Yes, 24 foot long trusses are pretty simple. Are you going to build them yourself, or have them built? Either way, be sure to calculate your design on the 20 foot span from wall to wall. The overhang isn't the important measurement.
Spacing your trusses for a metal roof will decide how many you have. I like to put them in on 4 foot centers, then use 2x4 purlins on there flat side. This makes them easy to attach and easier to find when attaching the metal panels.
You can go to 6 foot centers with 2x4's on edge, or 8 foot centers with 2x6's on edge. Spacing of the purlins is something you need to check first with your metal supplier. I like them every four feet.
What pitch are you going to use? 4:12 is what I like as it's steep enough too look nice, but not too steep to walk on. Even at that pitch, you have to have clean, white soled sneakers to walk on it. Dark soled shoes are harder and more slippery. Any dust at all will cause you to slide off the metal at a 4:12 pitch. If you go steeper, it just gets allot harder, to impossible to walk on it and do any work. I did one roof at 5:12 that was the absolute max that I could crawl up and down. Even then, I had more close calls then I want to remember.
Do you get snow? If so, ignore everything I've said as it doesn't apply to snow loads. For snow, you need an engineer to tell you how much load you can handle and how to design the roof.
Are you limited in size for your building? Will you have doors in your four bays? If so, then ten foot is kind of tight. I'd go 12 foot for each bay, and make the doors ten foot wide. That extra 8 feet in lenght isn't going to add very much to the overall cost of the project, but the extra space will be appreciated for the life of the building.
Eddie
