Flat roofs have a reputation for being problematic. The most common issue with flat roofs is leaking, usually due to poor workmanship during installation, age or lack of maintenance by the property owner.
If the installation process was carried out correctly and a membrane was installed, the roof only tends to last 20-30 years, a lot less time than your well-constructed tiled roof.
So as you may already know, flat roofs are not completely flat they are sloped but appear flatter to the human eye. The slop degree of the roof is important as to reduce the leaking issue and flow the rainfall water off the roof top.
Regular maintenance is a definite must with flat roofs as to improve the lifespan of them. A common problem is the build-up of leaves in the rainwater outlets, although leaf guards can be installed this only slows down the issue that still requires regular attendance to.