Florida Roof Sheathing Discrepancy (Wood Structural Panels)

[StrEng007]

TLDR: The Florida residential code requires thicker plywood roof panels than the Florida building code. I cannot tell if this is due to one being prescriptive while the other is analytical. I'd like to get some feedback from other Florida engineers.

Detailed discussion:
I wanted to confirm with the other Florida practicing engineers out there, it appears as though the 2023 FBC Residential general provisions have more conservative restrictions than the 2023 FBC Building HVHZ regarding roof wood structural panels.

Per the Building Section 2322.2.3, the minimum nominal thickness of roof sheathing is set at 19/32". Additionally, for a roof with truss framing at 2 FT o.c., the panel span rating must achieve 32/16. Per the APA, a panel span rating of 32/16 corresponds to a performance category of 15/32" thickness. So you basically have two criteria:

•Use 19/32" as a minimum (GOVERNS)
•Based on truss spacing, 15/32" is allowed

Per the Residential Section R803.2.2, the minimum performance category and span rating is determined based on wind exposure and wind velocity. Per Broward and Miami-Dade building jurisdictions, the design wind speed for typical residential structures is 170 to 175 MPH, with wind exposures C and D (varying distance from the coast).

Take a residential building in Miami, with V=175 MPH at exposure C. The required performance category defaults to 23/32" with a span rating at 48/24. In this situation, both the span rating align with the typical performance category.

So by default, any plywood used here cannot be less than 23/32" thick.

What's interesting is the Residential code requires a more conservative approach. Is this due strictly to the prescriptive or analytical methods? Meaning, does the Building code section assume there is an engineering analysis taking place while the Residential code assumes the spec is prescriptive? Sometimes I have difficulty understanding when the code is setting a prescriptive measure or setting a minimum check against an analytical approach.

Also, how many of you specify 19/32" or 23/32"? I'm used to specifying 5/8" or 3/4".

 

[lexpatrie]

Great, now I have to research this......

I remember the thickness change appearing in the 7th edition, Residential. (I have a slide preserved from a 2/23/2021, Shane Gillis presentation on the FBC, Residential, 7th edition showing what is probably the same table). It was a big like 7 hour series of presentations put on by s a few groups in Florida (including Gold Coast and the Florida Home Builder's association) and covering accessibility, energy code and the residential code and it got pretty involved. I watched it from my hotel in North Naples. My concern at the time was the (increased) mandatory thickness on the sheathing might cause conflicts on damaged roofs if the roof sheathing needed to be partially replaced.

I believe the thickness change in the FBC, Residential originated from industry groups, possibly a home builder's group like FAHB, so the FBC, Building simply hasn't had that code change proposal put forward or balloted. It's relatively rare for a building to go under the prescriptive provisions of the FBC, Building, if you ask me. Usually they get the engineer treatment.

The FBC, Building thickness tables are probably for prescriptive design, i.e. non-engineered, if, as an engineer, you can determine something thinner works via engineering analysis, it is fairly likely the thickness in the prescriptive provisions would not be applicable. You may get a more aware building official inquiring about it, but if the math checks out based on the wind speed/exposure category and the attachment schedule and span tables, I would expect it could be approved as an "alternate" even though it's really not, as the prescriptive provisions in Chapter 23 are intended for non-engineered non-residential structures. 2301- 2307 are general design requirements would apply to "all structures" but the prescriptive provisions (2308) are more of a stand-alone item for non-engineered structures, that tends to align with the residential code, but there are differences, which are probably not intentional.

Incidentally, the tables you are looking at, 2322, are for HVHZ specific areas, so mostly Miami-Dade and Broward (See Chapter 2 for that definition), although I've had a few people mention that Fort Myers has adopted them as well. Haven't verified that yet. There's a difference between HVHZ and wind borne debris region. WBDR is a larger area, HVHZ is very geographically contained.

 

[StrEng007]

lexpatrie,
Thanks for the heads up on this information. Most of the work I do is HVHZ and I've become so set in the way they do things... looking at general sections of the Code can be quite confusing if that makes any sense.

 

[lexpatrie]

Uh yeah. 100%. I really atruggled with a St. Augustine project back when...

It's the way ita written but I don't see any alternative that would be better. Do a normal design and cross check for HVHZ or do the HVHZ then work backwards?, or HVHZ only and presume it overrides all the important stuff?

It's a bit of a mess but what isn't? Get your hip waders on and get wading.

M-D Offered some really deep seminars earlier this year, watch for them if they return, or maybe there's a recording.