Which way should the purlin be oriented? (Detailing Tips: No. 5)

Have you ever considered a purlin’s orientation while detailing?

Pop-Quiz:

In the pic above, the structural engineer shows the purlin direction along the roof slope. Is this the correct way of orienting the purlin? i.e. Has the engineer made a mistake, and if so, why? (Answers below)

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Answer:

Definitely, this is an error on the structural drawing.  Generally, detaliers have to consider a purlin’s orientation before placing it. Why? When the load applies on roof sheets , a twisting moment will occurs on the purlins. If we place the purlin direction along the roof slope then it will cause instability. So, the purlin direction should be always kept against the roof slope (as indicated in pic), even if the  design indicates along the roof slope. Keep that in mind when you are detailing your next job. It’s a subtle but very important tip.