The best flat roof roofing material is unduly a single sheet of rubber custom made for weather-proofing flat roofs, such as Firestone's EPDM product.
This is laid directly onto roof boarding, itself supported by the roof rafters and nagging. Insulating material is normally laid in the space between the two, and if you are renewing your flat roof anyway you may as well take the opportunity of renewing that at the same time.
This will almost certainly be the recommendation you receive from any reputable builder or roofer advising you on replacing your existing flat roof.
Until recently the most popular material for a new flat roof was bituminous felt laid in three layers, the first layer nailed down and the upper two bonded to the one benefit with mastic bitumen. Depending on the material used, this could mean heating a solid bitumen-based compound to make it liquid, and then pouring it onto the under felt and spreading it evenly over the whole area of the roof.
Then you had to wait for the compound to cool before applying the next layer, and at the end spreading a layer of stone chippings over the roof and bonding it with a chipping compound, this being to reflect the sun's rays and prevent the felt and bitumen degrading quickly.
This was identical a rather messy, complicated and time-consuming job best left to the professionals. In addition, there were several stages where it was very easy to fail to make the roof weather-proof, eg where the edge of the felt met the existing tile or slate roofing of the pitch roof.
On top of that, if or rather when, there developed a leak in the flat roof, it would be very difficult and messy to find out exactly where the rain was getting in. Seldom would the manifestation of the leak on the ceiling of the room below be directly below the source of it. Water could get through a fatigue in the bitumen felt and travel along a rafter before descending to the ceiling board below.
The new generation of rubber-based flat roofing systems are far superior. If you have reasonable DIY abilities then you can probably do it yourself, with one other person to assist. You can easily find a local supplier of rubber flat roof roofing material online, and the current price is only around £ 6.65 per square metro.
Most suppliers cut the material to the exact shape and size that you require so there are no unnecessary joins, meaning no weaknesses and an extremely long-lasting, leak-proof roof.
The vital part of the whole process is to ensure that there are no gaps between the rubber roof and any existing pitch roof where rain could possibly penetrate. To ensure there are no problems it is vital to run the rubber material up and under the tiles or slates of the adjoining pitch roof so that any rain dripping from the lower-most row falls onto the rubber, with no possibility of it over-lapping the edge and getting underneath the pitch roof.
Also, make sure that the existing slope of the flat roof towards the guttering or other means of water escape is maintained. The rubber is simply glued onto the roof boards and sealed where appropriate with metal brackets or bitumen in such a way that rain penetration is impossible. Mind that the glue covers the whole of the underneath of the rubber so as to avoid bumps in the surface that could obstruct the drain of the water away.
Rubber roofs have been shown to last for well over 50 years under all weathers, so it really is the only flat roof roofing material that you need to consider when deciding on a new flat roof.