I am refitting an old shop above my garage to be an ADU. It was built in the late 70s / early 80s, and the truss system is 2x4 trusses on 22" centers. The trusses are professionally built & engineered trusses, with metal plates at the joints, so it wasn't a "hand-rolled" truss situation.

The span across the truss from end to end is about 28', with no bearing wall or support at midspan. It's held snow load and been fine (no visible sagging etc.) for 40 years, but that span seems pretty wide to me-- I'm concerned that adding 5/8" drywall to the lid will be a lot of additional weight on a truss system that already seems a little marginal.

I know this is more of an engineering question, but I'm wondering if this setup would raise concerns with the experienced builders around here. I had a contractor friend take a look and he didn't mention any issue, but I had an engineer friend say it looked like a pretty big span for that size of truss, though it'd "probably be fine". Any consensus on this or way to check the load?

  • The 2x4 dimension seems incorrect - 2"x4" or 2"x4'? Are you adding the ceiling panel, or cladding the walls? It makes a lot of differences.
    – r13
    Commented Jan 13, 2022 at 21:10
  • Trusses are usually installed 24” apart. How are you going to use the space as a room? If you install wallboard between the trusses, how are you going to install ceiling insulation first? (Btw, you know the bottom chord of the trusses are too weak for floor joists.)
    – Lee Sam
    Commented Jan 13, 2022 at 21:52
  • 1
    It's entirely in how it was designed. The fact that it's made from 2x4 material is normal and not a concern - the strength of a truss is in the overall shape, and if it was designed with 2x4's having adequate strength to do what it was designed to do, then it will do that. - My roof truss is 24 feet, 2x4, designed for a 50 lb snow load (including the case where it all blows off one side and becomes a snow-drift on the other side) and to carry drywall and insulation on the bottom chords. You might be able to track down documents from an old permit application.
    – Ecnerwal
    Commented Jan 14, 2022 at 3:07

1 Answer 1


There is no "general" answer here. Trusses are engineered products and when engineers, architects, or builders select them, the specifications are available to them from the manufacturer.

You're here after-the-fact and trying to get a specification for an unknown (at this time at least) product. That's not going to happen.

Some suggestions:

  1. Did you get any paperwork with the property that might have such documents included?
  2. Check the trusses and see if they have a manufacturer's label or stamp. This might help you track down the company and see if they can supply you with the needed information.
  3. Hire an engineer to evaluate the trusses and the structure and give you an answer to your question of whether or not these will handle the additional static load. BTW - unless your friend is a licensed professional engineer in your state and is experienced in this area, his opinion is effectively worthless.
  • 4
    "unless your friend is a licensed professional engineer in your state and is experienced in this area, his opinion is effectively worthless". Frankly, consider us all friends, none of whom are licensed engineers in your state. #3 is, most likely, the only legit option at this point.
    – FreeMan
    Commented Jan 13, 2022 at 18:55

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