I've got a three stall garage, and above it a largely underutilized attic space. Before winter sets in, I'd really like to have some kind of good, walkable flooring up there upon which I can put boxes and the like. This is not intended to be an extension of living space as the joists are 24'' OC and I've had a structural engineer tell me things need better support to be up to code for that kind of work. Fine, and that's in the plan someday. However, I still want to be able to make it semi-walkable and support boxes as right now without sagging or danger of falling through.

I've done some research and it seem as though 3/4-inch plywood is probably the most ideal for the job. So, I went and bought some. However, I discovered quite quickly the joists aren't perfectly spaced either, and so cutting to exactly 24-inchs puts the spacing off on the plywood.

I've read a bit more and discovered the concept of bridging and/or sistering, and I'm starting to wonder if that's the best decision here. Again, this flooring is just intended to support myself and boxes, not a piano and bed and entertainment center. So, I'm wondering if you'd recommend either bridging or sistering to aid in the spacing problem of the joists, and if that'd be considered good enough for the immediate task at hand. I'm also curious what you'd use to bridge/sister? What thickness of wood?

I'm personally leaning towards sistering the joists upon which I want to lay the plywood as opposed to bridging since the spacing is off already and it's annoying to have to measure the distance between each pair of joists.

Rambling question, but I'm curious of your thoughts.

Edit: I'm thinking option number "3" in this PDF: https://www.finehomebuilding.com/pdf/021184090.pdf

 

So, you've had a structural Engineer tell you that the attic can't support a storage load but you're going to do it anyway?

3/4" plywood and almost any sheeting is over spanned at 24" so I would expect it to sag between the joists after some time.

Sistering might address the structural issue. It doesn't really help the 24" spacing but by having 3" to hit does make it easier than 1 1/2" without sistering.

 

Assuming your garage is 36' wide and you want to add storage to one side, that's 18' - creating a large storage area, perhaps 400 square feet. Sistering should be an option if you're truly planning to run the joists the entire span. Instead of sistering, you can add joists at 12" OC. You already have joists at approx. 24" OC so you're adding 9 additional joists.

 

It's a little hard for us to give what would be considered as structural advice sight unseen.

We don't know sizes or spans so how could we possibly answer.

 

This is the question of the month, and every month,

If the intent is to store the Mr's Xmass, Halloween, or Easter decorations, put down some plywood and have at it.

No roofing system is ever engineered so close to the limits that some boxes cant be stored up there.

If your going to store a car worth of parts, which I have done on many occasions, them maybe something to consider!