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The ceiling joists in my garage are 2x8 spaced 24" o.c., and span 21'. The roof load appears to be carried by the walls, so the joists seem to only be in place to hold the drywall.

I'd like to use the space for light storage (holiday decorations, etc), but I'm not sure if the joists are up for the task. Obviously I'd want to add some decking; so items don't fall through the drywall, but that also adds to the weight the joists will have to carry.

Will I be able to do what I want, or will I have to upgrade the joists?

Since it's a garage (where cars have to park), I can't add mid span support without a huge undertaking.

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What's the discoloration on the fiberglass? Mold? –  Bryce Jan 18 at 17:30
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There is a nice article at familyhandyman.com/floor/repair/fixing-bouncy-floors/… which discusses a possible option: a beam below running perpendicular to the joists. Is it one large garage door or two smaller ones, below? –  Bryce Jan 18 at 17:44
    
@Bryce I considered a perpendicular beam, but that would eat into the headroom in the garage below. –  Tester101 Jan 19 at 0:18
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The perpendicular beam can be top or bottom. Top truss or bottom truss, all the same to gravity. Up top the truss could be a bench or partition wall. So what's below this? One garage door or two? –  Bryce Jan 20 at 22:33
    
@Bryce you should consider providing an answer, it sounds like you might be on to something. It's a two car garage, with a single wide door. –  Tester101 Jan 20 at 23:55

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Any 'credible and/or official sources' will tell you to hire a Structural Engineer, which is what I'll say also. There are too many variables and unknowns for Internet advice to suffice. Prepare well to minimize costs, supplying all dimensions and drawings, and schedule a site visit.

The Engineer may be able to give the details for a top truss run perpendicular to the existing joists. Gravity can't tell the difference between top and bottom. A top truss or beam will create a lump or ridge on the attic storage floor. The 2x8 joists would be held up with strap hangers. The two ends of the truss must be supported, which could be difficult if there's a two car garage door. You'd need to remove siding to slide the beam in place.

Or as @Ecnerwal wrote the engineer may prefer to retrofit the existing peaked roof into a series of trusses by adding web elements. These might go in very easily from the inside.

Family Handyman has an article that has a good analogous bottom support example. Prepare to DIY the work, not the engineering.

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Come on back and tell us how it all worked out! –  Bryce Jan 21 at 5:47
    
I should learn to follow my own advice. In this situation I would tell somebody else to consult an engineer, so I guess it's time for me to do the same. –  Tester101 Jan 21 at 11:25

The joists are almost certainly taking some tension load (keeping the walls from spreading) from the roof, at least, unless it's very strangely built.

As is, given a quick look at load calculators and those inputs, probably not. even reducing live and dead loads to 20PSF (minimum live load for a "storage attic" and a pretty low storage load) and allowing lots of deflection, it's too much for a 2x8.

Your best/most cost-effective approach would probably be to convert rafters and joists into a truss system by adding web elements between the rafters and joists. You'll need a structural engineer, but it's the least added material. My trusses spanning 24 feet are all 2x4 material, so it should not take much to make a 2x8 lower chord carry a reasonable load without support from below. So you'll spend some money on engineering and save a good deal in getting it done, while having some peace of mind. Storage truss

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Looking at the maximum joist span requirements for southern pine, 30 lbs/ft^2 live and 10 lbs/ft^2 dead load (the minimum values in my chart) I'm seeing a 2x8 at 24" spacing cap out between 12-13' (depending on wood grade, and this number will also vary by wood type). Best guess is that these joists were originally designed for an unfinished space where there was no load and the drywall was added later. The wood bracing you see connecting to the roof is for lateral support, effectively a shear wall, to keep the roof from pancaking.

Going down the table for the maximum joist spans, a 2x10 spaced every 12" is rated for the above loads for a 20' span (+/- depending on the wood grade). Reducing your load requirements would make this an acceptable for your purposes. The 2x8 will cap out between 15-16' depending on the grade if you reduced the spacing to 12".

This is all based on my "Handyman in your pocket" guide, ymmv, consult with a structural engineer to be sure, and other standard disclaimers apply.

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