Can I use 2 50mm sheets of celotex for my cold flat roof insulation rather than 1 100mm?

You can use two sheets of 50mm but it is not enough to comply with current regulations. You will not achieve the correct "U Value" which is the thermal resistance. In a nutshell your roof could end up with condensation (interstitial). Ideally if using Celotex you need a minimum of 120mm. On a warm roof you do not insulate between the joists since this would be a "hybrid roof" which falls outside of the "British Standards" for the warm roof specification. Best advice is to contact Celotex Technical. They are fast to respond by email (technical@celotex.co.uk). Tell them the type of insulation you have and what you intend to do with it. Ideally for a warm flat roof the make up is as follows from inside out: Plasterboard (standard - non foil backed) Joists 1st deck Vapour barrier (1000 gauge DPM) 120mm of solid insulation (CELOTEX GA or Kingspan TR26). 2nd deck (OSB T&G3 18mm) Waterproof finish - GRP / EPDM You also need Buiding Control (dotting the i's and crossing the t's). Regards Drew @ Fibretechs - Gloucestershire & Herefordshire

Yes, 2x50mm layers of insulation will be absolutely fine. The easiest way to introduce a vapour barrier is to use foil tape to seal the joints where the insulation meets the timbers. No need for insulated plasterboard.