Cold Roof Ventilation

[James T]

About to start construction of a flat roof but changes I have made along the way are throwing up problems with ventilation. 
 

My initial plans included a window looking out onto a flat warm roof but we had a change of heart and decided we wanted a Juliet balcony and to maximise the door size we changed to a cold roof system. We have created an opening (6ft 6) with a 150mm step above the joists. But in creating the 150mm step we have limited the space for abutment vents to create air flow above the insulation.
 

- The roof has a full surrounding parapet.

- Size 3.4m by 3.5m 

My options I’m working through are: 

1. I could run firrings perpendicular to the joists and then install mushroom vents in all 4 corners, this means air can flow through the full roof. My concern with this is building control might not approve this proposal as mushroom vents aren’t preferred method of ventilation.  
 

2. I could do abutment vents and only have 50mm up stand. Nervous about this not being enough as 150mm normally used to give better clearance below flashing. 
 

3. I can knock my 150mm step out and make a 300mm step and got back to a warm roof system Issue here is my door on my Juliet balcony is only 6ft tall and might look a bit odd. 
 

imagine below shows my first idea (ignore number of joists it’s actually 9)

 

Any advice appreciated. 
 

Thanks 

[SBMS]

2 hours ago, James T said:

About to start construction of a flat roof but changes I have made along the way are throwing up problems with ventilation. 
 

My initial plans included a window looking out onto a flat warm roof but we had a change of heart and decided we wanted a Juliet balcony and to maximise the door size we changed to a cold roof system. We have created an opening (6ft 6) with a 150mm step above the joists. But in creating the 150mm step we have limited the space for abutment vents to create air flow above the insulation.
 

- The roof has a full surrounding parapet.

- Size 3.4m by 3.5m 

My options I’m working through are: 

1. I could run firrings perpendicular to the joists and then install mushroom vents in all 4 corners, this means air can flow through the full roof. My concern with this is building control might not approve this proposal as mushroom vents aren’t preferred method of ventilation.  
 

2. I could do abutment vents and only have 50mm up stand. Nervous about this not being enough as 150mm normally used to give better clearance below flashing. 
 

3. I can knock my 150mm step out and make a 300mm step and got back to a warm roof system Issue here is my door on my Juliet balcony is only 6ft tall and might look a bit odd. 
 

imagine below shows my first idea (ignore number of joists it’s actually 9)

 

Any advice appreciated. 
 

Thanks 

100% option 3. We have full perimeter parapet and ended up going warm roof. That and 100mm isn’t really enough insulation. 

Edited Sunday at 13:24 by SBMS