Mac,
Building the parapet before joisting was one of your bigger mistakes. If your blocks are AAC we would not trust them with though bolts or rawlbolts and masonry to timber joist hangers will be no good due to insufficient courses above hangers plus you are split courses.to achieve a 2.4 storey height with masonry hangers.
Will come back to your joists at the end.
As you have to fall two ways, easiest way to do it is with the primary fall from flank to flank on reducing rips into a box gutter adjacent to flank wall by alley.. Box gutter built into joists, and to either fall front to back or back to front or to centre. Your choice.
For access, maintenance and repair only and a 2700 span, C16 47x150 regularised to 145mm at 600 centres will suffice. These will support 0.75kN/m2 snow load and a 1.0kN/m2 dead load, which is more than enough for your purpose.
To mark out and set out joists.
First joist against flank wall next to neighbours. Fifth joist 2400 centre of off block work same flank wall, then work back with 600 centres on 4, 3, 2,. This will leave smaller centre in 1 and 2, but doing it this way means that your decking and plasterboard will work with no waste or cuts. Joist 9 set 2400 centre from joist 5 and work back same way. Joist 10 set against alley wall and will leave a space of 225mm for box gutter between 9 and 10.
Minimum fall for flat roof is 1in80 but suggest you go 1in60, therefore fall from joist 1 to 9 at 1in 60 = 0166 x4800 = 80mm.cross fall.
In middle of joist number 1 put a packer piece 80mm deep. From the top of this packer string a tight line down and over joist number 9. Measure down from line to every joist and they are your reducing rips to be fixed on top of joists. On the thick rips at top of fall run a bead of Sticks like Shyte along joist before nailing home. You can either cut them your self or order them from a saw mill.
Back gutter. Fall 1in60 = .0125 x2700 = 45mm. Nail a 25x50 batten to the sides of joist 8 and 9. Batten to be level with top of joist at existing house end and 45mm below at back wall of extension. Deck this out on top of battens and will form a dropped back gutter in between joists. Form your aperture for chute and hopper in this corner.
Warm deck to achieve 0.18W/m2K you want celotex TD4000 126mm thick, this includes the 6mm ply bonded to insulation. This will comply for 3 layer felt, but if you want a single ply covering, then you need an additional 12mm ply on top.
Joists either carefully stitch drill pockets in to new existing blockwork and build in or use arbortech saw with plunge blade to form pockets. Hire charge about £70 for week end. Your choice. On existing house wall, plate same size as joists through-bolted to wall at say 600 centres with timber to timber joist hangers
Any thing you do not understand, then sing out.
Regards oldun
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