SteelPE,
Yes, I have read some similar points like yours. Actually this picture was taken from a cold roll-reforming company's catalog. In this catalog, it is said that the zed purlin tries to roll backwards when the roof slope is bigger than 10°， while in other cases it tries to roll forward...
Does any one could tell me the difference between these two rod arranging patterns in the following picture? When should the first pattern be used and when should the second pattern be used?
Many thanks in advance.
