The workshop building was constructed about 20 years ago, with independent rafters and cross beams, rather than engineered joists. I'm now paying the price because the 22' cross beams are sagging as much as 3.5" at the center.  An engineered joist would have had wood braces tying the rafter to the cross beam and use the position of the rafter to hold the cross beam straight and flat. 

It wasn't a big problem before, but now I want to drywall the ceiling, blow in cellulose insulation, and then hang air hose reels, hang a fine dust collector, and drop power and dust collection piping to some tools. Leaving the cross beams the way they are is just not going to work.

Channeling Tommy from "This Old House", I bought a small hydraulic jack and built a jig from two 2x4s and metal bottom and top brackets.  The top bracket screws into the cross beam to ensure it won't slip away.  I make sure the jig is plumb, then gently jack it up to the level I want, plus 1/8". Once released, I've found they sag back down about 1/8" so it goes right to the level I want.

As you can see in the picture, I cut two 41" 2x4 pieces, then tie the rafter to the cross beam with 5/8" carriage bolts. There were 13 cross beams to do and it was very hot dirty work, but really worth it in the end.  The attic crawlway is strong and flat, the ceiling is flat and ready for drywall and I can safely hang everything.