Questions regarding replacing flat roof with a pitch roof

zo · Posted: Fri Apr 29, 2011 8:01 pm

I am going to be replacing my flat extension roof with a sloping pitch roof.

The roofer is going to remove only 2 ft felt off the flat roof, away from the main wall. As the leak is towards the guttering, do I insist that the roofer strips the entire felt from the flat roof to see the water damage in case the wood has gone rotten? The roofer does not think it is essential.

Also, I want the best insulation possible in the low pitch roof which is over my very cold kitchen and the through lounge. Do I get Celotax or Kingspan? Will the roofers be able to fit that under the existing felt which they are not stripping. The roofer is advising me to go for the cheap insulating rolls which he can tuck under the old existing flat roof. If I were to insulate the roof with 300mm insulating roll, what thickness Celotax should I then go for?

What is the difference between Celotax and Kingspan? Where would I buy the cheapest Celotax/Kingspan?

Thank you.

Tuck1 · Posted: Fri Apr 29, 2011 10:54 pm

Cellotex and kingspan are just different brands of the same type of solid foam, foil faced insulation. Tell the roofer you would like the felt removed to inspect the underlying joists and you want him to install 270mm of rockwool insulation in the 'new' roof space. Also you need some sort of ventilation for the new roof space, either soffet vents or roof vents.

Remember however, what price & work you have been quoted for and what work was agreed in that quote.

zo · Posted: Sat Apr 30, 2011 7:14 am

Tuck1 wrote:

Cellotex and kingspan are just different brands of the same type of solid foam, foil faced insulation. Tell the roofer you would like the felt removed to inspect the underlying joists and you want him to install 270mm of rockwool insulation in the 'new' roof space. Also you need some sort of ventilation for the new roof space, either soffet vents or roof vents.

Remember however, what price & work you have been quoted for and what work was agreed in that quote.

The roofer is adamant that he is not going to strip off all the felt. I did try telling him to strip off all the felt but he is insisting that he doesn't need to.

mitch66 · Posted: Sat Apr 30, 2011 6:31 pm

Tell him to knob off and employ someone more helpful.

noseall · Posted: Sun May 01, 2011 7:59 am

Removing only enough deck is perfectly acceptable so long as it does not hinder either the roof construction or insulating processes.

It also means that the roofer will have somewhere to stand.

Insulate using say 150mm glass-wool between the joists and another 150mm layer across the joists in t'uther direction. This secondary layer will be ok across the deck that is remaining.

Using rigid board is fiddly and is an unnecessarily expensive way to insulate a roof space.

zo · Posted: Mon May 02, 2011 10:08 am

mitch66 wrote:

Tell him to knob off and employ someone more helpful.

I wish I could! I have contacted several roofers. This roofer has done my neighbours and has stood the test of time

Also, his was the cheapest quote.

zo · Posted: Mon May 02, 2011 10:11 am

noseall wrote:

Removing only enough deck is perfectly acceptable so long as it does not hinder either the roof construction or insulating processes.

It also means that the roofer will have somewhere to stand.

Insulate using say 150mm glass-wool between the joists and another 150mm layer across the joists in t'uther direction. This secondary layer will be ok across the deck that is remaining.

Using rigid board is fiddly and is an unnecessarily expensive way to insulate a roof space.

Thank you for responding. Does it make any difference to how the glass-wool layer is placed - across or on top of each other?

noseall · Posted: Mon May 02, 2011 10:56 am

The first layer that sits between the joists and against the ceiling can only go in one direction obviously.

For the sake of robustness it is then preferred that the next layer goes in the opposite direction to preserve the full efficiency of the insulation.

I guess you could stack it in the same direction but why would you?

jimAuk · Posted: Sat May 14, 2011 10:44 am

2 weeks today we are having our flat roof converted to a pitched sloping one and merged into the main house roof.

The FULL flat roof deck is coming off, but joists are staying.

Several reason why, like you are not hiding any problems, can insulate to the ceiling.

Also to get the pitch and not having it look like the new roof was not merged with the new, the eves over hans will be lower than the current flat roof.

We had or trusses pre made, and the truss company was very helpful in measuring up etc.