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.