Ridge Beam & Statics?

[ponderosae]

Perhaps a maximum allowable deflection would result in creep, at least where the framing isn't as balanced, and that supposedly lowers the amount of force necessary to shift a wood structure. I'd be more concerned about the ridge than the walls, where a ridge beam is lacking, or what would happen if one was under built, over loaded, etc.  It was more of a hypothetical question for me though, to consider the direction of forces involved (whether visible or internal, considering the properties of wood as well, which may facilitate those forces changing direction, and that's one reason why wooden moment frames are said not to work well, hypothetically that is).

Practically, I'd expect this to be more of a 'this old house' issue, where rafters may be undersized by current standards, and shear walls are lacking. It could be that some deflection in rafters merely neutralizes the outward force on walls, although a sagging ridge line seems to be a common problem with older roofs.

Or maybe it's common in ones built with larger rafters too. An article I was just reading gives me this impression.

Cathedral Ceiling & Roof Framing

"Inspecting a building constructed without rafter ties and that also lacks a structural ridge beam, we will often notice first that the ridge has sagged downwards, mostly in the center between the gable end walls...

Finally, when a cathedral or a vaulted ceiling (or any gable roof) is framed without either an engineered structural ridge, ceiling joists or rafter ties, or an engineered solution, some amount of ridge sag and wall kick out is guaranteed. In a worst case scenario such as the record snow loads of the winters of 2011 and 2015, the roof could collapse."

Since the rafters don't usually deflect a great deal, it would be more common for a roof ridge to sag in conjunction with the walls pushing out (as they say), which puts more force on a non-structural ridge board, and that sags in the middle as the rafters flatten out on top, pulling the ridge with them, I suppose (since nobody seems to say why it sags in the middle otherwise—it would seem strange to me if the rafters all butted against each other, without a ridge board, and only the middle ones sagged at the ridge line, because if they don't deflect much in the middle of their span, they certainly wouldn't be deflecting at the end of it).

I was actually deflecting some plastic straws that I taped together in order to observe the opposite reaction, and they can pull inward or tip free-standing 2x4 'walls', which are relatively heavy compared to the straws, so I think it's mostly about how stable the walls are, which determines that the roof would collapse before the walls, whether they were pushed or pulled on by rafters (and those are apparently much stiffer than straws)!  Yeah well roofs are silly that way (as if it was ever a good idea to leave out the ridge beam, and it's like the only beam known to man that gets misunderstood).

Something else I understand about it now (after reading the article above, and so on) is that it isn't merely a beam that sits under the rafters, because if those are not cut in a specific way to fit over the beam, it could split them apart, probably with much less force than it would take to make the roof fall in without a beam there, and the rafters also stabilize it laterally when fit around the sides of the beam. Otherwise, the rafters may be strapped over the top to keep the beam from splitting them apart, but this would still be testing the strength of that connection more than anything, so a proper fit is necessary for those to work together, instead of against each other (because I think as a general rule, you can't put a support in between a joint without splitting it, unless you fit the framing around it or reinforce that area).