Any roofers in the house?

Lestratcaster

Ok poor joke, but let's cut to the chase. As some of you may have seen I'm turning a garage into the back of my house into a music teaching room. Have had a few people over to come and have a look and the first thing that needs to be addressed is the roof. We've just had a quote from one roofer who wants just under £4k to do a fibreglass roof with fascias and guttering.

I didn't ask a the time about insulation, there's 2 ways, one via the roof and other via the ceiling. He said to do it via the roof he'd need to double deck it first adding another £1k odd. The ceiling method is a little more complicated apparently but seeing as I have no knowledge of this I'm not sure what way we should do it. Its obviously going to get very cold in the winter months and alot of the heat loss will be via the roof/ceiling.

Which way is better/cheaper?

Dominic

The going trade rate for a GRP fibreglass roof on prepared surface is £50 per sq m
 ideal surface prep is Sterling board -this gives the best bond to GRP resin -better than ply
a standard double garage is 16 x 8 foot which equates to about 13 sq meters  ie £700 max in fibreglass
You need to allow for labour and materials to form kerbing or for preformed GRP dressing flashings approx £350
 Is he removing and disposing of an existing surface ? if so add the cost of that
The best way to insulate in this case will be a polycyanurate foam board of 80mm min set with a vapour barrier beneath the sterling board OR you could buy perldeck which will do the whole job in one go.
Either way the insulation will cost approx £420 and the sterling board approx£140 with £200 labour to fit
PVC gutter, clips and downpipes together with 15 linear meters of upvc fascia board will cost £225 with a further £200 labour
  I value the works at £2250 cost ,allow profit element at 20 % ie £400 thus total of £2650 for an insulated roof .
If an existing surface is removed and disposed or if the present board is soft or rotted allow up to a further £600.
  What about insulation board for walls and floor (huge amount of SAPS calculated heat loss is to concrete floor - probably no less than to roof )
Quote you have is expensive 
What area are you ?
I have modelled the labour on suburban London basis.
All assumes that the garage is approx the size I quoted or adjust accordingly.
Total job is 2 or 3 days work.

Lestratcaster

He is removing the existing surface yes.

The roof size is 4.5m x 6.5m. Quote includes disposing of all rubbish as well.

And we are based in West London.

Dominic

Just about right then
check on the grade of the insulation and performance

Lestratcaster

He didn't say what insulation it'll be, he only said if I did it in the inside via the ceiling it'd be celotex. 

Sorry for brief replies I kinda don't know what I'm doing with all this.

Dominic

celotex is fine ;it's the thickness that matters ....you want 80 -100mm to be any true value
better on outside or inside of ceiling you can get cold bridging and interstitial condensation if you dont have 30mm air gap and cross ventilation
better to stick to exterior if u don't really know what you are doing 

Lestratcaster

He did mention about ventilation being a factor if we insulated on the inside. I think it will be easier to do in one go with the roof on if we went that way, a bit more but in the long run it'd pay off?

Jimbro66

Just one wee comment. Thermal insulation board has very little acoustic isolation value, which might be of consideration for a music room.