Flat Roof Extension-Parapet Height

[Mory_London]

Hi all,

I'm planning on having a flat roof extension (5x3m) with a parapet style walls but stuck on height requirements for the parapet wall.

I'd like to have the parapet style walls with either stone or metal coping to avoid having plastic trims/ visible gutter pipes/etc. but I'm having difficulty identifying the minimum wall height requirements above the flat roof surface. I have seen from NHBC guidelines that 150mm min is required for rain, splash and for external insulation to avoid thermal bridging - but I have also come across websites which state that if using a single product like GRP across your roof and up the 'parapet wall', the you could go as low as 50mm. I'd like to be able to go as low as possible to avoid impacting neighbours and keep costs down but also not affect performance.

I have attached below the kind of external look I'd like to have, and also attached a pic of a roof which definitely doesn't have a 150mm min upstand/ parapet - so what type of system does this have?

Any help would be very much appreciated - This is a self build (managing contractors) but I have zero experience but want to put in the time and learn. Thanks in advance.

 

[noseall]

height requirements for the parapet wall.

150mm above the deck (at its highest point).

 

[^woody^]

Seems like you don't want a parapet, just a raised kerb.

Whoever is designing the extension would design it.

 

[opps]

I think that is an EPDM rubber roof.

 

[Mory_London]

Seems like you don't want a parapet, just a raised kerb.

Whoever is designing the extension would design it.

Many thanks for your reply, would a raised kerb option be more susceptible to rainflow and thermal bridging issues compared to a 150mm+ parapet, or is it designed completely differently to avoid such issues? I’ve heard a lot of horror stories with architects/ designers not taking such issues into account properly.

 

[^woody^]

Many thanks for your reply, would a raised kerb option be more susceptible to rainflow and thermal bridging issues compared to a 150mm+ parapet, or is it designed completely differently to avoid such issues? I’ve heard a lot of horror stories with architects/ designers not taking such issues into account properly.

The roof and wall insulation should be continuous, so as to avoid any thermal bridging.

Effectively, this would be a flat roof but with the external leaf taken up to the deck and the joists stopped at the internal leaf.