a flat roof

huangyhg

a flat roof.
i have a house that is approx 988 sq ft (38x26wide) and has a flat roof. i want to change the roof of the house and plan of changing the roof with a pitch roof with bolted ceiling. my question is that what would be the size of the beam and span and what size of rafters and span i need.
thanks for your help
aqe1
hi!
your answere depends on what material you intend to use, how many columns and with what span you got to put the trusses on them, what degree of inclination is desired for the roof and how do you want to cover the pitched roof. there are actually many choices, from a wooden truss, to tappered steel girder or steel trusses. you can do covering by means of masonary material or steel or wooden sheets. you may also use sandwich pannels for covering. whatever your choice is, you will be needing detailed calculations and drawings that are approved by an authorised body. just avoid instant answeres as possible loss will cost you much!
i am planning to use wood, 2"x10" ridge and 2"x8" rafters  on 16" centers without ceiling joist because i want to have vaulted ceiling. now, my partition inside the existing house is not in the middle of 26'-0" width of the house. the partition that run from front to back is located on 12' from left of the house, if you are facing it, not centered. now, i noticed that the highest point of the roof is not centered either between the 26' width dimension.
if i understand the roof system, you want a vaulted (cathedral) type ceiling without ceiling joists.  i don't know your location, or loading requirements.  
do you have a 'collar' tie of any kind? or some element that prevents the supporting wall from being forced outwards?
without these, if you have a significant load (snow, for example, but there's likely not much of that in las vegas <g>) the 2x8@16 seem light for the span.
in our environs, because of insulation requirements, a couple of inches of airspace is recommended above the insulation to allow proper ventilation.
aqe1...i agree with dik that you need a collar beam.  this will give a flat appearance to a small part of your ceiling, though still "vaulted".
have you considered a vaulted "scissor" truss?  this gives vaulting, but at slightly different slope than the exterior surface of the roof (interior is obviously shallower slope).  this would be an efficient way to do what you want and the trusses could be spaced at 24" o.c.
ron
a roof with vaulted ceiling is not something you want designed in a chat room. even scissor trusses cause outward thrust on the walls. spend the money and hire a registered structural engineer to do the design and (especially) the details. make sure he/she is experienced in wood framing/residential work.

daski-
unless the type of scissor truss you are thinking of is different than the one i am, the truss reactions will cause negligible outward thrust on the walls.
i agree, get a structural pe to design your new roof. there are other considerations to take into account, if you have snow load and decide on the scissor truss design, make sure you get plenty of fascia vents for the roof, otherwise you'll get a flooded living room from the snow melting.
good luck.
ice damming is a common problem up here... to try to eliminate it, the main issue is that the insulation in the ceiling extend past the insulation in the wall (slightly) and that a suitable air space be left between the insulation in the ceiling and the roof sheathing.  there are proprietary thin 'corrugated' plastic foam 'sheets' that allow this venting to occur because of the reduced depth of the truss in this area.
anybody, is there someone (licensed structural engineer) you can refer? that would be a big help for a start.
i am located in west los angeles, ca 90025
also, i am getting tips from someone, saying, just run the
the inside partition of the house (front to rear) all the way to the roof and put double plate where the rafters of both sides of the roof will seat.
again, i appreciate everbody's reponse, that's a big learning experience on my part.