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Framing of residential home cathedral ceiling

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71corvette

Structural
Feb 26, 2003
105
A contractor friend of mine has asked me to evaluate a conceptual roof framing idea he’s come up with for a single family residential home he’ll be building in Vermont. Most of the roof framing will be very conventional, but one portion of the home will include a cathedral ceiling that spans the full width of the house (25’-6”). Rather than purchasing trusses he’d like to stick frame the entire roof. To do this he’s come up with a framing system that I’ve not seen constructed before (detail attached); essentially he’s proposing to use the rafters and ceiling joists together to form a scissor truss of sorts.

I plan to run some numbers on this concept tonight, but in my experience I’ve seen mostly trusses or structural ridge beams used in these types of applications. Given this I thought I’d solicit opinions and feedback from those of you here who may have dealt with similar circumstances in the past.

Detail:
Any thoughts would be appreciated.
Thanks!
 
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I dont think "framing by hand" is a waste of time. Hanging rafters is the way roof framing is done in rural VT more often than not for any type of custom or moderately high-end home. Labor here is cheap- younger framers make between $12 and $16 an hour. I understand the argument for trusses in urban areas or states with a higher pay scale, where a minimum wage factory worker can fab a truss cheaper than a prevailing wage carpenter can hang a rafter, but up here it doesn't really work like that.

Furthermore, you dont really need a structural ridge if you have collar ties (i get it, these arn't really collar ties), and you dont need ties if you have a structural ridge. Assuming you have a bearing upstairs (cutting down on ridge span) since this roof detail is only "for a portion of the home" just size your ridge beam correctly and you wont get much deflection and wont have to worry about your walls deflecting. Or just balloon frame it and there you go, you have ties anyway in the floor.

A scissor truss is cheating for a cathedral ceiling - true cathedral is much nicer when you have such a shallow pitch roof.I agree with Stillerz- bett just throw in a nice ridge beam and beef up your gable and make sure that connection can resist the huge uplift from all that tributary area.
 
Another benefit to stick framing is fire resistance. As an engineer who also is a volunteer fireman...use sticks! Ever try to cut a ventilation hole in a roof framed with trusses constructed of 2x2 fire treated toothpicks! Scary even at my weight!
 
I could see the bottom chords in compression if you model this as a pin-pin support and not a pin-roller.

I could also see the bottom chord in compression because it connects directly to the top chord. I think this would be similar to the reason that collar "ties" are often actually in compression unless they're a foot or so off the wall.
 
What brings to my mind that every kind of practice ends by showing its limitations. I was reading two weeks ago graphics statics and found myself at some difficulty (lack of patience has much to do with this thing, and we are being bombed daily with too much ... wasteful activity) following some of the cases (forces, construction, etc).

So, that's the beauty of the modern programs you can get soon and even check joint equilibrium etc ... yet we are forgetting abilities we had. To bring ahead the full ship of knowledge is demanding, and more in troubled seas.
 
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