Flat roof, couple of questions

[Jimmers]

Hello chaps.

I'm self-(re)building a small flat-roof porch/extension thing, got blockwork up all ok, but not sure what to do with wall plates: should I have one each on the inner and outer leaf of blocks with joists/rafters on level, then put firring things on top to get a run - or put the rafters onto the inner leaf only with the run built-in and use firrings on the inside to level the ceiling?

I'm planning on putting a lantern thing in the middle with about a metre of roof all around it if that makes a difference

 

[noseall]

Fit the joists level then fix firrings on top.

Fix the wall plate to the inner leaf only as you will have a (holding down) strapping issue if you don't.

 

[Jimmers]

Excellent, thanks