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Q.I am framing a gable roof with common rafters attached to a 2x12 non-structural ridge board. Must the rafters on either side of the ridge line up, or can they be staggered along the ridge board?

A.Christopher DeBlois, a structural engineer with Palmer Engineering Co. in Tucker, Ga., responds: Rafters can be safely staggered across a ridge board in most situations. Section 802 of the CABO One & Two Family Dwelling Code, "Roof Framing," does not explicitly require alignment of rafters on opposite sides of a ridge board. There are certain advantages to aligning the rafters, however, and unless you have a compelling reason not to, I recommend that you line them up. Not only does it make framing ceilings and collar ties easier, the roof framing simply looks more professional when the rafters align. Having said that, in many cases staggering the rafters does not create excessive stresses in the ridge board.

To satisfy myself that the ridge board will not be overstressed, I analyzed a somewhat conservative scenario. I chose a gable roof at a 6:12 pitch with 2x10 rafters spanning 16 feet from exterior walls to a 2x12 ridge board, to carry design loads of 30 pounds per square foot (psf) live load, and 15 psf dead load. In this scenario the unbraced rafter spans are relatively long, the design loads relatively high, and the roof pitch relatively shallow. I assumed that the ridge board is #3 southern yellow pine, the weakest grade available. Finally, I framed my sample roof with the maximum possible offset; each rafter is 8 inches from the two nearest rafters on the opposite side of the ridge. In spite of all of these conservative assumptions, I found that the 2x12 ridge is not overstressed in shear or in bending, and that predicted deflections are negligible.

Whenever you must splice the ridge board, line up rafters from both sides centered on the splice so that 3/4 inch of rafter material pushes on the ridge on each side of the break. As a rule of thumb, don’t offset rafters under any of the following circumstances:

  • when rafter spacing exceeds 16 inches
  • when the roof live load exceeds 30 psf
  • when the roof pitch is 4:12 or lower
  • when using a 1x ridge board, as permitted by CABO.