My next project is to build a storage shed (20'x17'). I want to minimize the footprint but maximize storage so I plan to use gambrel trusses for a usable attic. My span is 21' and that is where the problem lies. I understand the concept of gambrel trusses but not the engineering. I think I need to use 2X16 stock for that span but I cannot get stock that long so obviously I need to splice 12 footers. The problem is that I don't want any supports in the center so I don't know how to build it. I also can't run a perpendicular span down the middle to use as a hanger as there will be a double door at one end and thus no support. I'm guessing I need to splice the center joint with plates of some kind but I don't know what to use , how big they should be or how to attach them.
I also need help with the support members (2x4) as well. The goal is to maximize the usable space between the break points.
Can someone point me to a source for designing these trusses and getting the details right? Thanks.