Flat roof structural construction?

[GaryinAshby]

Okay, I am concious that I was inadvertantly hi-jacking someone elses post with my contant questions...... so:

Flat roof on a domestic extension. I am using 9"x 2" joists due to the span. "Builder-mate" suggested using 4"x 3" wall plates on top of the inner block wall to drop the joists onto & fasten them down using joist hangers. The BCO suggested using 9mm Master board to close the top of the cavity to prevent fire spreading etc, etc.

Woody has suggested the (logical) route of leaving the top of the cavity open and continuing the insulation over between the joists. This is going to be a cold deck system with Kingspan between the joists, but keeping a 50mm airspace between the underside of the deck and the top of the insulation. Rather than using a wall plate, simply dropping the joists onto the block work. If going this way, how would I tie the joists down onto the wall? Building the block/brickwork up between the joists will be difficult as I have to maintain the air flow over the top....

Anyone know if this should be okay with the BCO? Although the BCO is good she is very young (straight from college) so obviously doesn't have lots of experience behind her, relying on "text book stuff".

Thanks for any comments.

 

[noseall]

why don't you use 97mm kingspan insulated deck? it is pricey stuff but kills several birds with one stone.

 

[^woody^]

Typically in your cold deck roof, the joists will run across the walls and the fascia fixed the the joist ends and you fit a ventialtion strip to allow airflow above the insulation.

The insulation from the wall should meet the roof insulation to be continuous

The wall parallel to the joists is either bult up to the underside of the deck or you fit noggins across and continue the fascia around.

The joists are strapped down every 1.5m approx (you will probably have 3 straps per end) with minimum 1.2m twisted restraint straps. There is no need to fix evey joist, as the roof structure as a whole is held down. You can block up between the joists to prevent twisting - but leave the ventilation gap at the top. And don't forget your noggin or herringbone straps at joist mid-span

Whilst I agree with noseall that a warm deck is better, what I don't like is the massive roof and fascia depth that this gives with deep joists - it looks really ugly. When I do warm decks, I design the joist overhang and fascia to be a more pleasing 150-200 and not the 300-350 depth with a warm deck roof gives nowadays. The only problem is that it sometimes looks 'odd' with more brickwork between the window heads and the fascia

 

[noseall]

Whilst I agree with noseall that a warm deck is better, what I don't like is the massive roof and fascia depth that this gives with deep joists - it looks really ugly. When I do warm decks, I design the joist overhang and fascia to be a more pleasing 150-200 and not the 300-350 depth with a warm deck roof gives nowadays. The only problem is that it sometimes looks 'odd' with more brickwork between the window heads and the fascia

i know what you mean about the great expanse of fascia, particularly when factoring in the cocking fillet too.

we have run upvc soffit boarding around, but fitting short vertical lengths instead of horizontal. it is more pleasing to the eye and being upvc is not 'weather relevant', so long as the drip is done correctly.

 

[GaryinAshby]

Thanks, I did consider a warm deck, but with having to use 9" joists, plus the depth of firring, plus the 97mm Kingspan insul. deck, the fascias would have been huge. The other factor was that I didn't know how strong it would be with window cleaners ladders etc being plonked on top to clean the windows above.

I like the idea of dropping the joists straight onto the tops of the walls & continuing the wall insulation over. As the joists are 9" I could still build up two courses of bricks (6" high) between them on the outer wall and still maintain airflow over the top (3" gap between top of the bricks & deck). Continuing the insulation from cavity to roof will also give superior insulation properties.

Also it saves me having to buy 4" x 3" for the wall plates, Masterboard etc, etc.....!

 

[GaryinAshby]

Is there a decent web site that explains both flat and pitch roof construction? I can find loads on flat roof coverings, but very little on the actual construction of the roof itself.

 

[Deluks]

i know what you mean about the great expanse of fascia, particularly when factoring in the cocking fillet too.

LANGUAGE TIMOTHY!!