![]() ![]() 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. Install collar ties, purlins, sway braces, and other supports as needed or required by code once the rafters are securely connected. Traditional stick-framing techniques can be used to. ![]() The same problem does not present in a trussed roof because all of the upward support is located at the outsides of the triangle. I am constructing a workshop with timber roof. A hip rafter connects to the ridge at a 45-degree angle, as opposed to ordinary roof rafters, which run perpendicular to the peak of the roof. 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. The WFCM defines a rafter tie as a structural framing member located in the lower third of the attic space that ties rafters together to resist thrust from gravity loads on the roof. 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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