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Re: T1-11 siding

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Gerard -
It was (& still is) not uncommon to use T1-11 for shearwalls.  The capacity isn’t real high but, if there’s enough of it, it works O.K. As for problems, do a good visual inspection & watch for decay at the horizontal edges & improper nailing at the panel edges (it was fairly common to tack nail & then nail the lap joints with a single row of nails—basically zero capacity).  But it can often be re-nailed to get what you need. Just remember you’re nailing a strip of plywood that measures about 3/8” x 3/8” (that may be Redwood) so don’t try to push the capacity too hard).


The added weight of the stucco will require a full analysis & full compliance with current code. There probably aren’t any tie downs but strapping is often enough at upper floors.


Waterproofing is most likely going to be the biggest problem.  All the windows & doors will probably need to be replaced/reset.   These little jobs can be challenging.  You don’t want to stick your neck out, but if you get too conservative the client can’t afford to build it.  If push comes to shove, lose the stucco--I wouldn't consider it a "minor upgrade".  JLC listserver has a nice thread/photo essay on a 3 year old stucco house that's a complete disaster due to improper waterproofing.
Chuck Utzman, P.E.

Gerard Madden, SE wrote:

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.