When choosing the construction type of the roof you must already know all the purposes for which you will use it. Generally, the roof protects your house from bad weather, winds, rains and snows, but you can use it many other ways as well.
Flat roof insulation systems
If you decide to have the flat roof, you must know if you will use it just for the general protection, or you make a terrace or green roof (roof garden) out of it. Very well for insulating the flat roofs is the reverse insulation, where the thermal insulation is placed on the (traditional) roofing materials. In this case the thermal insuation (made from unabsorptive, very solid materials like extruded polystyrene in...
Even though we call them flat, usually they are not absolutely flat as some "minimum" slope is necessary to allow the rain water to flow off the roof. Therefor the roofs which pitch is below 10 degrees (standard is approximately 3 degrees) we call
the flat roofs. 
The flat roofs advantages are:
- the (residential) flat roof construction is less demanding than the construction of any sloped roof (which needs an advanced construction consisting of trusses, beams, joists, etc...); - for the same reason, the (residential flat roof construction is cheaper than the construction of any pitched roof typ...
