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rafter and ridge beam size on a cathedral ceiling

rafter and ridge beam size on a cathedral ceiling

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  #1  
Old 10-26-08, 12:07 AM
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rafter and ridge beam size on a cathedral ceiling

I'm adding a room to an existing structure in Southern California. Size, 15' by 23'. The roof will be 4:12, asphalt tiles, with an open catheral ceiling on the inside. The ridge beam will run the 15'. How do I size the rafters and ridge beam.
 
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Old 10-26-08, 08:41 AM
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Welcome to the forums! It would be difficult, if not impossible, for us to determine the answer to your question. It is always advisable to engage the advice of a structural engineer to make sure all the "gotcha's" are covered in your situation.
 
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Old 10-26-08, 02:36 PM
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While not impossible, it is difficult to give your what you need with the limited info you have provided. But ill get you started.
Ill assume that the gable end of your structure will be 15 feet across. To determine the unit line mesurement, you take the known mesurement run (1/2 the span) 7' 6'/90". Use Pythagrean to determine the "theoretical" length from the center of the ridge(thats why its theorhetical) to the seat of the birds mouth cut. If you succesfully do this and still wish to continue, respond in this thread and give me several more details.

How much projection from the exterior wall is your fascia(over hang)?

What building code must this adhere to. The size of the rafter (2x4,2x6) will depend upon snow loads. Or maybe you need hurricane staps..

More info would be required to give you and accurate cut list, and even if im theoretically perfect, this is a reno, your house isnt.

Hire a carpenter.
 
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Old 10-26-08, 08:00 PM
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So in essence, you are saying it is "impossible" to determine the answer to his questions, right?
 
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Old 10-26-08, 09:17 PM
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Clawfirst, the measurement run is about 13ft, this includes the length from the ridge to the wall plate, then 2 ft. overhang. The slope is 4:12. Snow is not a factor because it's in L.A. Not sure of the buidling code because I'm not a builder. I'm doing the plans myself. If I guess it and draw a 2 by 10 ridge beam and 2 by 8 rafters at 24" o.c. do you think the engineer will flag it when I go through plan check. The wieght calculation has to include roof shingles and 1/2" drywall on the inside. This may be a way to do it. Let me know what you think.
 
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Old 10-27-08, 04:01 PM
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Welcome Jerry,

In my area I would have to use 2x12 in order to get R-38 insulation value required by code. You could get away with 2x8 rafters for the span you are talking about, but as Chandler suggests, I would get an architect or engineer to at least review the structure.
 
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Old 10-27-08, 06:33 PM
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Originally Posted by Jerry4B View Post
Clawfirst, the measurement run is about 13ft, this includes the length from the ridge to the wall plate, then 2 ft. overhang. The slope is 4:12. Snow is not a factor because it's in L.A. Not sure of the buidling code because I'm not a builder. I'm doing the plans myself. If I guess it and draw a 2 by 10 ridge beam and 2 by 8 rafters at 24" o.c. do you think the engineer will flag it when I go through plan check. The wieght calculation has to include roof shingles and 1/2" drywall on the inside. This may be a way to do it. Let me know what you think.
Lol what i think doesnt matter. Buy a building code book. They cost $160 where I live(canada), and probably not a heck of a lot more where you are. There are so many differences in our codes i cant fathem what will pass there, or fail. 2x8, and 2x10 ridge is a massive overbuild. I cant stress enough that our codes are different and i dont know what will pass where you are. you guys use PT to strap masonary, that fails here(PT+fire=everyone dead). Buy a code book!!! If you need help interpreting what it says.. im listening.

And to the next poster. I would never, ever do a cathedral ceiling like that(unless you pay me enough). How do you vent that adequately?? I know it will still pass, but you know it wont vent well. Drop cathedral cieling is the only way to go.
 
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Old 10-27-08, 06:49 PM
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I'm just an old southern nail driver, so don't take me too seriously. But "PT+fire=everyone dead"??? What is the fear? That ACQ gives off toxic gasses in the crawlspace or basement, basically under the living space? I would really like a banter on that one. I mean, drapery+fire=quick acceleration.....or gas line under house+fire=big fire....forgive me.
 
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Old 10-27-08, 07:02 PM
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I'm with you chandler, pt doesn't give off any more toxic fumes than furnishings "carpet, drapes, furniture with foam cushions,etc" so where does that come in?
also the latest research i have read about venting for roofs is that color of roofing material matters more than venting as far as heating of the attic space, of course if you are talking about control of moisture moving up through the insulation then most new construction around here calls for controlling that from the inside to keep moisture out of the insulation.


life begins when the kids leave home and the dog dies
 
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Old 10-27-08, 07:22 PM
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Originally Posted by chandler View Post
I'm just an old southern nail driver, so don't take me too seriously. But "PT+fire=everyone dead"??? What is the fear? That ACQ gives off toxic gasses in the crawlspace or basement, basically under the living space? I would really like a banter on that one. I mean, drapery+fire=quick acceleration.....or gas line under house+fire=big fire....forgive me.
Was only making a point about code differences.If you are in a house and the framing is burning, you are all ready dead. And yes, i grabed may span charts and Here Id have to use at least a 2x8 #2 grade rafter.

Is this a bisecting gable,Thus creating a valley? or extending the existing gable?
 
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Old 10-27-08, 07:31 PM
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Originally Posted by Speedwrench View Post
I'm with you chandler, pt doesn't give off any more toxic fumes than furnishings "carpet, drapes, furniture with foam cushions,etc" so where does that come in?
also the latest research i have read about venting for roofs is that color of roofing material matters more than venting as far as heating of the attic space, of course if you are talking about control of moisture moving up through the insulation then most new construction around here calls for controlling that from the inside to keep moisture out of the insulation.


life begins when the kids leave home and the dog dies
The color , or colour as we say up here will only effect heat transfer to the "cold space" above your insulated cieling to your roof sheathing.
Now lets remember that this is a cathedral ceiling, and all joists will be filled(damn near) with insulation to get adequate R value. So now instead of one "cold space" we have in this case 5 or so seperate sealed spaces. Yet only one roof top vent required by law. The soffet venting will also be restricted if not completely blocked. See what im getting at.
Its nothing id worry about, but i wouldnt do that to my house.
 
  #12  
Old 10-28-08, 04:49 PM
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If the cathedral is insulated properly, baffles are installed above the insulation to allow for the free movement of air from the soffits to the ridge vent. You don't pack the space with insulation. This is a good argument for a larger span, as well as structural integrity.
 
 

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