I have not seen this covered specifically, so figured I would ask.

I have a steel building (zone 4) that I am insulating with rigid foam on the outside (most all of it getting foamular 150 1″ on the exterior side).

I ran into an interesting scenario with a lean-to style overhang (that will be enclosed down the road).  Due to some miscalculations, I need the roof panels to be elevated approximately 1/2″ for the flashing to fit properly.  I figured, why not grab some 1/2 XPS, put it down on the purlins, then I get some insulation, as well as fixing the roof height issue.

2 questions:

1.  Is 15 PSI XPS enough compressive strength for placing directly over steel purlins then covering with steel roofing?  More specifically, will I have issues with permament damage by walking on the roof after installation?
2.  I know there are rules for correct thickness of foam in walls to avoid condensation inside the cavities.  What about roofs?  Will adding this thin 1/2″ layer of XPS be a detriment if I do choose to add insulation to the roof cavities down the road?