Cathedral Ceiling - Beam Span

I'm building a screened in porch and I'd like to construct a cathedral ceiling. My questions is, how do I size the ridge beam accordingly to carry the load? Here are the specifics:

- Structure dimension: 12ft wide x 24ft long
- Roof pitch: 4.2/12 (straight gable)
- Beam needs to span 24ft
- Live in Atlanta so no snow load or high wind
- Building material is pressure treated pine
- The ridge beam will be supported by vertical columns that continue all the way to the sub floor system

Again, question is how large do I need to make the build-up the ridge beam? Are there span tables that can help? All of the beam span table I found are for floor beams.

The reason you don't find the ridge beam in the span tables is that the ridge beam is actually supported by the rafters. The ridge beam just needs to be sufficiently deep to give the rafters a place to meet and hold them in place. Thickness and load carrying strength is not really an issue. I have seen ridge beams made of 1X material, although I wouldn't do that. Structures built with trusses (engineered 2X4 triangulated rafters) have no ridge beam at all. If your rafters are 2X6, which they probably are, judging by your dimensions, a 2X6 or 2X8 (2X8 might look better if they will be exposed) will do just fine. You can scab two 12-14' boards together in the middle.

What you do have to pay attention to, is that your sidewalls are tied together so that the weight of the rafters does not spread them. Use a cross-tie from one side to the other every 4' (called purlins), like a ceiling joist, nailed to the foot of the rafter on each end. If that is not in the plan, then your ridge beam WILL need to support the weight. In such case I would suggest you consult with a local building supply (a real one, like contractors use, not Home Depot) that supplies contractors with engineered lumber and beams. Show them your plan and they should be able to fix you up.

Have fun.