Opinions on flat roof construction so far

Thinkingwtf · 7 Oct 2019

hi fellas.

I’m looking for informed opinions on a flat roof extension that I’m having done at the moment. The builder comes recommended and I believe has many years experience.

However I’m not sure the flat roof construction looks properly thought out to me. I’ve looked at many specs and discussions about warm flat roof design.

I’m not happy with how the roof has gone on at the moment. So I don’t want the roofer to come and felt it yet as i think it needs to be redone.

AFAIK standard construction for a warm flat roof is:

> waterproof covering
> osb deck
> insulation
> vapour control
> plywood sub deck

We’ve got:

> waterproof covering
> osb deck
> insulation

The insulation has been screwed through which isn’t ideal and there is no vapour barrier or sub-deck. So it seems poorly constructed.

Am I being overly concerned or can we just put up some foil backed boards?

oldbutnotdead · 7 Oct 2019

What does your building control officer think? (Presumably this is a PD job but you'll still need to comply with building regs)

VCL- if the joints between boards have been taped (with foil tape) then that makes a pretty good vcl

Screws through insulation- how else you going to fix the osb down? I'm not convinced that the insulation will support the roof properly sat on the rafters like that but i may be wrong.

^woody^ · 8 Oct 2019

You should have a board across the furrings to support the insulation. No vapour barrier.

And vertical insulation from wall/beam to the deck.

Thinkingwtf · 8 Oct 2019

Thanks for the replies fellas. Also what about the joists that are sat on the lintel? How would they be fixed? They are currently just resting on it

oldbutnotdead · 8 Oct 2019

They should be blocked (noggins or similar) at the ends to stop them twisting/racking and strapped to the lintel (is it steel?), not sure if builders band would be adequate

Notch7 · 8 Oct 2019

I tend to use 11mm OSB on top of the firrings.

Then min 120mm insulation -when insulation is all fitted, every joint should be covered with 75mm or 50mm foil tape -acts as a vapour barrier -strictly speaking on the wrong side, but its the easiest place to do it effectively.
some building inspectors ive used like to see foil backed plasterboard to add another layer of VPL on the wamr side where it is.

The roof needs to be laid to falls -for felt I would recommend 1 in 40 to prevent pooling

NB -firrings laid to falls around a lantern opening get rather complicated -the firrings need to done so the roof has 'hips' at the front-ie a high point running from corner of lantern to corner of fascia. at the back between lantern opening and house wall, the firrings need to continue to rise so that the highest point is at the middle of the roof (from house wall to back of lantern).

then 18mm OSB screwed down then EPDM / FELT etc.

there is no point screwing down the insulation -waste of time, there has to be a top deck board -and a necessity for felt.

there are modern construction methods allow for the insulation to be glued down and covered directly with the roof covering -but that needs special insulation.

^woody^ · 8 Oct 2019

Notch7 said:
I tend to use 11mm OSB on top of the firrings.

then 18mm OSB screwed

Shouldn't that be the other way around?

^woody^ · 8 Oct 2019

Thinkingwtf said:
Thanks for the replies fellas. Also what about the joists that are sat on the lintel? How would they be fixed? They are currently just resting on it

Two nails each.

A 5x1500mm twisted strap every 5th joist.

Thinkingwtf · 9 Oct 2019

^woody^ said:
Two nails each.

A 5x1500mm twisted strap every 5th joist.

Cheers Woody. I've taken a look at this but can't visualise what you are saying. How would the straps fix through the steel? Cheers buddy

Thinkingwtf · 9 Oct 2019

ah I see what you mean, knock in some noggings to fix to

^woody^ · 9 Oct 2019

TBH, if that's a lantern on that double joist, then you should only need to strap those double joists down at each end and that will hold the rest of the roof down.

Thinkingwtf · 15 Oct 2019

So it's coming on OK. I have a question about insulation. Do you guys think it is a good or bad idea to insulate between the joists with Rockwool? I have read about condensation issues with hybrid roofs, but I wasn't sure if it is a good opportunity to add extra insulation. There is 100mm cellotex to create the warm roof.

Notch7 · 15 Oct 2019

Thinkingwtf said:
There is 100mm cellotex to create the warm roof.

Usually a min of 120mm is considered necessary to achieve a 0.18 u value.

since you have a lantern it will be an extension with excess glazing and should have a SAPS test -thats if its open to the house and under B/regs

Thinkingwtf · 15 Oct 2019

yes I understand about the requirement for the 120mm. The builder has bascially thrown it up without much care but I am at the stage now where it can't go back. So thats why i was considering insulating below the roof deck.

The house is already open so I am not getting regs checked.

Leofric · 15 Oct 2019

Thinkingwtf said:
The house is already open so I am not getting regs checked.

Don't understand that, the extension should comply with building regs

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Opinions on flat roof construction so far

Leofric

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