In the photo on the right purlins with bracing are properly installed, but the required collar ties are missing. In the image on the left, it shows collar ties and purlins w/ it's bracing properly installed. Rafter ties (not serving as ceiling joists) that are running above and perpendicular with the joists can sit on the joists and also be fastened to them thus also serving as strong backs to help prevent ceiling joists from twisting and sagging. spacing or every three rafters on a 16" on center spacing. These rafter ties running perpendicular above the joists need to be a maximum of 48" on center apart, which would be every other rafter on a 24" o.c. Rafter ties when not also serving as ceiling joists are required to be at least a 2x4 material and should be on edge. When the ceiling joists are running perpendicular with the rafters then rafter ties are run above the joists in the direction of the rafters and are fastened to them. Ceiling joists serve as Rafter ties when they are running the same direction as the rafters and are tied together with them at the exterior wall. Spreading walls will create a sagging roof. Rafter ties are installed as low as possible as they are required to be in the lower third of the roof frame area to prevent exterior walls from spreading. Also, an engineered truss roof frame does not ordinarilly have a ridge. This is on a conventional stick frame (non truss) structure, as trusses do not require collar ties from a framing material as they are tied together with metal plates / gussets. Collar ties as per current IRC code are required to be at least 1x4(nominal) material but are most commonly at least 2x4's. Contrary to belief collar ties do not prevent wall separation. Collar ties prevent rafter separation and uplift. Collar ties are connections between rafters at the upper one third of the roof area just below the ridge and are required at every other rafter of a 24" on center spacing. Both are horizontal framing members that serve different purposes and are both important to a properly framed roof. Quite fascinating, actually.In the roof frame of a home, collar ties and rafter ties are commonly confused for one another. It's makes more sense when you see an actual Engineer's drawing of the compression and tension forces at work in roof construction. The upward support requirement at mid rafter of a truss is effectively transferred to the outside points via the strutting/web members, bottom chord and gang nails. The same problem does not present in a trussed roof because all of the upward support is located at the outsides of the triangle. A collar tie then becomes effective in negating this force, and they are most definitely in tension. However, as soon as you add a purlin run to the rafter span the upward force of the purlins/struts introduces a new "lateral spreading" force at the centre of the rafters, even though they are securely fixed at the bottom by the ceiling joist. If you added a collar tie to this single rafter span geometry it would indeed tend to act as a compression member, which is not the design purpose of a collar tie. You are quite right in that the ceiling joists will form the tension member of the roof triangle just as the bottom chord of a roof truss does, and prevents the rafters spreading apart at the base. In other words, when the roof construction involves one or more lines of under-purlins and struts to support the rafters in mid span. Collar ties are generally used when the rafters are continuous over more than one span.
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