uneven slope lenght roof main beam size

[lt]

I’m planning to build a 22’x 27’ (exterior dim) garage with a cathedral celling at the second floor (usable attic for additional storage and possible studio) with uneven roof slope lenght but having the same slope angle; 4/12. On the second floor the two walls running along 27’ won’t be of equal height, one will be 8’ and the other will be about 4’ height. The celling will be made of I beams of 16” x 18’ for the long span and 16” x 9’ for the short span, (all install at 24” c/c) (16” beams are needed for an insulation of R 40). The main central beam (which is not in the center of the 22’ but at about 2/3 of the lenght of the 22’ side) will be supporting all the ceiling I beams. If I want that main beam to be able to run along the 27’ lenght of the garage (- the thickness of the walls=26’) and being the only support of the whole roof ... what size does it have to be and what material shall I consider best; wood or metal ? Should I consider bracing some I beam together in a ‘A’ shape structure in order to give more rigidity laterally ?

Note: weight factor for snow is 60 psf over here. Wind factor is not really a consideration so weight should be calculated for maximum charge. The dim. surfaces of the roof are 9’ x 27’ and 18’ x 27’ both having a slope of 4/12 and will be recovered by standard painted corrugated metal sheets.

Best regards to all of you

 

[LPPE]

When you say ceiling is made of I beams, you mean engineered wood products, such as TJI's, right? Not actually steel W sections?
A 27' span for a wood beam supporting what sounds like a lot of load probably will not work. Maybe a flitch beam, a W14 section.

 

[ishvaaag]

An average wood 12x20 in tall is required to limit long term deflection to under 1 in. This is maybe too intrusive in the attic so maybe you would want.

A W14x30 braced atop every 2 ft seems strong enough and has about 1.25 in deflection or L/260 under my 0.84 klf total service level load, that can work for a roof.

 

[Trussdoc]

This seems like an ideal application for wood trusses at 2 feet on center. That would eliminate the massive center beam, as the trusses could easily span between the bearing walls. The downside is that you would end up with a higher structure (3-4 feet higher at the ridge)than you would with your scheme.

Not that I'm trying to sell trusses....

 

[LPPE]

If you use trusses, I dont think you'll get the deesired cathedral ceiling.