vent flat roof or not?

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Hi all,
Im going to GRP a 12sqm flat roof on extension which abuts neighbors extension on one side
I am doing work through building regs... and they want it insulated with 100mm king span between joists then 25mm under that.
I am guessing that they will accept 75mm between joists but this still only leaves me with around 25-40mm of space between insulation and roof, and then I have a problem of how to create adequate ventilation because even if I some how install vents its blocked off on neighbors side
My question is this: if i fill the whole void in with insulation and do not leave any space/air, will this not solve my problem of any possible sweating???
Thanks
legepe
 
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Full fill between the joists with celotex or similar fit it nice and snug foam fill any gaps along joist edges if required. Overboard the whole area with 25mm celotex or similar it must be foil backed insulation as this will act as your vapour barrier. Foil tape any joins between insulation boards. Plasterboard and skim.
 
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Why not fit a warm deck roof?
Ive had building inspector today... and thats what he said I should do..
Trouble is it will raise my side height to the neighbors extension by 100mm.. How do I get a proper finish/seal on their side?
 
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It's how you say it.

Inspectors are there to inspect not tell you what to do. You tell him that you can't do a warm roof because of (insert the problems) include the neighbour issues and boundary problems that it would create, and then tell him that your only option is to insulate with no gap.

Almost every single flat roof garage conversion is insulated full fill from below. It's nothing new or non-standard.
 
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Ok, I understand.. however, if i do it the cold roof method.. he wants me to still increase the roof height by putting in cross members then vent it from the front of extension to the back wall.. then construct up the back wall a space and have it vented there.. I think the warm roof method is going to be the easiest and less costly... I dont know of any other options as he is insisting on adequate air flow...
 
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That's it then, comply or contest.

I've never done that, and never will do. There are unvented options for any roof. You either point that out, or spend the money.
 
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Thing is it's a new roof, you can argue to do a hybrid for a garage conversion as it's existing and replacing the covering is uneconomical. So really this should have been thought about before you got to this stage and the problem dealt with during the design not once you're backed into a corner.
 
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I will ask the building inspector if its ok if I completely fill in the voids between joists with insulation then no airflow required...
I would imagine though that its pretty difficult to completely fill all space 100% no?
 
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It's nothing new or non-standard.
No, but it is not without its problems either.

If you do decide to full fill from below, make sure you put the 100mm layer tight against the deck prior to the electricians and plumbers doing their first fix.
Don't plan on having downlights.
Be prepared to have a fair thickness of material to have to deal with when fixing your light fittings - meaning long screws.
Be prepared for a LOT of Celotex cutting, taping and fitting above your head with Celotex dust raining in your face for days.
If your plumber wants to run pipes through the ceiling as well as the sparky, be prepared for absolute misery.
Accurately mark off your joist positions on the adjoining walls prior to starting any of the ceiling work.

You will need long screws to fix the plasterboard. You can buy long drywall screws but they are a bloody pain to fix as they cam-out a lot. It took us 5 long miserable days to complete the ceiling detail (on our current garage con') having to cut around pipes and cables etc.

Try not to chuckle if your neighbour renovates their side and goes for a warm deck later on.:(
 
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