Hi there, Andrew,
Thank you for your prompt about the membrane.
Having given that aspect some consideration, I have the beginning of a scheme in my head. It'll need some exploration on paper and phoning suppliers of material but it goes something like this:
[As they say in the Military, 'No plan survives contact with the enemy!'
1. Pray for dry and calm weather, repeat as necessary
2. Procure some insulation (Kingspan or similar),
3. Procure some timber (aka 'lumber') matching the thickness of the insulation,
4. Strip off the many layers of old roofing felt,
5. Examine the existing T&G boards and make good where necessary (or, strip off old T&G boards and replace with shuttering ply),
6. Lay out a heavy gauge polythene DPC over the roof, stapling to existing structural timbers,
7. Position three new timbers as purlins at top edge, bottom edge and midway up the roof, secure to existing rafters, (timbers to be pre-treated with preservative),
8. Add framing timbers between purlins at suitable intervals to stabilise the geometry, skew-nailed or otherwise fixed so as to not intrude into spaces,
9. Cut insulation to fit spaces in frame, seal cut edges with adhesive aluminium tape and fit in position,
10. Position four 1 metre wide sheets of box section steel sheeting, one sheet overlapping two boxes to adjust overall width to match roof,
11. Install eaves filler strips top and bottom. Attach sheets to purlins. top, bottom and midway, using self-drilling fixings with integral seals,
12. Contrive and attach some form of flashing to direct rainwater into existing gutter. (The upper surface of roof will be some 4" higher than the old one.)
As already stated, 'No plan survives contact with the enemy!'
I'll continue in this thread as it's still tin and still a shed, even if not the same shed!!
Doubtless, pix to follow, watch this space!