On ours, the builder hung electrical conduit and water pipes below the beams. To bridge everything, I placed 2 x 4's on their sides and on 16" centers. This gave me support for the can lights and the drywall.

Photos of the finished space can be viewed from "My Page"

I had the same situation. I have engineered celing joists that are 24" on center, and I was told to use 5/8" sheetrock to avoid sagging. I ended up going with ceiling tiles instead.

was hoping that I can use something like 3/4 to maybe 1". maybe the right term is strapping.

Don't use 1/2" it WILL sag and code is 5/8"! Go spend the extra few bucks to do the 5/8" and it will be money well spent!

James

I guess I agree with everyone's suggestion of 5/8 sheet rock.....unfortunately I already bought 1/2 and am installing 2x between the 2 I beam joists for support. I cant take it back, for I cut it in the store to bring home. Now im nervous about ..

If you want to stick to 1/2" then you could attach rows of 2x4's (install on their side so they only cost you 1 1/2" of space) perpindicular to your joists 16" OC. Like the other poster said then this would give you space should you ever need to run wiring or plumbing.