This video shows you how to cost a flat roof. Please note that EstimatorXpress uses a metre rate price for the Flat Roof Waterproofing system. 

This is set by default to £40 a metre to supply and fit the waterproofing system.  If you are using GRP or Fibreglass (for example) then you would need to increase this price  (typically £90 and upwards)

To change this price, you go into the PriceBook tab within the particular estimate, and select the Subcontract section of the pricebook.  Use the Mass Editing Tool - Edit prices, and adjust the price to suit.