Hi.
I live in NE England , am aware of sarking used in Scotland - a friend told me subject of Scottish Building Regs for housing - thermal insulation, his seemed to be a full 1" sawn thick. You will be aware in England - houses are built just using sarking felt & tanalised 2" x 1" battens. I have two brick / block double garage projects - these will be done with sarking timber under concrete tiles , mainly for safe access when tiling and better for security.
I don't know if the Scottish Planning & Building Regs are the same , providing you are not in a "Conservation" area (I am just outside) - you don't need to apply for anything providing the floor area is no greater than 30 m2 , apex height under 4m , eaves under 2.5m , within 1 m of your property (IIRC on your boundry providing flameproof materials) somewhere abt. 15ft off your house. If the curtilage of your property is sufficient - then you can have more sheds / garages providing you don't cover more than a certain percentage of your garded (IIRC 10%)
I decided to do a project IKEA flatpack , 18mm tanalised birch ply , the outside is to be finished with 18mm TG&V tanalised , 2" slab insulation then internal cladding of 18mm x 6" tanlised softwood (fencing boards) , 12mm sarking on roof , fake Welsh slates - the ply takes copper nails OK , the brick garages will have 22mm sarking ply and it takes galvanised tile nails OK.
Floor is just gratis reject bricks from a local brickworks (four layers of brick), I may put a 2" thick tanalised floor in later for better heat retention, there will be a interconnection to a small brick built bothy with a wood burning pot belly for a bit of winter heat.
Founds are only 9" hollows filled with concrete , more than adequate for sub-soil ground pressure.