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I am currently building a detached, single car garage and getting ready to sheath the roof. I had originally specified a ridge vent but am strongly considering switching to a gable vent. I have some reasons for making this choice which I'm not looking to discuss here.

My question is, if I install gable vents, will I need to modify my end trusses? These are the end truss plans:

enter image description here

Left as-is, the gable vent would be partially blocked by web ST2, which is untreated lumber. I'm not concerned about the limit to airflow, rather the fact that this web would be partially exposed to the elements.

If I do modify the truss, I assume this would be similar to window framing?

  • Twin 2x6 header
  • Twin 2x4 bottom plate/sill
  • King and jack studs left and right
  • Cripple studs centered above and below

Some details of the project:

  • 12' by 20' detached, single story, single car garage
  • I am not insulating at this time but plan to later on
  • I'm in Wisconsin, USA -- hot humid summers, cold wet winters
  • Trusses are factory made
  • Gable ends will have a 1' overhang
  • I added a large gable vent to a house and the framing did not interfere. – blacksmith37 Dec 27 '19 at 20:14
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    I myself would not sweat it, there will be plywood over the whole surface unifying everything. If it wasn't planned to have plywood, then it needs to be. The make the opening for the vent and add a horizontal 2X below the opening to stabilize the bottom edge. – Jack Dec 28 '19 at 7:31
  • Another curious question, what is going to support the 1' overhang? If there is nothing tying it back into the roof, it will sag rather badly. I have seen notches put into the top of the rafters to let in 2X4s to support the overhang. This is a typical detail that I have seen for gable ends, which is usually considered a non bearing wall, at least for the wall section below it. – Jack Dec 28 '19 at 7:35
  • If I'm reading the drawing correctly, the trusses are spec'd with 1.5x4 rafters (smaller than a nominal 2x4?) and can't be notched to inset 2x4's to support the overhang. – Eric Simpson Dec 30 '19 at 17:33
  • I don't think you need to worry about the trusses interfering with airflow through the gable vents. Even with a 2x4 (or two) in the way, the gable vent will still have much more free space area (FS) than perforated soffits. Are you planning on putting a van in the gable vent? – SteveSh Dec 30 '19 at 18:20
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The only circumstance under which I'd even remotely consider modifying a truss is if I had an engineer's stamp.

In this specific case, you're thinking about the opening as a simple downward carrying load, and I don't think either of us are qualified to make that assumption. (Trusses probably have side and up loads that aren't obvious to mere mortals.)

I'm not sure why you wouldn't just have an asymmetric vent, or split the vent between two smaller symmetrical vents.

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    I'm not a structural engineer, but my gut says that ST2 is likely in tension. I'd split the vent as suggested here rather than modify an engineered truss. – brhans Dec 27 '19 at 19:18
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How thick is the exterior wall & cladding? I have similar construction on the gable end of my home (albeit stick built, not engineered truss). The gable vent opening was simply cut into the exterior sheathing and the vent notched to fit around the (uncut!) vertical 2x4. Put the vent as high as practical to best vent the heat and use the overhang for coverage. (Personally, I'd prefer ridge & soffit vents but you're moving away from that.) The angled slats on the vent should direct the weather down and away from the building so I wouldn't worry about it bothering the truss.

Btw, are those members 1.5x4, smaller than a nominal 2x4? That would be even more reason to make the vent fit around the truss, not cut or modify the truss.

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