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The diagram below shows the problem and also the atypical way the roof was framed originally (see this question) This garage door was already in place since long before we bought the house and separate garage about ten years ago, I've noticed no visible problems until now as I'm looking into flooring the attic. I dont know how old the cracks are in the wall plate under joists 2 and 3 (joist 3 wasn't cut at all) There are smaller cracks in other places and there are some metal reinforcing strips screwed into the plate crosswise. enter image description here

One solution that came to mind was to add further joists on top of the upper plates over joists 1 and 2 and possibly 3, as in the diagram below.

new joist location Would the above be a valid way to proceed or not?

Possibly related, the two rafters attached to joist 3 have a very crude collar tie between them, quite high up in the attic, well above head height standing on the third joist. There aren't any others except one, ten or twelve joists further along. The tie isn't even level.

Thanks for reading and especially for any guidance offered.

Edit to add photos

Taking these, i realised that the diagrams above are incorrect, the upper plates are not directly over the lower ones, they lie approx 4" lateral which I think might explain the upward bowing. The second photo maybe gives some clue as to the quality of the 10" by 2" joists. You can also see the cracking in the lower plate in the third photo.

The wall to wall span is 16 feet and the joists are every 16"

cut away joists

close up of cut away on one joist

joists and rafters

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    The lower joists have two roles, first they are in tension to hold the weight of the roof triangle. Second they are the carrying element that carries weight of things in the attic or things hung from the ceiling (erm, garage door). With #1 and #2 compromised much of the heavy lifting is on #3. – Harper - Reinstate Monica Sep 16 '20 at 15:55
  • @Harper-ReinstateMonica.Thank you. Considering the second purpose you mentioned, first, wouldn't those weakened joists be still able to carry the weight since the vertical weight on them is all at the place where they rest on the wall plates and that is intact? Considering the tension forces, I don't understand the bowing on the second joist, it's almost like the tension is acting to compress rather than stretch the joist. Also and this might be stupid, is the fact that these weakened sections exist still, evidence of no real harm done, so far at least? – culchie Sep 17 '20 at 8:15
  • Do you have a picture? – pkamb Sep 17 '20 at 11:46
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    @pkamb, I've just edited the question to add some, thanks for your interest – culchie Sep 17 '20 at 18:29
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The wisest course of action is to consult a licensed structural engineer in your area for a remediation plan. They can tell you where you need to add structure to resolve the issue.

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  • As smart and helpful as the folks 'round here are, this really is the only way to be sure. They can offer some good suggestions, but it would be incumbent on the asker to ensure that these suggestions were reviewed by a local SE who knows the local code and can do a hands-on inspection. – FreeMan Oct 17 '20 at 23:37

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