AskCodeMan

Hi Jerry

I'm having a problem understanding the charts in R602 for exterior wall construction.

I have a wood framed gable end wall (1972) that was originally sheathed with 1/2" T-111 needed repair and some one repaired with 11/32 T-111.

I believe this to be incorrect for several reasons but can't find any reference to "minimum" thickness required.

Only thing I can come up with is reference to: Structural wood panels 3/8 or less shall be installed long side perpendicular to studs.....which of course wasn't done as it wouldn't match the vertical groves of the original. Thanks.

David

David,

To know "the minimum" thickness you would first need to know the year of the repair, the code applicable at the time of the repair, the wind pressures and loads applicable at that time, along with the framing spacing, etc.

Other than that, the minimum (from the current 2014, 5th Edition, Florida Building Code, Residential, is stated here:
- R703.3 Wood, hardboard and wood structural panel siding.
- - R703.3.4 Minimum thickness.
- - - Wood, hardboard and wood structural panel siding shall be of the minimum thickness specified in the AF&PA WFCM.

And as allowed in:
- TABLE R703.4 WEATHER–RESISTANT SIDING ATTACHMENT AND MINIMUM THICKNESS
- - (and all applicable notes)

The first two columns address "SIDING MATERIAL" and "NOMINAL THICKNESSa (inches)", going down to "Wood structural paneli ANSI/APA-PRP 210 sidingi(exterior grade)".

Note "a" (after Nominal Thickness) states: Based on stud spacing of 16 inches on center, where studs are spaced 24 inches, siding shall be applied to sheathing approved for that spacing.

Note "i" (after Wood structural panel) states: Three-eighths-inch plywood shall not be applied directly to studs spaced more than 16 inches on center when long dimension is parallel to studs. Plywood 1/2- inch or thinner shall not be applied directly to studs spaced more than 24 inches on center. The stud spacing shall not exceed the panel span rating provided by the manufacturer unless the panels are installed with the face grain perpendicular to the studs or over sheathing approved for that stud spacing.

Under the Nominal Thickness column, it says: "3/8 – 1/2"

The fourth column, headed "WATER-RESISTIVE BARRIER REQUIRED", says "Yes"

Hopefully there is a waterpresistive barrier there too.

I suspect that you read the above code sections, information and notes, and that 'Note i' is where you got this: "Only thing I can come up with is reference to: Structural wood panels 3/8 or less shall be installed long side perpendicular to studs".

You are correct that, based on today's code, 3/8 wood structural panel siding requires a maximum stud spacing of 16 inches on center when installed such that the "long dimension is parallel to the studs" (as you said it was installed).

The next part of 'Note i' states: "Plywood 1/2- inch or thinner shall not be applied directly to studs spaced more than 24 inches on center.", and I doubt that the studs are "spaced more than 24 inches on center", thus that part has no particular restriction or meaning to this.

However, the last part of 'Note i' is applicable: "The stud spacing shall not exceed the panel span rating provided by the manufacturer unless the panels are installed with the face grain perpendicular to the studs or over sheathing approved for that stud spacing."

I am presuming that there is no structural sheathing behind the wood pane siding.

Go here (place cursor over link, right click, select 'Open in new' tab or window): https://www.apawood.org/publication-search?q=f405, bottom of page 7: "APA Rated Siding is manufactured in panel Performance Categories of 11/32, 3/8. 7/16. 15/32, 1/2, 19/32, and 5/8."

That is telling me that 11/32 and 3/8 are different, and with the table stating "3/8", the "11/32" is not permitted to be used as "3/8", presuming, of course, that the "3/8" reference is the panel rating based on a stamp, not necessarily a panel thickness.

Page 9, second paragraph down: "The Span Rating for APA Rated Siding panels is for vertical installation;" also see label examples at bottom of page 9; second paragraph up from the figure: "APA Rated Siding is produced with Span Ratings of 16 and 24 oc."

The 11/32 is incorrect as it does not meet the minimum requirements for its installation requirements - which is the "minimum rating" (revised from "minimum thickness") for the panel(s) installed.

That works through to the "minimum" 'thickness' answer, but changes the answer to "minimum rating" as the panels would be required to be "rated" and would have an "APA Rated Siding" rating which needs to be met, and exceeding that rating means the panels are 'not thick enough' (unless an engineer did an engineering evaluation of the site specific installation and signed and sealed it as being installed in accordance with his/her engineering design). Also notice that it has a "Exterior" rating instead of the "Exterior 1" rating for roof and wall sheathing panels.