After stripping the workshop roof (see Mouldy Workshop Roof), I'm pondering options for improving what I put back on it.

Current construction is a half-brick wall with piers, 4.6m x 4.8m. The roof joists are 6x2 at 60cm centres, and are built into the walls with no sign of any restraints (but these can easily be added...). Roof wall is 1 in 40, which after allowing for the self-weight deflection fo the roof will become 1 in 60.

I'd like to build a pitched roof, but I don't think I can without going through planning.... Not impossible in future, but I'm not prepared to wait for it right now. And I'd need to keep the neighbours on side too.

So - does this plan sound reasonable?

Pack out the existing structure with timber running at 90 degrees to the joists. 6x3" at the high end, decreasing in depth as we move down the fall. 3" so they've got more base to sit on. This is my preferred solution, for simplicity. It boosts me up to a 1 in 20 fall. I can run the packing pieces through the table saw with a cheap blade to get them to the correct size.

Cover with 18mm OSB3 8x4 square edge sheets (I know they weigh a ton. I've got help), spaced with a 3mm expansion gap.
Fix with Turbo Outdoor PZ 5x70mm at 300mm centers. Or borrow a nailgun.
Batten the side edges to stop drips there.
Apply 1 layer of underlay + felt. Considering torch on, though I've never used it - plus I'd need a torch and gas. Is it easy to get on with? EDPM also a possibility, but concerned about application in low temperatures.