Abstract: Through explanations on the following 4 aspects： 1） a staggered layout of sag-rods in adjacent purlin bays eliminates nearly the axial rigidity of the sag-rod system； 2）the distortional deformation occurred on the sag-rod cross-section changes and increases the normal stresses of purlins， leading to difficulty in predicting the strength of purlins； 3） the lateral deformation of the purlin mostly away from the anchorage point of the sag-rods of the purlin system is very close to the deformation of a purlin without sag-rod； 4） the demand on the rigidity of the sag-rods in a parallel system increases parabolically. The conclusions were drawn that the current layout of sag-rods had little effect on the lateral internal forces， lateral deformation， and bending -torsional stability of purlins. Several suggestions were proposed， including stringent demands for the existing sag-rod system，the use of C sections to provide torsional restraints to purlins， the use of small angles instead of sag-rods at the lower flange of the purlins.