The ceiling joists in my garage are 2×8 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.
Best Answer
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.