[ben1216]

building a saltbox roof. front pitch is about a 12 and rear is about a 3.5 just for example. the rear run is going to be approx 18ft. i was going to make trusses out of 2x12x20. ridge beam a 2x12. i was thinking since im putting 2x12 i can safe money and not put ceiling rafters in and make a cathedral ceiling. should i feel comfortable with that 18ft span on the back? say i would go 16'OC. what would be the most cost efficient way to build this in the long run. not sacrificing building safety? even if ceiling rafters could be used. thank you

 

[3onthetree]

First, are you using the term "trusses" for the 2x12x20 "rafters," or are you actually going to make trusses? I would not recommend anyone make DIY trusses unless you are using a manufacturer's design drawings and have the capability of putting enough mechanical pressure on the mending plates.

If you have trusses there is no ridge beam. You can emulate a cathedral/vaulted ceiling with truss design, a common one is scissor truss.

If you want an open catheral ceiling with stick built, then you must use a ridge beam. Ceiling joists can be eliminated then. This ridge beam wil be much bigger than a 2x12, not to mention the depth must be a minimum size to cover the angled cut end of the rafter tying into it (i.e. 2x12 rafters need 2x14 ridge board depth depending on rafter angle).

The span of the rafters will determine the size, and this is based on loads from snow, seismic, and lateral forces they will take, so the 18' span is unknown.

Your best bet is to hire someone to design it for you.