Steel roof with purlins?

[schiada]

Looking into steel roofs and see the use of purlins and no sheathing. Knowing that steel can condense water would it be a good idea to put the bubble/alum between the top of the insulation and the bottom of the steel with a air gap? This seem to be used on most steel buildings. Or should i just use shesthing? Thanks,Randall.

[MountainDon]

It is my belief that it is better to sheath the roof with 4x8 OSB, 7/16 minimum as in code or 5/8 for a better roof. Then the weather barrier goes on and then the finish roofing.

Note , not all metal panels are rated as structural members which would be needed if you do choose the purlin route.

[Don_P]

If the underside of the metal is not exposed to nightime high humidity air and morning cool temps it will not sweat. Seal the top and bottom of the panels with their foam closure strips between the roof sheathing and the metal. The air gap, or trying to ventilate the underside of the metal is what causes the condensation. Ventilate the underside of the sheathing as normal if using fiberglass and moisture will not be a problem.

[schiada]

Thanks for the input!

Should the holes for the screws over sized for the steel panels? Moving around?

[n74tg]

Will your attic be ventilated?  If so, how will you keep nightime temps from dropping below the dew point temp.

I built my roof as you desire, but sprayed the inside with 2" minimum high density foam, and most important of
all, it is a sealed, unvented attic. 

You can read about it in my blog, address below. 

[glenn kangiser]

Use self drilling fasteners and the holes will be the right size for the screw. 

You can use a 2" UL Vinyl backed insulation that you just sandwich between the sheets and the purlins.  We used to do it all of the time building steel buildings.  There will be a pinch point at the purlin but not a problem and stops condensation problems.