Sheathing a Roof
You're 30 ft. in the air, walking on narrow framing members and manhandling sheets of plywood that'll catch the wind like canvas sails. You'd better know what you're doing.
Synopsis: A variety of tips that will make it easier to sheathe a roof with plywood. Among other topics, the author, a custom home builder in Massachusetts, discusses panel layout, explains how to nail off panels, and looks at the advantages of using roof staging. A sidebar rounds up comments from other carpenters via email to the magazine’s Breaktime forum.
My earliest roof-sheathing memories are of extra trips up tall ladders with a circular saw in tow on the end of a long, heavy extension cord. The plug would inevitably snag on the edge of the sheathing when I least expected it.
Besides the adrenaline rush of being jerked off balance, it always seemed ironic that to make the sheathing fit properly, I had to cut pieces from the sheets that I had just struggled to lift onto the roof, only to watch the cutoffs fall back to earth. Many years and many roofs later, I’ve developed much better methods for sheathing a roof accurately and efficiently.
A good sheathing job begins with proper rafter layout
To minimize waste and to simplify cutting, the sheathing courses should begin either with full or half sheets. But laying out the rafters isn’t as simple as duplicating the stud layout on the wall below. To get the starting pieces of sheathing to fall properly on the rafters, the layout must reflect the gable overhang, which, of course, varies depending on the rake detail.
Taking the gable overhang into account when I lay out rafters minimizes cutting and wasting plywood. With a simple rake detail, accounting for the difference in rafter layout is easy math. But when I add an overhang with 1x rake trim and crown molding, as many of our houses have, I stop trusting my math skills and make a full-scale drawing of the rake detail on a sheet of plywood. Not only will the drawing ensure that our rafter layout is right on, but my crew and I can also measure — for items such as the blocking for the overhangs — directly off the drawing.
Once we’re sure of the layout, we cut and assemble the roof carefully, paying special attention to orienting the crown, or convex curve of each rafter, to face the sky. One crown facing down can cause a dip in the roof plane that will be a pain to sheathe and can’t be hidden with shingles. When the roof framing is finished, we make sure that the gable is plumb and that the frame is square so that the ends of our sheets don’t wander off the rafters.
Measure from the top and the bottom to lay out sheathing courses
Before hoisting that first sheet up to the roof, we figure out how wide the final course of plywood will be when we reach the ridge. The object is to avoid having to install a narrow (less than 12 in. wide) and flimsy course at the peak.
We begin by measuring the length of the rafter plus any additional length needed to make up for the thickness of the ridge beam, the ridge vent and the fascia detail. If the measurement falls within a few inches of a 4-ft. increment, say 16 ft. 5 in., we shrink the bottom course 12 in. so that the top course becomes a much more substantial 17-in. width. The added strength for the final course more than makes up for any extra effort it might take to rip the sheathing.
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