A flat roof is the most cost-efficient roof shape as all room space can be used fully (below and above the roof) and as this roof allows easy revision/placement of solar panels  They also provide space for outdoor recreational use such as roof gardens. Applying a tough waterproofing membrane forms the ideal substrate for green roof planting schemes.
Traditionally flat roofs would use a tar and gravel based surface which, as long as there was no pooling of water, was sufficient to prevent penetration. Modern flat roofs tend to use a continuous membrane covering which can better resist pools of standing water. These membranes are applied as a continuous sheet where possible, though sealants and adhesives are available to allow for bonding multiple sheets and dealing with structures penetrating the roof surface.
In general, a flat roof lasts longer if it is properly maintained. The life expectancy of a flat roof can be proportional to the maintenance done on it. Some assessors use 10 years as an average life cycle, although this is dependent on the type of flat roof system in place. Some old tar and gravel roofers quietly acknowledge that unless a roof has been neglected for too long and there are many problems in many areas, a BUR (a built up roof of tar, paper and gravel) will last 20 - 30 years. There are BUR systems in place dating to the early 1900s.