Balanced flue - roof penetration / ducting

[bmj1]

Hi all,

 

We're looking at a balanced flue/gas fire system, to sit below our side extension, which as a flat roof immediately above it (I understand an open gas fire is not possible as no chimney).

 

We'll be sealing the roof in the next couple of weeks (deck/VCL/insulation/deck/liquid waterproofing layer).

 

What's the best way of doing the flue penetration through the flat roof ? Do I need the specific first fix items from the fireplace company now, or can we just run a duct for it - and if so - what size duct would make sense ?

 

Thank you so much for your kind advice.