If there is a ridge beam so that there is no truss action, ties may be
required to keep the roof from separating along the ridge, as in the examples
Jim Kestner and Thor Tandy cited. But, I can't visualize a tension force for
this case and would like to see how it is determined.
I've based my trussed roof designs on there being no need for a strap over
the ridge board to secure the ends of rafters for a tension force. But this
doesn't seem to be the case you have in mind.
It has been common among some So. Cal. engineers to include a strap over the
ridge as a matter of good design, even for trussed systems, without analysis.
I believe that, in seismic zones, there is another force that is often not
accounted for, and can become critical in a steeply sloped roof acting as a
diaphragm bracing massive walls. The two planes of the roof are actually two
separate diaphragms that tend to have incompatible deformations at the ridge,
one displacing laterally and upward, the other displacing laterally in the
same direction but downward, resulting in a shear force across the ridge.
The action of this force would be to un-zip from the downward-displacing
diaphragm a nailed over-the-ridge strap. In trussed roofs for which the
ridge connection has no designed capacity for gravity-induced shear at the
ridge, this may be an important force to consider. I can cite nether failure
examples nor an analysis approach for this effect -- but I do include
across-the-ridge shear capacity in my designs for retrofits of buildings with
trussed roofs that brace heavy walls. Has anyone else thought about this?