Many small fasteners distributing the load over a larger area is a better way to connect the tie in most situations. Wood is not particularly strong per unit of area and has a real tough time with tension perpendicular to grain. The bolts shown are not typically the way a conventional residential roof would be framed. An engineer can design a roof with rafter ties on wider spacings, look at the ridge and/or wall plate as a beam if doing that. The prescriptive provisions of the building code require rafter ties on EACH rafter pair and collar ties every 4′. The upper collar tie does not experience the tension that the lower rafter tie is resisting, if that tension is resisted in the upper third the roof is probably on the ground. The rafter ties are incorrectly labelled as collar ties. Figure 6 illustrates this computation.Ī couple of notes that might help clarify things įig 1 illustrates rafter ties. There is a bit of calculus work associated with modeling the moment due to the distributed load along roof. The mechanics of architecture - free Google Ebook that actually works this problem on pg 121.Īppendix Calculation Detail on Moment from a Distributed Load.18th Annual Eastern Conference of the Timber Framers Guild (Article: Pity the Poor Rafter Pair).

It is not intended to be a tutorial on designing rafter or collar ties. Note that this post reviews the mathematics performed by a licensed structural engineer in a construction industry publication. from wind or asymmetrical snow load) rafter ties are used to counteract the outward thrust of the roof. I also learned that rafter and collar ties have different purposes: collar ties help secure the rafters to the ridge beam during periods of unbalanced loads (i.e. In fact, so many fasteners are needed that the joint may weaken because of having so many fasteners in a relatively small area. The tension is so high that a large number of nails/screws would be needed for each joint. These forces make securing the ties to the rafters an engineering challenge. This exercise has shown me that rafter and collar ties are subject to enormous tension forces. I have duplicated a worked example from JLC magazine. Using basic statics, I have derived the equations used in in determining the tension in rafter and collar ties. Note that this example assumed that the dead load had already been projected onto the horizontal plane.įigure 5: JLC Example in Mathcad. I plugged the JLC example into Mathcad to verify that I get the result that JLC obtained (Figure 5). Where w is the unit loading (lbf/ft) along the rafter, expressed as force projected horizontally per unit length. This equation can be modified to produce Equation 4, another commonly seen result. 3Įquation 3 is presented in the JLC article. This form is correct, but a more useful form for this discussion substitutes the roof slope and height for L. non-rotating) situation, the moment about ridge beam connection exerted by the tension tie must equal the moment exerted by the support reaction and the distributed load (Equation 2). For details on computing the moment due to the distributed load, see the Appendix at the bottom of this post.įor a static (i.e. Purlins are often used in conjunction with rafter ties (see Appendix below for a figure).įigure 4: Free Body Diagram of the Rafter.Moments are summed about a point on the ridge of the roof. The rafter ties can generate tremendous force on the rafter to which it is connected. The number of ties required per rafter will vary by situation. See a structural engineer if you have a real problem that needs to be solved.įor those who are not familiar with rafter ties, I have included Figure 1 as an illustration of what they are and how they are connected to the rafters. Instead, I am reviewing the basic mathematics and physics used in doing this sort of work. Note that this post does not presume to train anyone on how to do structural engineering.

I also noticed that the same equation is also built into an online calculator. I became curious where the equation used in this article came from and I decided to derive it myself. I had forgotten about this old farm building until I saw an article in the Journal of Light Construction (JLC) on the problems associated with using rafter ties on a roof. This also was a sign of tremendous tension. I noticed that steel cables has been hidden behind the rafter ties to try to reinforce them, but the screws attaching the cables to the rafters were also pulling out.

The rafter ties were clearly pulling away from their rafters because of the tension forces they were experiencing when under snow load. I noticed that the rafter ties had pulled away from the rafters they were nailed to. As a boy, I remember seeing a dilapidated farm building that had a set of rafter ties in its attic.

Roof rafter collar ties