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Structural Gable Truss for Garage Door Opening

maine_tyler | Posted in General Questions on

Basic structural framing question here:
Is anyone familiar with the practice of using structural trusses (not gable trusses requiring full bearing support) so that a large barn door (18′ opening) below doesn’t need a ridiculous header?

Anything to consider when doing it this way? Can a structural truss still be ordered ‘dropped’ for 2×4 lookouts (18″-24″ overhang planned).

Seems easy enough, but haven’t brought it up with the truss company yet…
Live-in-attic truss for what it’s worth. Barn.

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  1. Expert Member


    Yes the common trusses can bear on a truss. It might not be the gable truss, they may prefer to run a girder truss inside of it instead. That's the sort of stuff you can leave them to decide, but yes it's very doable, and the best place for the header, where height isn't a limitation.

    Edit: I may have read too much into your question. If the gable truss isn't picking up any other loads, then it's ever easier. It's just a structural truss with backing added at 16" oc. for the sheathing - and yes in either situation it can be dropped.

    1. maine_tyler | | #2

      Thanks Malcolm.
      Yeah I basically just mean using a regular truss on the gable rather than a so-called 'gable truss' requiring a bearing wall beneath.
      I can't quite picture how they can 'drop' a full regular truss and i just want to make sure what I'm asking isn't too non-standard.

      1. Expert Member
        MALCOLM TAYLOR | | #3


        The gable trusses I get are just one more common truss with vertical blocking added in between the chords. Must be a regional thing. Anyway, they bear just like any other truss. The main force at the opening will be horizontal, so it's worth doubling or tripling the top plates over the doors.

        1. Expert Member
          Michael Maines | | #4

          Malcolm, here in the northeast, gable trusses don't have diagonal webs, only vertical nailers, so they need full support under the bottom chord unless specified otherwise.

          Tyler, by "dropping" the truss, they would just frame the truss at a different dimension and would not include rafter tail extensions. The truss heels would bear on the same top plate as the other trusses. You would just need to show them what you want for gable end lookout dimensions.

          Are you planning to have windows or a door in the gable ends? The truss designer needs to know about any loads, and might adjust the design to be most efficient with what you want to put into that wall. They may double the end trusses to have more lateral strength as well.

          I forget where in Maine you are but Aroostook Truss seems to be supplying a lot of the state these days and they have good engineers on staff. They performed a near-miracle on a recent project of mine, figuring out how to build a very complicated, high-performance roof. You can contact them directly but usually you go through a lumberyard.

          1. Expert Member
            MALCOLM TAYLOR | | #5

            Another option would be to stick frame the gable.

  2. maine_tyler | | #6

    Thank you Michael and Malcolm.

    There will be a window in the gable.

    My thinking is that with a large 18'+ opening, having a structural truss above—even though just a gable end—would be superior to using a massive header supporting stick framing.

    It seems that 'dropping' a structural truss would cut into it's bearing members, but perhaps I'm just not picturing the specific set-up where it bears on the wall, or perhaps they just engineer around the issue. I don't want to ask/pay for a premium upgrade though, which is the gist of my question here. Ultimately I need to talk to the truss folks.

    As far as lateral/horizontal loads... are we talking loads placed against the broad side of the gable such as wind? My assumption was that the floor diaphragm (covering the bottom chord of the attic truss) would do the lions share of resisting such a load.

    In the end I will probably still want a decent sized header simply to support the door hardware (sliding) and to help with the shear resistance of that wall by extending the header to the ends and heavy nailing it for moment resistance with the wall legs (wall legs to be 4' or so each side).

    1. Expert Member
      Michael Maines | | #7

      Tyler, I think I understand your concern about dropping the trusses. I almost always use raised-heel trusses so lowering the top chord a few inches would not be a big deal. Is your top chord already sitting on the bottom chord at the exterior wall? If so, they may suggest creating pockets where the gable trusses sit so they plane out in line with the other trusses.

      For an 18' attic truss meant for living space, the bottom chord will be large--or, more likely, they'll suggest an open-web truss as the bottom chord, at least between the "walls" of the attic truss.

      Regarding lateral loads, walls need to resist wind blowing directly on them, typically in Maine somewhere around 25 psf, and the studs or struts need to be able to resist that load without excessive deflection or total failure.

      This section of the code deals with framing and shear bracing walls, which with big door openings can get complicated: But it's better to know that your structure is sound than to guess and wonder.

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