Framing of residential home cathedral ceiling
Framing of residential home cathedral ceiling
(OP)
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: http:// files.engi neering.co m/getfile. aspx?folde r=3001d7bf -1483-46f6 -abad-307f 70feb517&a mp;file=00 000001.JPG
Any thoughts would be appreciated.
Thanks!
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: http://
Any thoughts would be appreciated.
Thanks!






RE: Framing of residential home cathedral ceiling
RE: Framing of residential home cathedral ceiling
RE: Framing of residential home cathedral ceiling
RE: Framing of residential home cathedral ceiling
Mike McCann
MMC Engineering
RE: Framing of residential home cathedral ceiling
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RE: Framing of residential home cathedral ceiling
With or without the vertical member, the structure can be analyzed and appropriate connections can be made so that it is structurally capable of carrying all loads applied.
BA
RE: Framing of residential home cathedral ceiling
RE: Framing of residential home cathedral ceiling
Aloha bruddahs,
Keawe
RE: Framing of residential home cathedral ceiling
What you WILL have is horizontal displacements at the top of the bearing walls. The magnitude will depend on the stiffness of the trusses.
BA
RE: Framing of residential home cathedral ceiling
I ran some numbers on this last night and was finding some rather large horizontal deflections at the truss supports. In addition, the connection design would be a bear. In the end I think it may be easier and cheaper to either buy trusses or purchase a structural ridge beam.
Thanks again for your thoughts.
RE: Framing of residential home cathedral ceiling
Trusses would be, cheaper, 100 x's easier to construct, and make for a much neater job since making a ceiling like this is not as easy as it looks.
Getting a nice flat ceiling using less than perfect lumber can be a real bitch.
It's just a waste of material.
RE: Framing of residential home cathedral ceiling
I think you will find that you will need a larger member for the bottom chord than the top chord. I quickly modeled your "truss" in RISA 3D. It indicates (if my model is correct) that a 2x6 SPF #1 BC won't be anywhere near adequate. I used a balanced snow load (Ps) of 42 psf which is come for VT. Another thing to watch is there are some pretty substaintial loads at the joints. It will take more than a handful of nails or single bolt to resist.
I quickly added a web member from the peak to the intersection of the BC's and that reduced the bending moment considerably in the BC. I can't spend anymore time to convince myself why this is.
I also happen to have a scissor truss design from a company out of Canada sitting in front of me. It has a 2x8 BC and a 2x6 TC. Some of the joint loads are huge, 6000# plus. 32' bldg, 4' o.c. truss.
RE: Framing of residential home cathedral ceiling
It is not obvious to me why the bottom chord should have large bending moments with or without the suggested king post. Except for ceiling load, the bottom chord is a tension member. I would expect the top chord to be subject to significant bending moments and thus larger than the bottom chord.
BA
RE: Framing of residential home cathedral ceiling
I still contend that any time spent on this will ultimately be seen as a waste.
Get the trusses!!!!
RE: Framing of residential home cathedral ceiling
I will say, I see truss companies model scissor trusses with pin-pin support conditions regularly. I think they should be modeled closer to a pin-roller configuration unless we account for the thrust load. Tough to do on top of a wood frame wall with a vaulted ceiling. As BA pointed out.
For the detail shown in the OP, I think in a pin-pin configuration the bottom chord will load in compression and maybe be larger than the top chord. Check the support reactions, can you truly resist that horizontal load?
RE: Framing of residential home cathedral ceiling
In any case, my contractor friend called a lumber yard today and they designed a structural ridge beam for him. The total price was $250 for the beam (2-14" deep LVL's). In the end this was a much simplier solution than engineering the full roof system.
RE: Framing of residential home cathedral ceiling
But your assembly isn't a series of pin connected members. Both the top and bottom chords are continuous through the joints. This is also the reason for the moment in the BC.
RE: Framing of residential home cathedral ceiling
BA
RE: Framing of residential home cathedral ceiling
Use a nice deep rafter for insulaating and you'll have a nice wide open space.
RE: Framing of residential home cathedral ceiling
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.
RE: Framing of residential home cathedral ceiling
RE: Framing of residential home cathedral ceiling
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.
RE: Framing of residential home cathedral ceiling
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.