Hi, I think I have the answer to your question. Your roof has a very low slope and the rafters are channel-shaped with purlins resting on top of them Assuming that the roof is designed and constructed properly, it is likely that the current setup will be sufficient. The purlins resting on top of the rafters will help to distribute the load of the roof sheet evenly across the rafters. Besides, the low slope of the roof means that the wind load acting on the roof will be relatively low.

The member designated as 'rafter' on the drawing is not a rafter; it is a beam. Correct me if I'm wrong, but a rafter is a sloping member describing the slope of the roof. In the present case, it's too flat to use the term rafter at all; it is just a beam.

Please note that the penalty for double posting on Eng-Tips is to be boiled in oil!

Hi! I think that it is not clear from the information provided whether the current roof structure is sufficient for the intended purpose, and it would be best to consult with a structural engineer to ensure if the roof fits structurally and fullfill with local building codes and regulations.

In general, the design of a roof structure depends on a variety of factors, including the size and shape of the roof, the expected loads (such as wind and snow), the materials used, and the local building codes and regulations. Having rafters in both directions can provide additional stability, but it may not be necessary depending on the specific design and conditions.

It's important to note that even small changes in the roof slope or structural design can have a significant impact on the roof's performance and overall safety. It's always best to consult with a licensed professional to ensure that any changes or modifications meet local building codes and regulations and are structurally sound.

I hope I help you! Good luck :)