Rafter spans

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I am building a workkshop and have a question about the size of the rafters.

The rafters will be supporting an 18mm OSB roof with an EDPM membrane on top of that.

The span of the roof will be 3m.

I have checked various span tables and they suggest I can use 6by2 rafters. We are not a significant snow loading area.

I just want to check with experts here because when I described this to an engineer (a friend rather than on a professional advice basis) - he said that 6by2 was excessive and that 4by2 would be fine. He ws most insistant about it.

Are the span tables designed for worst case scenario and could I in fact get away with 4by2. Or should I stick with 6by2.

Thanks
 
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I would stick with 6x2, there’s more meat to work with and it will be more stable.

Once you’ve finished constructing you may want to hang items from ceiling, and if you are insulating and plasterboarding these will all add an overhead to the roof.

Your friend can be as insistent as he likes, but it’s your workshop.
 
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As its not to be constructed under building regs you can get away with whatever size you want. Building regs don't what everybody's roof collapsing when those 1 in 50 years snow storms come, in your case no ones bothered except you so its all about your take on risk and what you are prepared to accept. To put it into perspective using 50 x 100 the strength capabilities would be reduced by half of 50 x 150 and deflection 3 times greater. Or, the allowable snow loading would need to be 3 times less so that the 50 x 100 stays within allowable strength/deflection parameters. As Clint Eastwood would say 'You've got to ask yourself one question. Do I feel lucky?
 
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i would be happy with 4x2"
remember to avoid the need for planning it has to be below 2.5m near the fence and 2" will be eating into your head space
 
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I have gone for the 6X2 just to be on the safe side. I dont want any bow in the roof. Costs more but of the life of the workshop it is not too painful.
 

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