Flat roof insulation / replacing felt question

[DIYerHELP]

Hello All,

We have a chalet style house with a flat roof over the bedrooms, built in 1968 and very poorly insulated.

What is the best option for me to insulate the flat roof please? I would prefer insulation can be externally as the felt needs replacing. Concerned regarding the loading to existing roof.

Reasons for not wanting to take up existing board on flat roof: nail in and seems to add strength and support / rigid construction for the flat roof.

Thanks for any views / responses.

 

[DIYerHELP]

Quick update:

After a few hours research, going with Celotex TD3000, looks quick and easy to install and can achieve effective thermal insulation for my flat roof.