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Roof Sheathing Thickness

R

Rooftopgolfer

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Dec 4, 2020
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Tulsa, OK
My state uses the 2015 IRC. 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" oc supports. Otherwise the plywood needs to be a minimum of 7/16". Can anyone provide additional information, confirmation, or dissent? Thanks
 
The American Plywood Association has pamphlet titled, APA Engineered Wood Construction Guide. Like the codes, you need to look at the footnotes. The answer is... it depends. Table 33 on page 79 gives the full story. 3/8 plywood at 24", yes, clips are required. 7/16 plywood at 24", No clips required.
 
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Table R803.1 is for lumber wood sheathing.
TABLE R503.2.1.1(1) ALLOWABLE SPANS AND LOADS FOR WOOD STRUCTURAL PANELS FOR ROOF AND SUBFLOOR SHEATHING AND COMBINATION SUBFLOOR UNDERLAYMENT gives allowable spans for rated plywood.
 
Even if the code says 3/8 plywood complies with the code you may be better served by using at least 1/2' plywood. Check the plywood rating since not all 3/8" plywood is the same.
 
Rick18071 said:
Table R503.2.1.1 is for floors. Table R803.1 is for roof sheathing which requires 5/8 plywood with rafters 24" o.c. or less.
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R803.1 is for lumber sheathing. Real wood. Plywood is a wood structural panel covered in R803.2.2 which points you to R503.2.1.1

There is a lot of cross over between chapters 5, 6, and 8, so don't just go with the chapter titles of floor, wall, roof.
 
I found out the hard way many moons ago that 3/8" or 1/2" plywood roof sheathing will buckle and warp, even with clips, so I have specified 5/8" ever since. OSB might be more dimensionally stable, but plywood has better nail-holding ability.
 
The plywood should have a stamp that tells you the span. 24/0 means 24" oc on roof (may need clips) and 24/16 means 24" oc for roof and 16" oc for floor. If you like springy roofs and floors, these minimums are for you.
 
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