hip roof converted to a cathedral ceiling

[lubos1984]

Hello Everyone,

I am working on a project where a client wants to convert a section of a house to a cathedral ceiling. Right now its a square hip roof with ceiling joists. The ceiling joists would be removed. As well the roof has intermediate beams to support the 2x6 rafters since some of them span over 20'. My questions are:
1. Size new rafters - this seems straight forward as i would evaluate the rafters based on the unsupported span for the snow / dead loads and size new ones appropriately.
2. 2x8 HIP. I always assumed that a typical hip functions similar to a ridge board. If the ceiling joists are removed should the hip be evaluated as if its a ridge beam as you would for a gable cathedral roof ?
3. Do the exterior walls need some sort of lateral support now that you remove ceiling joists ?

I've added a picture with some measurements.

Thank you all for your help.

 

[jayrod12]

So you'd be putting in a big horking beam below that support right? Or a column down to something stiff?

If the support isn't stiff enough, you'll still have the roof rafters pushing out on the walls.

If no ceiling joists to tie the walls to each other, the hip framing would have to act like a beam, not just a ridge board.

 

[lubos1984]

The hip would connect to a ridge beam. At the connection there would be a support. I was planning to sister new rafters and then put a hip beam under each hip ? If I do that do I still need to worry about lateral support at the exterior walls. Or would this be no different now from a ridge beam gable. Thanks

 

[jayrod12]

Correct, if there's adequate vertical support (including it being stiff enough to prevent substantial deflection of the support) it would act the same as a ridge beamed gable roof where you don't need the ceiling joists as tension ties.