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 bracing.