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We are insulating our garage and need to build a ceiling to separate the attic and the main garage area. The span of the bottom chord of our trusses is 22 feet. Will our truss system support the weight of the drywall and insulation?

Here are some pictures of our garage roof, which was built in 1970.

our garage truss system

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    Those are factory built trusses. The odds of them not being able to support some insulation and drywall are very low. You'd have to ask the builder (or whoever you bought the trusses from) to know if they're designed to support the additional weight of storage. From what I can see, I'd guess not, but a wide pic showing the full (or at least a half) span would probably give the engineering/builder types here the ability to make a decently educated guess. Adding additional ceiling joists for hanging drywall is definitely overkill.
    – FreeMan
    Dec 15, 2020 at 20:25
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    The main problem you will have is that the span between trusses is fairly large, and drywall is likely to sag (especially if insulation is laid on top of it). This probably wouldn't be a structural problem but it would be ugly.
    – Hot Licks
    Dec 16, 2020 at 15:01

3 Answers 3

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Your engineered trusses are almost certainly designed for insulation and drywall loads. I've never encountered a situation where they weren't, and I've been involved in residential construction almost since that garage was built. I wouldn't hesitate to finish the ceiling.

Use no-sag 1/2" drywall to reduce weight if you like. Just use some sense when loading the attic for storage. Spread the load, keep heavier items near the wall, and don't go nuts. A few hundred lbs. is about what you can reasonably do. If you ever get a zillion feet of snow at a time consider removing some near the middle.

Secondly, a proper solution to a situation where you did need to bolster the roof system doesn't involve a bunch more joists. It would likely be a single large beam down the center, directly under the vertical truss members. You'd then still attach drywall to the existing ceiling framing and wrap the beam. No need to lower your entire garage ceiling.

That said, beams under monolithic engineered trusses are very unorthodox. This type of truss has an expected seasonal deflection (called "truss lift")--as they heat and cool and absorb moisture and dry out they rise and fall at the ceiling line. A beam would be likely to have a gap over it half the time.

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Yes, your trusses will support insulation and gypsum board.

The Building Code requires the bottom chord of trusses and ceiling joists to support a minimum of 10 lbs. per square foot. (See IRC Table R301.5)

Gypsum board weighs about 3 lbs. per square foot and insulation (depending on the thickness and type) weighs between .3 and .5 lbs. per square foot per inch thick.

Your trusses were made by a truss manufacturer certified in your state. This requirement is built into their software for the construction of trusses.

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    I would doubt that any builders were using software to design trusses in 1970. I would however, expect that the math used included enough slack for the load.
    – Ben
    Dec 16, 2020 at 17:32
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You have truss bottom chords not "rafters" nor "joists".

If you have the plans/documents they will undoubtedly list the bottom chord loading in pounds per square foot.

A 22 foot 2x6 will behave like a wet noodle if it's not part of a truss. On the other hand, a 22 foot 2x4 that's part of a truss can carry quite a lot of load.

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