I can help
Do you have any plans you could share with me?
Or rough sketches with the proposed configuration, could work.
It could possibly be done. There are a couple of limits that need to be followed. I am attaching the 2012 IRC Code Section that covers Braced Rafters and Raised Rafters.
1. Your proposed sloped ceiling would need to act as purlin bracing and meet the 45 degree minimum requirement.
2. Your raised ceiling joists would be limited to being raised 1/3 of the HR value shown in my attachment.
Let me know if you have any questions
Of course, your added beams that will be accepting the purlin brace loads would have to be sized accordingly.
Sorry, I was looking at your sketch as the proposed.
That can work as long as the roof rafters are sized accordingly and you do not raise your ceiling joists more than the 1/3 of the HR value shown in my attachment.
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I can size them for you. I have a few questions:
1. What is the horizontal distance between the ridge board and wall the roof rafters will rest on (span of rafter)?
2. What city and sate do you live in for possible snow load consideration?
3. What type of roofing is on the house?
I’d like to point out that a Professional Engineer’s standard of care typically includes a site visit to assess field conditions and get an overall understanding of the structure. This can obviously not be accomplished through the internet. The information provided here is meant for planning purposes only (general sizing and budgeting) and is based on the information provided by you. All loading cases considered are for vertical loads only, no lateral analysis has been completed. The information should be verified by a professional engineer who can visit the site to ensure that potentially important information has not been overlooked or omitted.
You can use conventional lumber 2x6's to sister to the existing 2x6's. This will give you double 2x6's spaced at 24" o.c.
I would add the additional 2x6 rafters to one side of the existing 2x6 rafters and then move the ceiling joists up the other side of the existing 2x6 rafter.
Since your "HR" value is 48", you are limited to 48"/3, or 16" to move your ceiling joists up above the wall top plate.
If you could rate my answer, that would be great. Thanks