Extension flat roof height queries

[Deluks]

Hi all, long time no see.


I'm planning to build a rear extension under PD.

3 bedroom semi.

Roughly 4mW x 4.5mL up against the boundary, with the adjoining wall becoming a new party wall.
Neighbours have agreed in principle, I told them they could use it to form part of their own extension in the future.

Flat roof, small parapet wall on boundary side.

Clear span of joists will be 4200 (ish)

For light and space I would like to retain the ceiling height as it is in the existing downstairs. 2.6m, also putting in a flat roof light in the centre of the roof. So joists will likely need double or tripling up.

I would also like the eaves to overhang the extension slightly, maybe by a foot on two sides

The height from existing ceiling to patio as measured outside is 2725mm

I've worked out the roof thickness as 456mm

15 PB and skim + 195 joist + 70 furring strip + 18 deck + 156 insulation (inc. 6mm ply) + 2mm membrane (EPDM)

This gives a total height of 3181mm. (what have I missed?)

This seems excessive to me, not only will the roof end up looking very chunky and high in relation to the rest of the house, it would go over the pd eaves limit of 3m, and until the neighbours build their extension, would make them feel like they're living in the West Bank.

I could probably get away with 170x72 C24s to shave off 25mm

I've considered a cold roof, but that idea seems about as popular as 1970's glam rock stars, warm deck seems to be the 'in' thing.

I'm not sure of what hybrid options are available, or how much I can get away with putting insulation between the joists.

Or am I worrying too much, and it'll look fine and no one will care about the extra height and I'll win the lottery and marry a model?

 

[^woody^]

Nothing wrong with the Sweet or Bolan

It avoids stupid roof depths.

 

[Notch7]

You could use 2no I beams of say 152 height.

Then your joists could be 95mm

By the time you've added the firrings the steel beams would only stick into the insulation a little bit.

You would need a SE to calculate - but if it enables your extension under PD it would be worth it.

I thought insulation needed to be 125mm thick to achieve 0.18u value of warm roof

 

[Deluks]

Nothing wrong with the Sweet or Bolan

I was thinking more of the dodgy geezer with the wig, but Bolan gets a pass.

I'm finding a lot of info online implying that cold roof construction has too many issues.

Could it be that it's more difficult to construct, and builders just want an easy life, and suppliers don't want any comebacks on their products?

The other issue is that a service void would be needed, and ideally, insulation needed under the joists, so the depth saving becomes minimal if the roof joists need to be higher to account for this. (In order to maintain the existing ceiling height)

 

[Deluks]

You could use 2no I beams of say 152 height.
...the steel beams would only stick into the insulation a little bit.

You would need a SE to calculate - but if it enables your extension under PD it would be worth it.

I thought insulation needed to be 125mm thick to achieve 0.18u value of warm roof

Extra cost and labour involved.

2 beams back to back or dividing the span into thirds?

I'm confused about the 150/120 rule.

I was under the impression that it's 150 for new builds and 120 for refurbishment of an existing roof?
It's a new extension on an existing property, or is it a refurbishment of an existing property?