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. Collar ties should be placed in the upper third of the rafters near the roof’s ridge. The same problem does not present in a trussed roof because all of the upward support is located at the outsides of the triangle. 'Taking time off work tomorrow so that I can close off the roof because of coming rain and snow,' David noted in. ![]() 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. It forms part of a continuous load path to resist wind and seismic forces. This video will provide you with a few illustrations and three-dimensional images on how you can attach the roof framing for a home addition to an existi. 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. A rafter tie aka hurricane tie, provides a direct connection between the roof rafter and the wall framing. The original structure is 24x24 with 6' rafters spaced about 20' apart on a 6/12 roof. 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 can be omitted when suitable rafter to ridge attachment methods are used. Its not an Amish barn raising, but raising your own walls and roof is still a deeply satisfying experience.Interested in learning more - lots of videos to c. Collar ties are generally used when the rafters are continuous over more than one span. Stand stepladders at opposing walls inside the garage. Rafter ties prevent the walls from spreading, collar ties prevent the roof from lifting and separating at the ridge line.
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