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7/16 OSB or 15/32 Plywood For Roof Sheathing

steveray said:
I ASSUME you mean rafters 16" OC, not rafter span...or 16" sheathing span anyway....(well 14.5", but lets not get crazy ;) )


View attachment 12186
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OK, now we are getting somewhere.
R803.1 and Table R803.1 set a minimum thickness of 5/8 sheathing for a 24" span, yet Table 503.2.2.2(1) shown above allows 7/16" at 24" O.C. without edge clips. This is a conflict in the code and the more restrictive applies, therefore, if you have trusses at 24" O.C., you must use 5/8" sheathing.

Does everyone agree?
 
jar546 said:
R803.1 and Table R803.1 set a minimum thickness of 5/8 sheathing for a 24" span, yet Table 503.2.2.2(1) shown above allows 7/16" at 24" O.C. without edge clips. This is a conflict in the code and the more restrictive applies, therefore, if you have trusses at 24" O.C., you must use 5/8" sheathing.
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Table R803.1 is for "lumber sheathing". It does not apply to wood structural panel sheathing, for which spans are covered in R803.2.2. Florida has amended R803.2.2 and has its own table; the unamended IRC R803.2.2 just refers you to Table R503.2.1.1(1).

Cheers, Wayne
 
wwhitney said:
Table R803.1 is for "lumber sheathing". It does not apply to wood structural panel sheathing, for which spans are covered in R803.2.2. Florida has amended R803.2.2 and has its own table; the unamended IRC R803.2.2 just refers you to Table R503.2.1.1(1).

Cheers, Wayne
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So Florida is messed up?
 
wwhitney said:
Table R803.1 is for "lumber sheathing". It does not apply to wood structural panel sheathing, for which spans are covered in R803.2.2. Florida has amended R803.2.2 and has its own table; the unamended IRC R803.2.2 just refers you to Table R503.2.1.1(1).

Cheers, Wayne
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Thanks Wayne. I was not being specific to Florida, but let me recap your answer.

Lumber Sheathing, as listed in R803 would be for individual boards, not engineered plywood or OSB which are typically in 4'x8' sheets. So when I built a covered porch in 1989 and used 1-1/2' T&G so the underside would have better aesthetics, I was using lumber sheathing, not structural sheathing.

On the other hand, wood structural panel sheathing would be the OSB and plywood sheets that we typically see.

Therefore 803.2.2 specifically for roof sheathing points back to R503.2.1.1(1) which, although is under Floor Sheathing, applies to Roof Sheathing.

Does that sound correct as an explanation?
 
jar546 said:
Thanks Wayne. I was not being specific to Florida, but let me recap your answer.

Lumber Sheathing, as listed in R803 would be for individual boards, not engineered plywood or OSB which are typically in 4'x8' sheets. So when I built a covered porch in 1989 and used 1-1/2' T&G so the underside would have better aesthetics, I was using lumber sheathing, not structural sheathing.

On the other hand, wood structural panel sheathing would be the OSB and plywood sheets that we typically see.

Therefore 803.2.2 specifically for roof sheathing points back to R503.2.1.1(1) which, although is under Floor Sheathing, applies to Roof Sheathing.

Does that sound correct as an explanation?
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Yes....Is that what Chat GPT gave you?
 
Manufacture rating can be used

R104.11 Alternative Materials, Design and Methods of Construction and Equipment
are not intended to prevent the installation of any material or to prohibit any design or method of construction not specifically prescribed , provided that any such alternative has been approved. An alternative material, design or method of construction shall be approved where the building official finds that the proposed design is satisfactory and complies with the intent of the provisions , and that the material, method or work offered is, for the purpose intended, at least the equivalent of that prescribed in quality, strength, effectiveness, fire resistance, durability and safety.
 
steveray said:
So Florida is messed up?
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No, Florida only refers back to R503 for identification and grading. Florida has its own minimum structural panel sheathing table in 803 which is based on wind zone and exposure level. Florida does not refer back to 503 for roof sheathing spans.
 
Also one thing to be aware of is OSB being cut less than 24-inches in width, we see this a lot at the ridge.
 
steveray said:
Just busting you a bit..and seeing if you were still testing that stuff....the lumber vs. sheet goods clarified it a bit and Florida amending their own Table sealed the deal...
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Still testing out the AI for sure. It can be a real pain in the butt sometimes and there are accuracy issues so sometimes I just create the outline myself.
 
Wood structural panels are marked for their span rating. I believe the code allows us to accept that. However, I seem to recall that the installation instructions or manufacturer's literature (or maybe the American Wood Council?) calls for ply-clips between joists when rafters are spaced at 24 inches for many grades and thicknesses of sheathing panels.
 
"Show me where it is code compliant to install 1/2" nominal OSB or Plywood on a roof deck with a rafter span of 16" O.C."

Everywhere but Florida?

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2018 IRC R803.2.2 Allowable Spans

"The maximum allowable spans for wood structural panel roof sheathing shall not exceed the values set forth in Table R503.2.1.1(1), or APA E30."

IBC Table 2304.8(3) actually allows 3/8" plywood or OSB with a 24/0 span rating to be used for rafters or trusses 24" OC as long as plyclips are used.
 
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