#Roof rafter calculator how to#
#Roof rafter calculator free#

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.

#Roof rafter calculator free#

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.

#Roof rafter calculator how to#

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.