Roof Decking in Greater Vancouver, B.C.

The roof deck is an integral part of any roofing project designed to serve two major functions on a home.  It is a structural component built to transfer the weight of live and dead loads to the supporting members of your home and to provide a suitable substrate for the roofing system to be installed. Live loads typically include snow, rain, ice, workers, tools and equipment while dead loads may include such things as the decking, roofing system, skylights and solar panels.

 

Most residential home’s in greater Vancouver, with a sloped roofing system (typically 4/12 pitch or greater) will have a wood roof deck. Plywood is the preferred wood roof deck as it provides a durable, smooth surface for the Roofing Products installed. Other typical wood roof decks found on residential buildings include board decks (ship-lap boards, or tongue and groove boards), and oriented strand boards.

Board decks can propose problems in re-roofing your home, predominantly with ship-lap decks as the board’s cup and twist over time which will create deflections in the deck. These deflections cause buckles to develop in the new roofing system that may create leakage and void manufacturer warranties. Expansion and contraction of boards can create movement in the roofing system and over time, twisting in the boards can loosen the fasteners holding the boards to the rafters and allow for movement in the deck. These movements in the deck can as well create buckles in the new roofing system. Another problem when installing some asphalt fiberglass products to a board deck is that there is a specific area where the shingles must be secured (nail zone) and it is likely that many times during the installation the nail zone will occur over a space between boards forcing the installer to place nails outside of the specified nail zone. Installing products outside of manufacturer guidelines can void warranties. For these reasons, we highly recommended when re-roofing your home that any board decks are covered with a minimum 3/8th plywood.

Oriented Strand Board (OSB) decks as well can create problems. Moisture and condensation quickly penetrate these decks and degenerate the integrity of the product. Buildings that have not been ventilated sufficiently or that have had leakage will in many cases be found to have spongy and weak sheeting if OSB has been used. In many cases, particularly in wet climates like greater Vancouver, the decking will have to be replaced with plywood.

Cedar roofs typically have a strapped roof deck consisting of boards installed at intervals that accommodate the nailing exposure of the product being installed. An example could be 1*4 lumber installed at 7 ½” on center to accommodate a 7 ½ exposure of 18” shakes. This would leave spaces of approximately 4” between boards. Many older homes in greater Vancouver will have 1*6 or even 1*8 installed. When converting your roofing system to accommodate asphalt shingles, a minimum 3/8th plywood can be used to recover the strapped roof deck with additional strapping installed where the plywood edges don’t land on the strapping.