Purlin size

[jake07]

Hi has anyone came accross a 4"x2" purlin running on flat supported every 400mm in a dormer 45d pitched roof. I am alittle concerned over its strenght properties. Can anyone advise

 

[dik]

If I understand your post, this type of arrangement is typically used to provide lateral stability to the framing members in lieu of sheathing.

Dik

 

[jake07]

Yes this is offering support to rafters and inturn slates.However, as to how much support and how capable it is, is the question. A normal arrangment would be 9"x3" vertical purlin

 

[dik]

I don't know how much support is provided... likely determined by the fasteners... but 2x4 compression members spanning 16" or 24" likely have a capacity of several thousand pounds... far more than the fasteners. The nailing likely has a capacity of 300 or 400 lbs @ 16' or 24"... sufficient to provide lateral support.

Dik