- Q. Does using purlins
and struts at midspan allow you to cut the roof span in half
compared with what’s given in a rafter table?
A. Yes. Actually, in theory,
a rafter with purlin support at midspan could be a
little longerthan twice
the maximum allowable single span length. This is due to the
effect of moment continuity
across the support. This means that
the roof load on one side of the purlin has a slight lifting
effect on the other side of the purlin.
When using purlins, you must be careful that the struts are
properly supported. Always carefully trace the load path down
to the ground, verifying the adequacy of each element.
To be most effective, struts should be installed as close to
vertical as possible so as not to create lateral forces that
have to be dealt with. This will depend, of course, on the
location of the bearing wall below. And keep in mind that when
the struts get longer than 6 feet, they may require lateral