I believe BA bought in in kit form from his local timber merchant. Instructions not included
The wall studs will be 3x2 or 4x2 (in inches) timber, cost about a quid per metre.
You can buy chipboard flooring panels from B&Q for about a fiver, about £35 worth will cover a floor the same size as BA's. Or plywood (better) will cost about £80
A floor shouldn't need to be weatherproof, unless you mean laying the felt on the ground with the chipboard on top, in which case no. The ideal base would be 4" concrete, or concrete/brick pillars supporting the timber frame.
i planned the shed around the space avalable shiplap size and 8x4"sheets to minimise waist
9 sheets of 18mm exterior ply do both the floor and the roof
all the structural timbers are 3x2" [only treated for the floor joists ]
the spacings are 2ft centres for floor and walls
and any where between 2 and 4ft spacing for the roof
if you can join screwfix and look for me on carpenrty i can give you the thread that discusses all facets
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