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I'm looking to drywall and insulate my detached 3 car garage that measures 40' x 25'. The ceiling joists are 4' OC and I've realized thay may be a problem.

The previous owner contracted a garage contruction company to build this detached garage in May of 2000. I've recently been researching and have figured out that I will definitely need to add strapping or some way to ensure that the drywall won't sag.

Main question is: Is this construction capable of bearing the weight of the drywall and insulation?

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    Almost certainly going to need to retrofit more joists/bottom chords - or actually convert those rafters and "kinda trusses" into a full set of "real site-built trusses" to bear the load, unless you can get info from the "garage company" claiming adequate bearing for the "kinda trusses" on that spacing. If you are rural you might be able to "cowboy it" - in more regulated areas you may need to involve an engineer and get stamped plans for the retrofit, and in rural areas you might want to if you are not a cowboy, or vernacular building aficionado.
    – Ecnerwal
    Apr 8 '21 at 22:34
  • I am rural, so I'm sure I'd be fine there. I just want to make sure it doesn't all come crashing down on me. I am contemplating using the lightweight 1\2 drywall and some insulation.
    – CSarge
    Apr 8 '21 at 22:53
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    Well, the risk of not getting advice from a pro is precisely the risk that it could come crashing down, but the joy of being adequately rural is that you can choose to take that risk, or you can choose to hire knowledge if you are not confident that you have educated yourself to limit the risk. Sometimes the agricultural extension folks have some VERY handy references for "building farm buildings" which include such things as site built trusses rather than factory built ones, etc. I can say that the span is no problem for a proper set of trusses; I have one that wide. But they were engineered.
    – Ecnerwal
    Apr 8 '21 at 22:57
  • Also @CSarge if the goal is to add insulation above the ceiling, you'll need to make sure you have a soffit vent (it looks like you have an apex vent already, from what I can tell) so air can get in and circulate on the cold side of the insulation to prevent condensation forming. Apr 8 '21 at 23:17
  • Yeah, it does have soffit vents running on both of the 40' sides. And two apex vents. The goal is absolutely to put rolled R-30 insulation above the ceiling. Just trying to figure out how to get a ceiling put in some way or another.
    – CSarge
    Apr 8 '21 at 23:33
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No, you would need to reframe the ceiling with timbers that are continuous and rated to bear the load of a ceiling, rather than simple rafter ties.

Timbers that are butted together like that are in tension - their job is to stop the roof from spreading outwards. Adding the weight of quite a significant amount of drywall underneath would make them sag potentially dangerously.

Here's a relevant example of U.S. building code:

Ceiling joists shall be continuous or securely joined where they meet over interior partitions and fastened to adjacent rafters in accordance with Tables 2308.10.4.1 and 2304.9.1 to provide a continuous rafter tie across the building where such joists are parallel to the rafters. Ceiling joists shall have a bearing surface of not less than 11/2 inches (38 mm) on the top plate at each end.

Because the secure join in this case is not over an interior partition, they are not to code as ceiling joists.

The walls also look like they might be too thin to support the weight of all the additional joisting and the ceiling itself too, although it's hard to tell that from the photos (they're probably fine, but do check).

Alternative ceiling options

As you shouldn't use those rafter ties as load bearers, here's some options for how the ceiling can potentially work instead:

  • A warm roof:

enter image description here

Warm roofs have the insulation either above (ideal) or between the rafters. It's really (really) important that there is no air gap between the insulation and the outside roof deck, and that you control moisture with an effective continuous moisture barrier. This type of roof is beneficial here though as the weight is directed down through the walls, rather than on the rafter ties. This would mean you end up with an open roof space (i.e. your existing rafter ties would be visible).

  • A suspended ceiling

A ceiling suspended from the rafters (not the rafter ties) would also direct its weight through the rafters and down the walls. Plenty of decorative options with these, but given their common usage in office buildings have a certain feel attached to them which isn't for everybody.

  • Insert additional joists

Use additional lightweight continuous joists (such as engineered I-joists) and only attach the ceiling to them if at all possible. Keeping the weight down, using metal or PVC ceiling boards would still allow for insulation above.

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Like others have said you definitely need to re-enforce the trusses. You will also likely need to add additional trusses. The maximum unsupported span for drywall is 24". It's hard to tell but looking at the wall assuming the studs are on 16" spacing it looks like the trusses are spaced further apart than 24".

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  • Question states that they are 48" OC. Eyeball examination is that they are "truss-like" not actual trusses, though they could be retrofitted to be trusses, and the rafter pairs in between could also be so retrofitted to give a more reasonable 24" spacing, and better load carrying from being actual trusses, not collar ties with some webs that are not quite enough to make them trusses.
    – Ecnerwal
    Apr 8 '21 at 23:28
  • I'm definitely not confident enough to add trusses in a DIY fashion. I've contacted the original builder to see what they rate the loads to be, but no answer so far.
    – CSarge
    Apr 8 '21 at 23:41
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    Yes, the walls are 16" OC. The "trusses" are 48" OC. I had previously planned on scabbing on some 2x4's to create the 24" required for sag-free drywall, but now I'm quickly realizing that is a bad idea given the weight.
    – CSarge
    Apr 8 '21 at 23:44

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