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I am planning to insulate my unfinished garage, which leads to adding a ceiling. There are 4 2x6 joists spacing around 50”. How do I add more joists to support drywall and fiberglass rolls? Location north California. 20 feet span seems too long even for none storage ceiling. Do I need to add a beam from back wall to garage door? joists hang on beam. The other ways is to hang to roof joists? The problem is the roof is not flat, contains many planes. I updated pictures. Hopefully it could show the structure of the roof. Additional question, how do I ceiling the flue pipe area? The huge duct from coil also block one joist.

Roof from inside Flue pipes Diagram

enter image description here

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    Going to need more support for 2x6s at 20 feet. Kind of surprise you do not notice a sag in them. Probably will need a beam, but should have an structural engineer to give right sizes and supports for beam. Are those 2x6s tied/attached to the roof rafters?
    – crip659
    Oct 4 '21 at 22:29
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    A picture of the existing joist structure from the inside, and of the garage door side from the outside would help a lot to determine the shape of the garage and what's going on here. I can only guess that this is a very old structure, because if those joists are truly 44" to 54" apart, that doesn't come anywhere near meeting current building codes. Especially for CA with their somewhat frequent earthquakes. It's likely you're going to need a structural engineer to take a look at this and approve plans to update it if you want to do anything to it.
    – FreeMan
    Oct 4 '21 at 22:30
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    Your 2x6's that run side to side just function as tension ties that are there to prevent the roof from flattening out and pushing the walls apart. They are not meant to carry any kind of weight. FWIW, my old garage was framed similarly (built in the early 60's, I think), but the joists ran 24+ feet front to back.
    – SteveSh
    Oct 5 '21 at 0:58
  • From the code up.codes/viewer/california/ca-residential-code-2016/chapter/8/… , I can do 12 inch spacing, #2 Fir almost get 20f span. Also I am thinking about medal sheet (roofing). Probably avoid adding joists? Oct 5 '21 at 2:57
  • If this is not a new house, the capability of the roof to stand the loads has already been proven, the only thing the additional joist needs to carry is the weight of the ceiling and the insulation. I think adding another 2x6 between the existing joists shall work. You need to get the nod from a structural engineer though.
    – r13
    Nov 4 '21 at 14:08
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You can span 20' for a ceiling with 9½" engineered (TJI) joists on 16" or 19.2" centers. Won't be cheap, but it would be a quick, clean solution. You'd probably have to hang them on a ledger, which means you'd lose some ceiling height, and you'd need to brace the tops against lateral movement.

More information from Weyerhaeuser

As SteveSh commented, the existing "joists" are actually rafter ties in tension. Don't get the impression that they indicate that a 2x6 will span that distance under any load at all without major deflection. I'd guess that those are already quite sagged.

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Whether or not the roof structure can support the weight of drywall and insulation is a question we probably can't answer for you.

Assuming that it can, however, we can make suggestions as to how to support the drywall. One possibility is to add more wood structure so that the drywall is supported at, say, 24 inch spacing.

Another possibility is to build a suspended drywall ceiling and hang it from whatever rafters or other structure is convenient and suitable. This might work well since you mention that the roof contains many planes, but (we presume) the ceiling should be on a single plane. I wrote a little about suspended drywall ceilings in an answer on another question.

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I thought one of my garages was cheap with 2X6 on 32" centers. I added a 2X6 between each making it 16" centers to put in decking for light storage. With your 20 ft span and 2X6 ; 16" centers with 2X6 is the least you can do.

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    2x6 is too small for 20 ft span at 16" on center; At that distance and spread, the maximum span allowed is anywhere from 13' 9" to 18' 9", depending on the species and grade of wood you use.
    – TylerH
    Dec 31 '21 at 1:48
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I would go with your suggestion of "add a beam". Cut the existing joists in the center, removing ~4" of the joist. This would allow you to install a double-up 2x12 or 2x14 glulam type of beam (actual size would depend on your load analysis), then attach the cut back joists to the beam with hangars. You would also need appropriately sized posts to support the beam at each end, and ensure that what the posts bear on is solid (like the foundation wall). Oh, and of course, you need to build a temp wall on both sides of the center of the cut out joists to keep them from falling.

Something like this:

enter image description here

If you can afford the reduction in headroom along the center of the garage, you can forego cutting the joists and just place the beam under the center of the joists, so the joists rest on the beam.

Added tension tie info

brhans brought up a good point about the fact that the joist hangars shown above are not rated for a lateral load - the joist pulling away from the beam. Some tension ties that connect the two halves of the joist together would solve that problem. Here's a page from the Simpson catalog showing typical tension ties.

enter image description here

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  • Is the beam from back wall to the garage door? How do I add post to garage door? Oct 5 '21 at 0:26
  • Yes, front to back. Do you have a single or double garage doors?
    – SteveSh
    Oct 5 '21 at 0:29
  • Single garage door. Oct 5 '21 at 0:35
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    Agree about needing a SE for this. Though some lumber yards are able to sign off on some of the simpler designs, using load/span tables from the engineered lumber manufacturer.
    – SteveSh
    Oct 5 '21 at 1:23
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    It looks to me as though the existing boards are under tension and preventing the rod from pushing the walls apart. As such, cutting them all down the center at the same time sends like a very bad idea... Even if it doesn't immediately all come crashing down, using joist hangers to attach the cut ends to a new beam won't restore the structure to its original configurationsince joist hangers aren't intended to resist horizontal forces like that. This seems like a very bad plan to me.
    – brhans
    Oct 5 '21 at 8:34

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