Ranch house to Cathedral Ceiling
The current issue of FH has an article about making a cathedral ceiling in a ranch house. The house they use has simple roof refters, rather than trusses.
As part of the conversion, the ceiling joists are cut. This has me concerned; perhaps someone could clear up my misunderstandings about how a roof works.
As I see it, a roof presses ‘down.’ Since the roof is suspended over the house the rafters serve to transfer this weight to the outside (and other bearing) walls. This means that some of the weight will be seen by the walls as a force pushing the walls outwards.
I thought that a major function of the ‘bottom chord’, or ceiling joist, was to counter this outward force by tying opposing outer walls together … the force pushing ‘North” is countered by a force pushing “South.” Remove these joists – as was done in the conversion – and I see nothing to counter those forces.
Even the addition of a ‘structural ridge’ (as seen in the article) has me a bit perplexed. Is the author making the roof a glorified umbrella, supported by the center posts alone? If so, isn’t some sort of horizontal bracing needed, to keep the ‘umbrella’ ‘open?’
Finally, I understand ‘truss’ = triangle. A roof of open rafters, with joists, is but a simple truss … a single triangle.
SO … is the article describing a structurally insane practice – or can someone better explain these things to me?
Replies
You might want to search this one. It has already blown up into quite a nasty debate...
great question, I was wondering the same thing. I will be looking forward to seeing a response.
The addition of a ridge beam eliminates the outward push of the rafters. All vectors become downward vectors shared by the walls and the beam. WIthout the beam, the center of the roof can sag and thus the rafter must push outward.
OK ... so now roof loads want to make the roof swing 'in.' I don't see anything in the article to prevent this, but for the (again) outside walls.
Yes, the beam makes it a glorified umbrella.
Bob's next test date: 12/10/07
As DoRight says, once the ridge becomes loadbearing there are no horizontal (in this case outward) forces acting on the roof. The forces on the roof joists are the same in a pitched roof as they are if the roof is flat. The two walls take half the loading, the beam takes the other half.
Whether the beam used in the article is sufficient to take up the loading at the ridge and prevent outward thrust at the walls is another question.
Is it safe to say that this sort of thing ought not be done without first having a chat with an engineer? That 'Joe Handyman" ought to stay away from doing it on his own?
Well up here our Building Code includes span tables for various loads so that you can size joists and beams. As long as there aren't a lot of point loads involved it's not too hard.
I didn't read the article but to answer a few of your questions, a structural ridge in a cathedral roof is there for the very purpose of bearing the load transferred to it by the rafters. It is engineered not to sag from the downward and horizontal loads put upon it thus it will not transfer a horizontal load to the walls so they will not be pushed out. The ridge and the walls now share the load.
Yes, horizontal bracing is needed to the structural ridge, that's one of the things that the rafters do.
Your description of a "glorified umbrella" is a good one.
"Yes, horizontal bracing is needed to the structural ridge, that's one of the things that the rafters do."
I'm not sure I get you here. What horizontal bracing do the rafters do? There is no horizontal force being applied to the structural ridge.
The rafters brace by not allowing the beam to twist under load.
DoRight said it