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Structural Considerations for TJI Larsen Truss PGH build

WLSchebaum | Posted in Expert Exchange Q&A on

Hello all,  I’m looking to build a project with 9-1/2″TJI exterior trusses in order to achieve a solid continuous insulation layer outboard of the structural sheathing. The  main structural 2×4 wall in board of that will hopefully be insulated with TimberHP bats.  My question concerning the TJI truss system has to do with structural issues.  We’re looking to install the triple pane windows at the exterior WRB layer which will be Mento or similar at the exterior of the TJI.  I’m curious to hear from folks who have built this wall system before what tips and tricks help expedite the process and if there is anything to consider regarding load paths and what not.  I’ve built a number of double stud walls as well as projects with large amounts of rigid exterior insulation but this will be the first truss wall system . Thanks in advance.

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  1. Expert Member
    Michael Maines | | #1

    If the TJIs are not supported at the bottom, they can easily support the weight of insulation and cladding but not of windows and doors. For those, you need to build structural plywood or OSB bucks, so the windows are hung from the structural framing, and doors need to bear on floor framing. It's best for performance and structure to have the windows inset.

    If you support the bottom of the TJIs, they can support the weight of the windows--Unity Homes uses TJIs as their wall framing--but they are not code-compliant as wall framing unless they are part of a manufactured system such as Unity's.

  2. WLSchebaum | | #2

    Michael, when you refer to plywood bucks are you meaning a style of buck similar to what one would build on a double stud wall or something else? As I have the plan currently drawn ( see attached) there is 4" of bearing for the TJI on the foundation. I'm also exploring the idea of extending the floor system out to catch the bottom of the TJI. I'm not opposed to shifting the windows towards the center of the wall for structural and performance reasons, I just like the clean planer aspect of the windows integrated with the WRB and not having to make that transition in to the center of the window buck. I plan to consult with my engineers regardless of where I put the windows on the structural needs of the TJI wall system. They have experience with that type of wall assembly so I'm looking forward to their ideas.

    Also, as you'll see on the attached image. I wasn't able to persuade my clients to keep the concrete to the exterior of the control layers so we are going with 5" of Roxul comfortboard. Not shown on the drawing is the concrete free slab on top of 12" of Glavel. I'm looking forward to that!

    1. Expert Member
      Michael Maines | | #3

      Yes, I mean a through-wall buck like you would do with a double stud wall; the bucks are fastened to the structural framing and basically cantilevered out over the TJIs. With your design I don't think that would be necessary, but licensed engineers can be very conservative so you'll have to see what they say.

      Good luck with the concrete-free slab! I lose plenty of battles with clients (and contractors) as well; exterior foundation insulation works just fine, especially Roxul/Rockwool.

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