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I am looking at some tables of allowable roof loads for southern yellow pine: http://www.southernpine.com/app/uploads/AL_27-32L.pdf

Rafters are typically at an angle to horizontal. I'm wondering if the figures for "clear opening" in the tables refer to the length of a rafter or beam between supports, or to the horizontal "projection" of that opening.

In other words, if my rafters are at a 30-degree angle to horizontal, and are 16ft long between supports, is my clear opening 16ft, or is it 16ft times the cosine of 30 degrees, or a little under 14ft ?

  • The figure in the link does not help, as it is for the beam supporting the left-hand end of the rafters, and that beam is horizontal – RustyShackleford Nov 14 at 5:20
  • I thought clear span is the distance between supports. When I order trusses I give them the distance between walls and the overhang I want. Nothing in the middle is clear span, if I have an interior wall that can reduce the price of the truss. But where it may be a 30’ clear in one area only 15 in others because of a hallway and that saves 30-50 per truss. – Ed Beal Nov 14 at 15:22
  • @EdBeal If you give the location of an interior wall to the truss manufacturers, it had better be a bearing wall. Trusses are built with “camber” so they don’t sit on interior walls. In fact, the truss manufacturers show a “slip joint” for interior walls so they do not become bearing walls. – Lee Sam Nov 14 at 16:09
  • I'm not doing trusses, just a simple rafter. – RustyShackleford Nov 14 at 17:37
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Yes, the “clear opening” is the horizontal distance from support to support. (We often call this the clear span.)

In your case, the clear opening is about 14 feet. In fact, the clear span is the horizontal distance from FACE of support to FACE of support. It doesn’t matter the length of the member or if it’s a 4:12 pitch or a 12:12 pitch.

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