doubt absorb and hold in moisture. Decks that are subjected to rains and/or dews just before roofing may be filled with excess moisture and need to be dried out before using. This moisture if not dried out can cause deck movement and shingle blistering.
Wood board roof decking is acceptable when it is at a maximum at 6″ wide for wood boards. Wood boards that are wider than 6″ are subject to movement and splitting. Movement can damage the shingles. Splitting reduces the nail holding capacity of deck boards. 1″ thick, thinner boards are not dimensionally strong enough to hold roofing loads. 1/8″ separation at rafter seams to allow the roof decks for expansion and contraction. Deck boards 1/8″ maximum spacing between consecutive boards up the roof to allow for expansion and contraction.
Plywood and oriented strand roof board decking details are to use plywood or OSB recommended by the APA. APA is The Engineered Wood Association, which sets standards for acceptable plywood and OSB (oriented strand board). APA is printed on accepted decking for roofs. Minimum of 3/8″ thick to meet the fire rating requirements (UL). The thicker decks make for stronger and higher load bearing decks. Keeping 1/8″ separation at rafters, plywood or OSB jammed together at rafter seams will make the plywood buckle. Concerning OSB and local building codes, there are local and state building codes that do not allow the use of OSB as roof decking. You should always adhere and abide by the local codes. Treated or fireproofed plywood and lumber: Wood treated with formaldehyde and other chemicals have the potential to damage roofing materials and metal components.
Insulated roof decks are not generally considered for shingles. Shingles must not be secured directly to any insulation or insulated deck systems unless authorized by GAF-Elf Contractor Services. These decks are prone to expansion and contraction which is too much movement. It can also be difficult to achieve proper ventilation with some of these decks.
Roof Deck systems with a GAF-Elk approval, as long as they meet your local code requirements are as follows: Loadmaster Shingle Decks, Tech Shield or equivalent Radiant Barrier Decking Systems with vapor permeable, perforated foil backing, Homasote Co. Deck, 2″ minimum only, Thermasote from the Homasote Company, Span Rock Gypsum Plank from USG 2″ minimum, Hunter Vented Nail Base, Vent-Top Thermacal from the Cornell Corp., Atlas.