The cathedral rafters do not act like a truss member which would behave as you are are assuming with a vertical and horizontal component to the reaction. The loads on the rafters are vertical and the reactions from walls and the ridge beams are also vertical mostly equal and opposite. You need to reconsider your free body diagram. They are only problems if you fail to consider them properly. These issues are not flaws as much as things you must be aware of and design for. So for this I say, the design shown is perfectly reasonable as long as you design for these potential issues. The sort of system I am accustomed to is much more complex, involving cross beams in the walls, bracing, and intermediate support of the ridge so that failure at any one point will not cause the entire structure to collapse.īut obviously the poster is after a design like what we are shown. So if the ridge fails or heavily deflects, not only will the rafters collapse but the walls will be pulled inward so the entire building fails. This sort of design fastens the rafters to the walls, which also serves to brace the walls in lieu of any sort of tying beam. SO if that ridge fails at any point, the entire structure will topple in on itself. The potential problem by this design is the massive span of the ridge without support. The building shown does certainly push the limits of design but does not yet exceed them, though I personally would probably not design a roof structure like that. This is true of you have a poorly designed ridge system. Going back to the potentially catastrophic failure. As such, they are allowed to slide if need be. In central Europe, these forces are further alleviated by forgoing any attachment of the rafters to the plate, rather they extend over them without any type of seat or nailing or any such. What has been shown is a greatly simplified version of the structural ridge roof. The building shown has no such bracing, rather relying on the great depth of the rafters to take care of this. So for us, we would brace our ridge post to the cross beams and to the ridge to handle these forces. Other sorts of loading due to winds and asymmetry are handled by bracing. ![]() So the rafters oppose each other at the peak. We have for every action an equal but opposite reaction. Remember that in roof loading, only the most basic physics are involved (though in a very complicated and dynamic way) so here this is a simple instance of Newton's law. This is easy of course for me, because in German they have completely different and unrelated names (Rafen and Sparren) and are not thought of to be the same thing at all. It is beneficial to think of the rafters in this situation as being a completely different type of timber than are rafters without a ridge support. In practice this is a non-issue, unless of course you have a carpenter who has built such a system with no clue of how it works.Įssentially when you have a structural ridge beam, you exchange your typical means of calculating rafter loads with a totally new set of equations. I must disagree with the point made about the structural ridge not being ideal, due to a single point of potential failure. Of course, I cannot resist a discussion on Structural ridge beams! Each must assess needs and risks and weigh them accordingly. This is in know way a criticism of what other have done. ![]() I would reserve it for when no other traditional framing solution would work. It is also subject to high load forces at the joints and potential failure should any racking occur. Things like variance in lumber weight, wind load, and drifitng snow on the roof can all cause a stucture constructed like this to see horizontal load factors at the ridgebeam and at the eave.Ī ridgepost and ridgebeam construction is not ideal in my mind because it has a single point of catastrophic failure. ![]() Notice I said in theory as loading of a roof is almost never uniform and symmetrical. In a computer model, laboratory or theoretical setting this is possible and basically each side balances the other out and thrust is controlled at the ridgeline, with the eave wall having to carry only one half the shared vertical load per side. To have no horizontal thrust load, perfectly even roof loading must occur. In a structural Ridgebeam and Ridgepost configuration, the ends of the rafters are notched and permanently secured and affixed to the ridgebeam with approved joinery or fasteners.
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