Styrofoam over OSB Roof Deck, Box Profile Steel on Top

Hello--I am building a 28' x 12' workshop/"she cave" with 2 x 4 walls, 2 x 6 rafters 16" o/c and 6.5/12 pitch (28.44 degree slope) and can't seem to get a straight answer on installing styrofoam EPS70 insulation on top of OSB roof deck or on the exterior of the building as most structures built with wood in England are covered by brick. I'm from the USA and want to do it the US way, but can't get the products/brands I see available in the US!

For the walls, I plan to clad with OSB/ply, breather membrane, 1" of EPS, batons, then rough wood "shed"siding; for this I'd rather use a crinkly membrane like drain-wrap under the EPS but don't see any available here. Would plain housewrap do?

For the roof, I want an open ceiling inside because of the 8'2" eaves height restriction, I don't have much height inside. I've sheathed the roof with 5/16 OSB and would like to add 2" of EPS on top, then 34/1000 PVC coated Box Profile metal sheets above (0.55mil thick, 1 meter cover each), but I hear they constantly drip onto the deck, so I need underlayments of some sort. What I really need to know is: Is this feasible or is there a better way to do this?

What type of vapor/moisture control membranes do I need, and where in the sandwich they should be placed?
Do I need to use batons/strapping and counter-batons and if so, how should they be laid? I would be very grateful for your sound advice! Thanks.

All sounds good except for the fact that EPS is pretty vapor permeable. Unless you are going to incorporate some sort of vapor control layer to the inside of the framing, I would be more included to want to use XPS here or at least EPS with some sort of facing on it.

Hello and thanks for the reply. The EPS here doesn't come with facing on it, it's straight styrofoam and I can't afford Celotex or Kingspan rigid boards. For the roof, what sort of membrane should I lay on the OSB prior to putting the EPS on top? Should I lay some thin strapping on the OSB over the membrane, then lay EPS? Do I need to cover the EPS then with another membrane? I am still confused as there are so many different types of breather-waterproof products out there.

For the inside walls, I was going to use 2" of fiberglass/rockwool and put clear plastic sheet over, but I've read using plastic isn't a good idea anymore, so I really don't know what to use over the batt insulation.

You could put peal and stick (ice/water) over the roof to air seal it prior to the foam or some synthetic underlayment.

You will want to strap the OSB to give the metal installers and easier go of attaching the metal. The metal can sweat on the backside so it preferable to have some sort of underlayment to deal with any condensation moisture.