I will explain the necessary steps for the alignment of the purlings over the beams:
-after the beams and the purlins are in place, you need to set the eccentricity option on (0,z-) for the beams and (0,z+) for the purlins;
-to align the purlin, you must define a point to guide the alignment. The point must be placed in the certain plane, after you want to align the purlin;
-select the purlin and in the Properties bar set the "Orintation" parameters: for the "Point" option, insert the "1" value (so the purlin will have the Y axis orientated towards the point defined) and the "Angle" must be set on "90" (so the orientation of the purlin will be the on intended).