Cold flat roof

[iwanttodiy]

hi guys

My builder has quoted a flat roof, and based on how he has described how he will build it, looks like he is aiming to build a cold flat roof and put insulation between the joists.

Looking at recommendations, cold flat roofs are no longer recommended for new constructions due to condensation risk.

Unfortunately as it’s going to be a permitted development, I dont have the luxury of roof height for an extra 100mm above the joists.

If we do go with the cold roof route, I plan on asking the builder how he anticipates venting it to prevent condensation. Is there anything else I need to check with him? Interestingly he’s building my roof the same as others he’s built, thus I assume he builds all his roofs as cold roofs.

 

[noseall]

I assume he builds all his roofs as cold roofs.

Sounds like a dinosaur, unless......

If he is diligent in his approach to how the roof is vented or the positioning and sealing of the insulation, there is no reason to think that the roof can't be successful.
It sounds to me like you are asking the right questions.

I cant think of any reason why I would not opt for a warm deck roof though when completely re-furbishing a flattie.

 

[iwanttodiy]

Not even a refeb, it’s a new roof for a new extension. In theory there is no excuse to not use the latest recommended standards by default.

I’ll ask him, there isn’t exactly much additional work for a flat roof, just roof height is a problem.

How tolerant are BC with regards to going over slightly? I think based on my designs, I will be 3.1 - 3.2m off ground level with a warm roof and a parapet wall housing a box gutter.

 

[John D v2.0]

BC won't care at all about the height specifically unless it changed the load or insulation., If you build exactly as per the drawings them they can't not approve it. if you change the design later and it doesn't comply with building regs then you can't say that they approved the design. But generally a change to warm roof as long as you calculate u values correctly should be ok.

 

[iwanttodiy]

I see, I guess I need to double check with planning.

 

[iwanttodiy]

Having checked with neighbours, they seemed to prefer the idea of higher parapet walls, as it stops water and gunk from dropping into their side of the garden.

 

[freddiemercurystwin]

What do the drawings say? Oh wait ....

 

[iwanttodiy]

I designed the drawings, but on consultation with the roofer, he wanted to do a box gutter and hence the need for a parapet wall. So I’m making modifications to the design to suit.

 

[^woody^]

Nothing wrong with cold roofs they will still keep the room warm.

 

[^woody^]

Looking at recommendations

From where?

 

[iwanttodiy]

http://www.ikopolymeric.com/best-practices-flat-roof-build-ups/

 

[^woody^]

Lol

 

[iwanttodiy]

Is there any thing wrong with what I’ve picked up on that site? Other material seem to point designs towards a warm roof anyway?

 

[pilsbury]

So, from my alcohol induced understanding, PD says eaves can be 3m, ridge height 4m. I have a bastardised pitched/flat roof..... the eaves are 3m, the top of the warm deck is well over.

 

[iwanttodiy]

Hmm interesting, but seems your eaves are still 3m thus within PD. And maybe the top of the flat deck is managed by the ridge height limit?

I’ve asked my SE to look at the height and reduce the joist sizes. As my roof is being made out of steel, I guess we can get away with 150mm joists, which saves me 50mm. He also suggested why not mix a cold and warm roof and put some on top, i.e 10-25mm and 50mm in the joists, reduce the height of the parapet wall from 500mm to 200mm. Now the theoretical height of the top of the parapet wall from ground is now just under 3m and planning are not going to go crazy with a few mm over, and worst case of they do, I can remove a course of bricks.