Thermocol (3") and water proofing for top floor roof

[dev13]

I am looking to make good thermal proofing for the top floor roof, I understood from TERI website that 3'' (75mm) Expanded Polystyrene (Thermocol) would be a good one to embed before roof top plastering/waterproofing.
3 inch thermocol is effective in decreasing heat absorption...but I am looking to combine the Thermocol with professional water proofing such as synthetic waterproofing membranes.
the building has a flat concrete roof with inverted beams.

If you know a good water proofing expert who has experience in the Thermocol embedded solution, that would be ideal. Please suggest.