By L.D. Hogan, RRC, FRCI, PE
Abstract
This is the twelfth in a series of articles examining various deck types. Of the numerous considerations necessary when selecting a roof system, the type of decking is among the most important. With the variety of decks to be encountered (both new and old), it is incumbent upon roofing experts to be the authority on these matters. This article will explore features of older wood plank decks that are not of a tongue-and-grove profile (commonly termed “board sheathing”).



Elsewhere, the author has encountered two layers of shingles installed over original wood shakes, as well as other bizarre combinations; an old wood plank deck usually lurks below this mass of materials. As more layers of roofing are added, there is an increasingly remote chance for water to percolate its way through to the roof deck. This can give the false impression that the roof is performing adequately. Stacking roof layers in this manner is a departure from rational thinking—and very likely a code deviation unless there has been formal approval by petitioning for a variance consideration.







As outlined earlier, wood shrinks as it dries—just like many other construction materials (Figure 10). Note that shrinkage of wood planks is seldom uniform. The occurrence of wide gaps will require a judgment call regarding the proper course of action (Figure 11). Note that if deck planks are 6 in. wide and a 5⁄8-in. gap is present between planks, there is a 9.4% chance that any random nail will occur in a joint opening; where joints are ¾ in. wide, there is an 11.1% chance for nails in a gap. That is a lot of wayward nails for the new roof covering.
Summary Remarks
Many board sheathing decks are still around, and the wide gaps can create fastening problems. Those practicing in restoration and historical renovation are sure to eventually be involved with this type of substrate. It would be wise to anticipate at least some corrective work, possibly to include a complete overlay with plywood or OSB sheathing. In that event, there should be verification of load-carrying capacity of the roof framing. Finally, edge attachment measures for any new overlay sheathing should be honored just as in new construction.
Lyle Hogan is owner and principal engineer of Fincastle Engineering, Inc., Greensboro, NC. He is a registered engineer in five states, a Registered Roof Consultant, a Fellow of RCI, and an ICC structural masonry inspector. He has designed and administered roofing projects in half of the U.S. using a variety of systems. Hogan has received RCI’s Lifetime Achievement Award and its Michael DeFrancesco, William C. Correll, and Richard M. Horowitz Awards.
Interesting and informative article! the slapping on and layering of seemingly random roofing materials was odd.
Great post, exactly this will hinge on matters of rafter/support type and spacing, slope, potential for rooftop snowdrifts, etc.