Hello there; It is important which two elements you choose, in the section with triangular geometry, purlins can be thrown. First select the beam that you did not mark in the image, then select the right triangle long arm. It will fall in love as in the picture. In order to throw a purlin by selecting the two beams you have marked, you must apply the independent selection instead of the perpendicular selection to the first piece in the offset section from the purlin menu. Good work.