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.
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