Another home improvement question for our resident experts...

What are some best practices/requirements for proper drainage around the outside a building structure? We have a detached, finished garage that measures 25' x 16'. It's on a slight slope, and I'd like to move rainwater away from it. Some contractors suggested digging a trench along the back 16' side and down one of the 26' sides, laying down a perforated PVC pipe in the trench, and then covering all of that with 3/4" rock. The big difference between estimates was in the depth of the trench -- one guy quoted 8"-12" and another 18"-24". The latter jives better with what I've seen online.

What do you all think? What else should I know about this kind of project?

Thanks!