Roof rafters without collar ties3/12/2024 ![]() ![]() Ridge straps are not designed in this module, but may be added to control the uplift separation. However, a ridge strap has no effect upon gravity load scenario of the walls being pushed outward. However, a tie in any position will help in both scenarios.Īlternatively instead of a collar tie, a ridge strap can also be used to keep the rafters from separating at the ridge. A tie located in the bottom third of the roof, usually called a "rafter tie" (and which may also serve a dual purpose as a ceiling joist), is most efficient in preventing the walls from splaying outwards under downward loading. Ties prevent the walls from being pushed outwards by the roof under downward or gravity loads.Ī tie located in the top third of the roof, usually called a "collar tie" or "collar beam", is most efficient for preventing the rafters from separating at the ridge in uplift loading. Ties prevent the rafters from separating at the ridge in wind uplift loads andĢ. In the design of a gable roof, ties serve two purposes, as illustrated in the graphic on the right:ġ. Using the calculator involves three essential steps, and one optional step in which the design can be fine-tuned. All load combinations are checked, and both uplift and downward forces are considered. ![]() Eventually someone will cut the ties, leave them out, nail them improperly, etc.The wood roof ties calculator is capable of designing the ties and rafter for a gable roof having any combination of collar ties, rafter ties, or ridge straps. ![]() Honestly if this was a real design my advice is always keep it simple and design those rafters to bear all the weight by themselves. But with unbalanced loads I suspect the heavier side will transfer some load to the lessor loaded side. If your loading on each side of the roof is balanced than it will be in either compression or tension. As far as the collar tie a lot depends on the loading. Never mind sorry I see that your wrote that it is a steel ridge beam so I assume it is structural. A frame trusses simply do not have enough stiffness. And I suspect it happens a lot more than people think. I can think of at least four instances where I have seen this. I've seen far too many A-framed rafters or trusses thrust the tops of exterior walls outward. If the ridge beam is not supported than you need to have continuous ceiling joists. I'd like to reiterate what another poster stated already. RE: Roof Rafters Question 1503-44 (Petroleum) 14 Jun 21 21:28 A birdsmouth at the top is not a good idea as it will greatly reduce the shear capacity of your rafter. I would add triangles (beveled plate) below the rafters at the steel beam as noted on your sketch. So, your rafter span would be the horizontal distance from the wall to the steel ridge beam. Also, as noted above, the rafter span is typically taken as the horizontal projection of the rafter. Your detail shows a steel ridge beam, so I would assume that beam is sized to carry the vertical loads at the peak and rafter thrust is not an issue for you (therefore no rafter tie or ceiling joist is needed). A collar tie is not required if other means are used to prevent separation due to uplift (like a ridge strap). It is not for dealing with rafter thrust, that is a rafter tie or ceiling joist, which is required to be in the lower 1/3 of the rafter (again per the IRC). In general (and using the definitions used by the IRC), a collar tie is placed in the upper 1/3 of the rafters and is there to prevent issues with separation due to uplift as noted above. ![]() There have been multiple posts on the subject of collar ties (and the differences in verbiage in different parts of the world). ![]()
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