Is 1/2-inch plywood enough on 24-inch trusses under a metal roof?

An owner-builder in Canada asked on Reddit's r/Homebuilding whether the 1/2-inch plywood already bought for a 3:12 roof on trusses 24 inches apart, under 28-gauge standing seam, will sag. The engineer had called for at least 1/2 inch. Table 9.23.16.7.-A of the Part 9 rules, in the British Columbia Building Code's version, sets the minimum for plywood on supports 600 mm (about 24 inches) apart at 9.5 mm with supported edges and 12.5 mm without. Canadian 1/2-inch sheathing plywood is 12.5 mm, so it passes with no H-clips or blocking. Where edge support is needed, the code accepts metal H clips or 38 by 38 mm blocking.

The span rating stamped on each panel gives the same answer. In APA's roof table, a 32/16 panel (15/32 inch, sold as 1/2 inch in the U.S.) may span 32 inches with edge support and 28 inches without; the first number is the roof span, the second the floor span. At 24 inches the table allows 65 psf of live load, assuming 10 psf of dead load, against 120 psf for a 40/20 panel (19/32 inch, sold as 5/8). APA's deflection limits are 1/240 of the span under live load and 1/180 under total load. Compare 65 psf, about 3.1 kPa, with the specified roof snow load for the site. If the snow load comes close to that figure, a 5/8-inch deck nearly doubles the margin.

The metal roof sets its own minimum. Best Buy Metals' standing seam guide accepts 15/32-inch APA plywood or 7/16-inch OSB as the solid deck, fastened with #10 by 1-inch pancake-head screws long enough to pass through the sheathing. It also requires the deck to stay within 1/4 inch of flat over 20 feet from eave to ridge, to limit panel distortion. To reduce oil canning, the guide recommends heavier gauges, narrower panels and striations; its own panels come in 26 and 24 gauge, heavier than the 28 gauge planned here. Read the installation guide for the panel actually ordered, since deck minimums and clip spacing differ by system.