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RE: T1-11 siding

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I asked our helpdesk to respond:
You can also go to our website, and search for TT-018 (mentioned by someone else) and Q370, which covers our recommendations for stucco over wood structural panels.
There is several very significant difference between our recommendations and those mentioned by various people who responded.  The design values for panel siding is based on the thickness at the point of nailing.  So, if underlap thickness is 5/16, and the bottom nail is nailed through the underlap, then use the 5/16" number.  If the point of nailing is through the fat part of the panel, then one can use the full thickness of the panel.  Even though grooves have been cut into the panel, in general, the governing issue for shear walls is the strength of the nails, and not the shear through thickness strength of panels.
Finally, the non-Str I values should be used.  A Str I panel is most definitely not "needed" for a shearwall.  In fact, many parts of the country (including Washington State), Str. I panels are quite difficult to find.

Thomas D. Skaggs, Ph.D., P.E.
Senior Engineer
APA - The Engineered Wood Association
7011 S. 19th Street
Tacoma, WA 98466
ph: 253/565-6600
fx: 253/620-7235

### start helpdesk comment  ###
The shear values for 303 siding panels installed directly to studs or over gypsum sheathing are the same as those specified in Table 25-II-I-1 of the 1997 Uniform Building Code, or IBC Table 2306.4.1 for plywood panel siding.  Thickness at point of nailing at panel edges determines allowable shear wall values. Fastener corrosion may be an issue in a project as old as this one. 
I am not aware of any historic problems or drawbacks specific to T1-11. The majority of complaints regarding plywood siding products are directly related to improper installation, finishing or maintenance. As long as T1-11 is properly installed, finished and maintained it performs as well as any other wood siding product. Attached is APA's Technical Topic, Weathering Effects on Plywood Siding, which provides some basic weathering information.
Generally speaking, if T1-11 siding shows no evidence of decay or delamination and also looks and feels sound, it is probably OK.
Installing stucco over T1-11 siding is an unanticipated application so we don't have recommendations. Typically T1-11 siding carries a 16 oc Span Rating and should remain flat when installed at 16 inches oc. The major concern with a grooved siding panels is buckling. If the existing siding panels are in good shape, and the wall is flat, it may be OK. It's a judgment call. Attached is APA's Data File, Installation Of Stucco Exterior Finish Over Wood Structural Panel Wall Sheathing, which provides our general stucco recommendations.

From: Gerard Madden, SE [mailto:gmadden(--nospam--at)]
Sent: Monday, May 01, 2006 11:17
To: seaint(--nospam--at)
Subject: T1-11 siding

I just received plans to do some minor upgrades to an existing home. The contractor who sent me the plans indicates that they are going to place stucco over existing T1-11 exterior siding on the upper level two levels of the residence as one part of the modifications. I know there are some drawbacks with T1-11 that are escaping me at this point, but IIRC, they are likely due to exposure to the weather. I realize that the increased weight of the stucco (10 psf) to the walls will increase the seismic load, but I don’t think by that much. The home was built to the 1973 UBC.


I’m hoping someone can direct me to some information or can share their thoughts on:

  • Shear values for T1-11
  • Summary of the historic problems with T1-11
  • How to visually observe if these problems are present
  • Suitability of placing stucco over the T1-11


I’m just in the proposal phase at this point, and don’t have much except a few pictures and the original plans. It was 5/8” T1-11, can’t seem to find any nail spacings on the drawings.