Reinforcing 2x6 attic joists above garage for conversion to living space?

I have seen similar questions online, but not my situation specifically. I have an attached garage with 2x6 attic joists and 2x6 roof rafters. The joists span 20' and are 16" on center. '. Each joist is connected by a vertical 2x4 to the roof rafter above it positioned about 4' from the wall. There is a 1/2" plywood sub-floor on top of the joists. I want to finish this space with drywall walls and ceiling and also a drywall ceiling in the garage (under the joists). I know the 20' span is well over the limit for a 2x6, but was wondering how much the vertical 2x4's help compensate for that. Would sistering another 2x6 to each joist be sufficient support for the finished room above? I have attached pictures.
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Unless the roof structure was specifically designed with those vertical 2x4's (like an engineered attic truss), I wouldn't count on them supporting any weight. Especially if they're just nailed on.
You would need to sister 2x12's to the joists to span 20', or add a support beam/posts down the middle and span the 10' distance with 2x8's.
You will also need 3/4" plywood on the attic floor to span 16" spacing. I have 5/8" on my garage attic floor and it flexes way too much for anything other than light storage.
Good luck,
Anthony Watson www.mountainsoftware.com www.watsondiy.com
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replying to HerHusband , mpadlo wrote:

HerHusband, Thanks for your reply. Do the 2x12's have to rest on the top sill plates or just be attached to the 2x6's?
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Ideally, yes, the 2x12's should be supported on the top plates. The top ends could be cut at an angle to match the rafter slopes, but the angled portion shouldn't extend much beyond the inside of the wall. Of course, this installation would require the removal of your existing plywood floor in the attic (since the new joists will be taller).
You might be able to sister the 2x12's to your existing 2x6's with the top edges even. Then add additional supports under each end of the 2x12's. This would allow you to keep the existing floor in place, but would require a lot of additional studs to support the new 2x12's.
Regardless of which way you go, the ends of the 2x12's need to be supported so they can carry the load of the attic floor.
Also, you can't just notch the bottom of the 2x12's to match the 2x6's. While this may offer some additional support, the 2x12's could split at the notch, effectively making it just another 2x6.
Good luck,
Anthony Watson www.mountainsoftware.com www.watsondiy.com
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replying to HerHusband , Mpadlo wrote:

Thank you Anthony. The ceiling is already kind of low in that area so I don't think I'm willing to give up 6 inches in headroom to place the 2x12s on top of the sill plates. I'm going to incur the additional cost of 2 by 4 studs under the notched 2 by 12s. The cost of the studs is worth the trade for the 6 inches in head room. Thanks for your help!
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