The attic above my garage was modified poorly for attic storage. I’m looking to redo the Joists in order to properly store above my garage. The Current Joists are 21 feet long and are only 2x8s. Would 2x12s spaced 12″ apart with 3 or for blocks between joists be adequate? There is not currently any beam to support this load. I would be sheathing this space with 3/4″ plywood. I have no experience with home building or construction, just a DIY guy. I live in central California if that makes any difference. thank you all in advance.
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It’s not a straight forward answer unfortunately and you will need to refer to your local code authority, but for general guidelines follow table R502.3.1 of the IRC (can Google it) as this is the base line standard for building code. It will all depend upon your use, species of lumber, spacing and etc.
Whiplash it looks like you will need to support your joist at mid span from below with a built up/ engineered beam.
A good lumber yard should be able to have a joist manufacturer run the calculations for you.
Got it, I'll see if I can find someone to inspect and advise me on where to run the beam. thank you all for the advice.