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.