Slope on a RCC flat roof

Slope can be done using cement-concrete screed itself however 

The purpose of mud + straw is to provide heat insulation in summers. Problem is if there is crack in top cement layer mud will absorb water and this water will remain there for months. Bitumen is for waterproofing but it is a poor material as the cold winters cause cracks and shrikages.

Other better modern alternatives are

(1)  Use chemical sealants for water proofing instead of bitumen and heat reflective white tiles on the top.

Use Laticrete / Foscrock/ Pidilite newcoat 3 layers on clean roof slab or similar water proofing chemical and put a layer of white ceramic tiles or heat reflecting white tiles at top this will give you advantage of 3 to 5 degree Celsius in the room.  As you know white colour reflects sunlight best it is a good choice in India. Search for heat reflective tile on internet and check of there is any supplier in your area else normal white ceramic will do. Slope can be done while putting mortar for tile laying.

Make sure that you leave small drain holes in parapet to drain out water seeping under tile layer from the edge.

Terracotta tiles can also be used for water proofing above RCC in similar manner.

In MP instead of mud  broken brick / terracotta tile with mortar to give slope and put cement slurry. This is also called Kharanja. It is better then mud and straw however heavy.

If you have big budget use Chemical sealants + matki / kulhad + white heat reflective tiles.

In my opinion method 1 is best however follow instructions on label of waterproofing sealant rather then going by what contractor or mason says.