Advice required on warm roof construction with parapet

[rango]

Hello!

I'm planning to build a single storey extension measuring approx. 4.5 x 5.5m and despite scouring the web for answers, I've found very little information concerning the detailed construction of a warm roof where a parapet has been constructed using a single skin of brickwork.

Fortunately, my research also led me to this forum which seems to attract a lot of good advice from some very knowledgeable people, so here's to hoping somebody can clarify a few things for me!

Working from bottom to top (please refer to attachment):

Joists
I've calculated the maximum span across internal leaves of blockwork to be 4.42m, hence I've selected 200mm joists.

Joist hangers
I intend to use 140mm joist hangers as illustrated. I also inserted a row of coursing units to accommodate the joist hanger at the desired height.

Firrings
I'm aiming for a fall of 1:60 (based on a length of 4.5m), giving a maximum height of 80mm to the firrings.

Timber subdeck
I plan to use 12mm OSB3. I believe this should a) float on top of the firrings (i.e. not be secured) and b) terminate 25mm before the start of the blockwork.

Insulation
I have calculated that in order to meet current building regs, I will require 120mm of celotex insulation or similar.

Unfortunately, this is where things starts to get a bit messy as I'm unclear at what point I should span the blockwork and beyond. Ultimately, when this does occur, I see a problem. There will inevitably be some 'building up/down' to do where the material spanning the blockwork does not sit flush with the top of the blockwork itself, for instance. I'm struggling to see how this can be achieved?

I understand that I will also need a vapour control barrier (not shown on this diagram) directly beneath the insulation with the foil facing down. Does this requires bonding with the top membrane to form an airtight parcel of the insulation or can it just be lapped underneath the upstand? I presume the upstands will need insulating too?

Timber deck
Should I leave a 25mm gap between the upper timber deck and the single skin of brickwork forming the parapet?

Waterproof membrane
I have opted for an EPDM covering. I understand this should lap under the flashing as pictured?

I appreciate I'm asking a lot here, which is why I've tried to break things up into sections and explain things as clearly as I can. I'm not expecting somebody to address each and every one of my queries (although that would be nice!). I'd be very grateful for any assistance, even if it's just to point me in the right direction where I can hopefully find some answers.

Thanks All!

[rango]

Joist hangers
I intend to use 140mm joist hangers as illustrated. I also inserted a row of coursing units to accommodate the joist hanger at the desired height.

I've just realised that my joist hangers are sized incorrectly. They should be 190mm. I'm still learning!

[rango]

Further to my previous posts, I've made a few changes to make things a little more readable. I appreciate the annotations on the first image were a bit small, so I apologise if that's what put people off responding. Furthermore, I'll try asking some more direct questions...

1) Do I need to leave 25mm expansion gaps between both OSB3 decks and the parapet wall?
2) Should the firrings terminate in line with the end of joists or extend beyond?
3) Do I need to insulate the upstand? This doesn't appear to be necessary IMO
4) I assume all voids beneath the subdeck should be packed with mineral wool?
5) Do I need a vapour barrier on top of the celotex too (belt and braces)?
6) Are cant strips required? Aren't they at risk of falling down the expansion gap?
7) Any other issues?

[wes56]

wrong. thats not how to build a flat roof.
why the parapets? they are usually lots of trouble over time.so are tile creasings.

[rango]

Hi Wes. I appreciate your honesty. We’re in a conservation area. The adjacent garage is built in this way. I’ll upload a picture tomorrow. Any advice on how to achieve this finish with a warm roof construction? Best regards, David

[rango]

I've attached an image showing the adjacent garage with a single skin of brickwork forming the parapet wall.