Warm roof with corrugated metal sheets

[Kombi Camper]

I am about to start building a timber frame outbuilding but am slightly going round in circles on how I achieve the look I would like. I want to clad the external walls in black corrugated metal sheets and I'd like to do the roof to match. I could do a rubber roof but it's not really the look I want (rustic ish). 

I am hooked on the idea of a warm roof for all of it's benefits (easy to lay whole sheets of PIR, vapor barrier, less thermal bridging etc). But I don't know how I can fix the metal sheets to the roof without screwing battens all the way through the sandwich (osb, vcl, pir, osb) into the joists below. 

The building will be a place for the family to hang out, cook, log burner etc so I wonder if I am over thinking/engineering the build. It will be occupied and heated sporadically so I wonder if I shoudl just do a cold roof and out the PIR between the joists, min 50mm gap above the PIR to the sheet and vented front and back. I have a worjshop with a metal roof and it drips a lot when there's very low temps, - that's probably why I am hooked on the warm roof too. Any advice would be gratefully received, even just ideas. 

[ProDave]

Same as with a tiled roof.  Counter battens then battens over the membrane and fix your wriggly tin sheets to the battens.

[Kombi Camper]

Just now, ProDave said:

Same as with a tiled roof.  Counter battens then battens over the membrane and fix your wriggly tin sheets to the battens.

so just screw the battens down through the sandwich to the joist? I realise that I will puncture the VCL doing that but is it really a big deal in this scenario where it's basically an occasional room?

[saveasteading]

53 minutes ago, Kombi Camper said:

screw the battens down through the sandwich to the joist? I realise that I will puncture the VCL

If you are using a composite pir and metal panel, then yes you screw straight through. Its not like tiles though in that you are not leaving any ventilation gap underneath or the insulation wold become pointless.

These panels are also the vapour barrier and you don't need another underneath. Using the correct screw it  seals its own drill hole.

Which direction are the joists? If up the slope, then you will need counter-battens.  BUT these panels span  a long way so you don't need many supports or fixings.

[Kombi Camper]

10 minutes ago, saveasteading said:

If you are using a composite pir and metal panel, then yes you screw straight through. Its not like tiles though in that you are not leaving any ventilation gap underneath or the insulation wold become pointless.

These panels are also the vapour barrier and you don't need another underneath. Using the correct screw it  seals its own drill hole.

Which direction are the joists? If up the slope, then you will need counter-battens.  BUT these panels span  a long way so you don't need many supports or fixings.

I'm not using insulated panels. I will lay 18mm osb on top of my joists (rafters) then a vapour barrier, then 100mm PIR foil backed both side, then finish off with 11mm osb (I think I can dispense with the 11mm osb as I'm not having a rubber roof so I don't need a nice smooth surface). My question is about screwing through that sandwhich to fix my battens down into the joists (rafters). Hope that makes sense. I know it's not good to puncture the vapour barrier but I'm 55 and only need the building to last 20 odd years if I'm lucky 

[saveasteading]

14 minutes ago, Kombi Camper said:

I'm not using insulated panels

Ok. It was the term sandwich that i picked up as being a metal/pir sandwich.

 

A screw through a batten and then the vapour barrier on osb etc does not cause a leak.