Flat roof overhang

[OllyH]

Hi all,

 

We are currently building a single storey flat warm roof dwelling.

 

We have large glazing which is south facing so have a 1.2m roof overhang designed also.

 

The Easi joists can span beyond our SIP to create the overhang.  Just wondered your thoughts on cold bridging and how to minimise this.  I’m going to put PIR between the joists to fill the void.  Should I just pack out the Easi Joist itself too?

 

Thanks

[mvincentd]

I think if you put PIR in the joist space of a warm flat roof you'll need to research hybrid roofs to ensure you're not heading for issues with interstitial condensation.  You'll probably uncover numerous historical posts on here if you do a search.

If you mean just to put the PIR in the external overhang part, that's ok.

I regret not discovering the 'tony tray' in time as it makes relatively light work of making the joist space airtight, which can otherwise be 'fiddly' depending on how everything is detailed.  This factor alone might turn me against posi's that 'shoot through' (making Tony tray impossible), in favour of a separate 'bolt-on' structure, perhaps based on some custom metal brackets given the 1.2m size.

[OllyH]

Thanks for the reply.

 

The original design was for a bolt on additional structure, this was tapered however.  We changed this to square edge so thought it would then be easier Construction wise to continue the joists through instead.

 

I’ve been told to look at Proclima membranes and Siga tapes as a moisture barrier and with PIR between the joists to limit cold bridging.

 

i look the Tony trays up too.

 

Thanks

 

Olly