Adding rafter ties to a roof3/17/2024 ![]() ![]() ![]() The ridge beam shown below, at the living room of a 1950s modern-style ranch house in Gainesville, has cracked at mid-span and is slowly heading towards failure after the crack opened further after a recent repair. While the joists commonly attach to the side of a ridge beam, they can also sit on top of it, as shown in the diagram below, which creates an interesting look when the roof structure is exposed as part of the design.īecause a ridge beam often carries a significant area of roof load, they can exhibit structural distress due to a defect in wood, impact on the roof, or use of an undersize beam. A ridge board was not always required and the photo above, of the attic in a 1920s era house, shows a roof framed without one.Ī ridge beam carries the loads of the rafters connected to it and must be both strong enough to carry the weight and well supported at the bearing points at each end, to transfer the loads down to the ground. This means that the ridge board for 2圆 rafters, for example, must be more than the nominal 6-inch height of the rafters, because the diagonal cut at the ridge end of the rafter makes the vertical dimension of the face a larger dimension. The ridge board is usually just a nominal one inch or two inches wide (1x or 2x lumber) and must be tall enough so that the ends of the joists make full contact with it along their face. The ridge board is installed to provide a bearing point where the rafters meet, and it also helps with alignment along the ridge. The difference between them is that a ridge beam is a structural member that bears half of the live and dead loads of the rafters on either side of it, but a ridge board is not structural. ![]() A detailed discussion by theoretical structural guys will help on this, particularly for the characteristics of wood.Yes, either a ridge board or a ridge beam is necessary, and required by the building code, where roof rafters meet at the the center of their span. Put the rafter in steep slope and failure may be wholly compression, not tension. In addition the stress diagram in the rafter under a failure test that may not be pure triangular compression or tension as typically diagrammed for bending. The inclination puts added compression in the rafter, in addition to that caused by bending. When it comes to analyzing the stresses in the rafters, it is not just calling bending the critical factor before failure. Put in more of these at lower heights and you can get away with a lighter rafter, effectively. That effectively reduces the length of the rafter as a beam carrying load. However, under loads the rafter tends to sag and this puts the tie into compression. The writer gives these collar ties a function of tension in case of uplift. Here is an explanation that I don't agree with. I agree with JAE's three options as stated in his post above.īA RE: Collar + Rafter Ties Roof Framing - No Load Bearing walls needed, right? msquared48 (Structural) 21 Apr 20 09:02 The type of wood would not change the fact that a 2圆 cannot span 26'. It is quite possible that the existing roof could support the existing ceiling by means of wire or wood hangers, but this would rely on competent nailing and splicing of the bottom ties (ceiling joists), which may or may not be the case. I do not know the type of wood the rafter ties/ceiling joists are made from-perhaps I can look for a marking.I believe you are correct in the two comments highlighted in yellow. The spanned length is somewhere around 25'. I'm fairly confident it's not needed except possibly to support the ceiling drywall and plaster. ![]() There is an interior wall but believe it is non-load bearing and in-place to provide space for more kitchen cabinets, refrigerator, and oven. ![]()
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