How Flat Do the Purlins Need to Be?

I've got the trusses set on a new pole barn. After initially setting the trusses we pulled a string across from gable to gable (about two feet up from the end of the tails) and then slid the remaining trusses forward or back until they just touched the string. It would seem, in theory, that I would now have a perfectly flat plane for the roof. Yea, right. [img]/forums/images/icons/frown.gif[/img] So now that theory has gone out the window, and reality has once again taken control, the question is just how flat do I need to shim the purllins? I'm going to use metal roofing, probably one of the styles that mimic the classic old standing seam roofs. If I pull a string from gable to gable on each run and shim the purlins to it, how close do I need to get? 1/4"? 1/8"? And yes, I know, I'm being anal-retentive, and a perfectionist. But it's a whole lot easier to shim them now than to try to go back and do it after the roof is on.

Thanks,
Kevin

Shim the purlins? I've never heard of anyone doing that.

I would just lay the purlins "on edge" on top of your trusses and nail away. You aren't installing tile on the roof, just metal sheeting.

Just make sure that your truss are correctly spaced, bottoms and tops are straight and that the truss is standing tall, not leaning to one side ...i.e., align them and as "Gator" said, nail away. I am assuming the walls to which you are attaching the truss are plum and flat on top. Oh yes, if you were down here you should make sure the hurricane straps are on in acord with the code.
Leo