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Roof sheathing attaches to the surface of the rafters or trusses to form the structural skin of the roof. It spans the rafters to support the roofing and, in the case of panel sheathing such as plywood or OSB, it acts as a diaphragm with the walls to resist horizontal loads.
There has been substantial debate over the minimum acceptable thickness of plywood when used as roof sheathing. There has been a lot of mis-quoting of the wrong section of the code. After research I believe that you have to get to Table R503.2.1.1 which show that 3/8" plywood is only acceptable if there are panel clips spaced equally between 24 ...
Most buildings need roof sheathing to support the roofing material and add rigidity to the rafters or trusses. Working with plywood or OSB sheathing is a pretty straightforward process, but smart techniques can make the job easier, faster, and safer. We’ve collected our best articles and videos on the topic to help you out.
Roof staging makes the upper courses go on quickly and easily Staging goes on before the courses go up. After the first course of sheathing is nailed off, roof brackets are nailed in and planks are set before the next course is started. Staging provides a safe place to stand as well as a place to rest the sheets for the next course.
Structurally, in most cases, exterior wall framing can easily be 24 in. o.c. as well. (The exception, according to the 2015 IRC, is 2×6 walls supporting two full floors plus a roof assembly above, or 2×4 walls supporting one floor plus a roof assembly above.) Zip sheathing can typically be installed either vertically or horizontally.
When combining exterior foam insulation with fiberglass, cellulose, or mineral-wool roof insulation, the foam layer must be thick enough to keep the sheathing temperature above the dew point. Otherwise, water vapor can condense on the back side of the sheathing, elevating the moisture content of the materials and possibly resulting in mold and rot.
This pair of numbers tells you the maximum joist spacing for which the sheathing was designed, in inches. The first number (24) applies only to using the material on a roof, and the second (16) applies to using it on a floor. Suppose you purchase conventional, commodity-grade 3/4-in. sheathing stamped 24/16 to use as a subfloor.
7/16 OSB really isn't adequate for your roof, even if you're running rafters on 16" centers. You might get by with 7/16 plywood, but even that would be iffy. I put 5/8 OSB on the roof of the 8x12 shed I built this summer (16" centered rafters). It worked fine. It's a shed. And hey, it was on sale.
Unclemike42 suggests that Jeff skip the new layer of sheathing simply because it will mean additional weight on the roof. A layer of 3/8-in. OSB weighs about 1.2 lb. per sq. ft., says Oldhand. Some basic checking on rafter sizes and spans should tell Jeff whether the added weight is going to be a problem.
As per code in my area (Ottawa, Canada), I plan on using 3/8″ plywood for sheathing. The roof covering will be asphalt shingles. I just read somewhere (actually in Taunton Press’ Roofing with Asphalt Shingles book) that one should consider using sheathing thicker than code to avoid sagging between rafters.