Beams only have a form of out-of-plane instability.There has never been such a thing as in-plane instability for beams. In the case of columns, when there are axial forces, the out-of-plane and in-plane lengths are different, so there are in-plane and out-of-plane instability calculations.Rigid beams, although it is called a beam, more or less always have a part of the internal force is axial, so it should be used strictly speaking, the model of the column, that is, according to the compression bending member of the in-plane and out-of-plane have to be counted as stable.But when the roof slope is small, the axial force is small, can be ignored, so the model of the beam can be used, that is, do not have to calculate the in-plane stability.The meaning of the door code (P33, Article 6.1.6-1) is that when the roof slope is small, the inclined beam members only need to calculate the strength in-plane, but still need to calculate the stability out-of-plane.