Extension Building Regs drawings

[pudding]

Hi, I've just had the draft building regs drawings from our structural engineers for our extension this week, based on my designs and specs after a few posts on here about it. The plan is to send them off to the LABC for full BR approval. The extension is planned 2 rooms, with flat green roof, timber framed and cedar clad to match our house. Heating is planned to used an air2air heat pump and air con units hung on the walls in the 2 rooms, and also I'm planning on using low power electric UFH with a floor stat only to take the chill off the floor tiles rather than actually heat the whole space.

 

If anyone if feeling particularly bored or has lots of time on their hands, I'd really appreciate any feedback and comments. I've already noticed a few little discrepancies to change but nothing major. Below are links to the 2 docs I've been sent so far.

 

https://1drv.ms/b/s!AkHh6rqs0aKmn4BjENnbHQ-uaVAo0g?e=VpwkbZ

 

https://1drv.ms/b/s!AkHh6rqs0aKmn4BkpFMFIxqIkpMFbQ?e=iHD3oo

[Dreadnaught]

I looked at your drawings with a particular eye on the roof make-up. This is because I am presently specifying the flat roof for my forthcoming build so it is foremost in my thoughts..

 

I don't know if you are aware but yours is an example of a what's called a "hybrid" roof, a hybrid of a cold roof and warm roof. You have some insulation in among the structural elements and some insulation above the structural elements.

 

I don't wish to alarm you but you might be interested to note that Bauder say "This type of roof construction should not be used unless a full condensation risk has been undertaken". The reason for this is the lower layer of insulation can cool the void increasing the risk of condensation within it. And it is of course moisture, more than almost anything else, that destroys buildings.

 

Here's a link: https://www.bauder.co.uk/technical-centre/technical-literature/design-guides/flat-roof-design-considerations/thermal-design-insulation-and-consideration

 

Did your architect do a dew-point (condensation risk) analysis of your roof, I wonder? Or perhaps its a standard details and so they are relying upon someone else having already done it?

[pudding]

Hi, thanks for the comments.

 

Yes I was aware it is a hybrid roof. There is no architect involved, I'm coming up with the designs and specs here. As well over 2/3rds of the insulation is above the rafter, as per these links below, then I dont think there will be any issues

- http://software.trada.co.uk/techinfo/library/send/3DC53CCF-9477-4190-9287-B4260CDA169D/How to design a flat roof/index.pdf

- https://www.partel.ie/news/Flat-roof-construction---an-expert-guide-to-moisture-management/702

 

I carried a u-value calculation which includes a simple static dew point calc in there and the dew point is not reached until well through the PIR layer so there is no condensation risk to the structural timbers - https://1drv.ms/b/s!AkHh6rqs0aKmnul2ftYxythaIORavQ?e=uZ6cBN