The cost of a flat concrete roof will depend upon a number of different things. Each must be taken into consideration when you sit down to work out the budget you need to put aside for completing the job.
Work out how much concrete you will need to cover the surface area of the roof. Measure the dimensions on the sides of the roof and multiply them to get a squared figure. This will allow you to work out how much concrete you need to order as well as allowing you to formulate a price figure.
You cannot simply pour concrete onto a new roof. You need to work out what type of insulation you will use to layer over the top surface before you can put down the concrete. This insulation can be very expensive so it's important you work out an accurate cost for how much you are going to need.
As the roof is flat, you will have to install some sort of reinforcement to strengthen the structure. You need to factor in the price of metal supports and other concrete reinforcement components.
The materials currently on the roof will need to be removed unless the roof is brand new. This can mean factoring the cost of additional tools or services to make removal possible without causing significant damage to the structural integrity of the roof.