Roof purlin sizing? Calcs and/or where to get them done?

Hi!

I'm putting a roof over a garage. The double pitch roof spans 5.7m with 35° pitch. The rafter length is therefore around 3.5m. The purlins will be around 1.5m from the apex and around 2m from the base of the rafters. The purlins span 4.2m between the supporting walls. They will be made from fresh sawn douglas fir. The roof will be done in clay pantiles.

There'll be one purlin per pitch and a ridgeboard.

Was thinking around 200mm (deep) x 150mm (wide) but don't want to go overboard if that's not necessary. Are there any rough calcs that can be done to see what is most likely needed or are there any quick online sites that will work it out? Could do with ordering the wood asap.

Thanks in advance for any help!

 

Would a single span rafter not be more convenient? Possibly a 175mm x 50mm for instance?
My guess is that a steel beam would be better suited as a purlin. Have you considered roof trusses?
A local structural engineer could design your roof in his lunch break.

 

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