how do I know what size of lumber do I need for a cathedral ceiling?
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the room is 20 L and 14 8 1/2 W, roof pitch 4/12, the high of the wood wall framing 8′. it is under a slab foundation. I want a cathedral ceiling with a ridge board that is supported at both end by a post that ends on a header, one header is for a 9′ patio sliding door that it takes you from the new addition to he garden and the other is for an opening of 10′ wide from the kitchen to the new addition.
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The rafter sizes can be gotten from tables in the building code, but you may need someone to size the ridge beam and headers for you, if the ridge beam load bears on headers rather than directly to the foundation. I would talk to the building department and see what they are willing to accept when you turn in your plans. If you are not getting a permit, then you may be able to get a beam size from the beam supplier via the lumberyard. There are also beam calc apps out there.
David's advice is spot on. Lumber yards my way calculate loads at no charge when selling the materials.