I think I am confused by your post. You say you want to weld a plate perpendicular to the flange so you can "screw/nail" the purlin. I presume you have a steel WF shape you plan to weld to, so presumably you are going to weld a steel plate to the WF shape. How would you nail a purlin to a steel plate? Maybe if you post a diagram of what you are planning that would help.

 

Diagram or picture would be great.
I think maybe the 'trusses' are metal and the wood will sit on top and be what a metal roof is screwed to? But then the wood would go right over the metal and not just have 2" bearing so I am confused.

 

Ok, sorry for the confusion. So my trusses are going to be steel beam like a metal kit building but the rest of the pole building is just like a traditional pole barn. My 2x6 purlins are DF @ 10' spans and will butt together on the 4" wide flange of the truss. So I'll weld a plate sticking up perpendicular from the top flange to anchor the purlin. If I do this the 2x6 will only land on 2" or slightly less of the beam but if that is not enough I can form sheet to make an angle and weld it to The flange so I can have the purlin bear on more than just the 2". Did that clear it up at all? Thanks, 1

 

Based on the fact that most joist hangers have 2&1/4" to 2.5"" bearing. I'd cut some 2" angle iron and weld it on each side to give you 8" overall, so 4" bearing.