A flat roof is not actually flat. It is a few degrees. Low pitched roofs can be nine to twelve degrees.
The advantage of flat and low pitches roofs is that they can span great distances and are cheap to put up. Because they generally have metal as their roof cover they are light. Being light means that the number of trusses installed to hold them is less. Roof battens are less too. So the whole roof structure is light.
Flat roofs work great in sheds, and factories. Commercially most offices use flat roofs. The quality of roof sheeting is generally higher in commercial roofing.
In a domestic situation the flat roof has meant that greater indoor ceiling spans can be attained, walls can easily go higher, and cost savings are available. And to a great degree they can be very stylish and modern looking.
The disadvantages of low pitched roofs is that they leak more easily than a higher pitched roof. A high pitched roof has gravity pulling the water away. A flat roof is using overflow as the means of water runoff. A pitched roof will get water off the roof faster and a flat roof will get the water off slower. So a flatter roof has more water on the roof. While there is more water on a roof it has more time to be blown into unsealed flashing corners, and then run down inside the building.
Inspectors cannot see behind a flashing. They cannot always tell if the flashing will hold away water or not. Often during a staged inspection an inspector can pick up problems before they happen. But after the construction, it is hard to tell.
Water can get blown back up a flat roof more easily than a higher pitched roof. That water then can back up to a flashing and get blown into and under the flashing more easily.
For ten years Canberra had a very dry period. During that time the technology to keep water out from a roof was partially lost as roofing tradesmen did not expect their flashings to be subjected to heavy rains. In 2020 they were hit by heavy water and some of their roofs leaked. Over the next year those tradesmen regained much of that ability to flash a flat roof more correctly to weather heavy rain.
The flashing shown here is open and not well made. Water can get in. While the roof is dry inside now, the roof sheet should be turned down more so water does not blow back up. The top flashing does not run the water off but rather will have a tendency to hold the water.
Decades ago architects were often seen on these sites, checking the quality of construction. Today, that expertise is not wanted due to the cost. Back then the tradesperson had to build to the architects specification. The architect would inspect often. Today, for economic reasons, that expertise has gone.
Often flat roofs are accompanied by big box gutters. These are traditional when the design of the buildings do not have the guttering exposed to simply overflow outside. Box gutters are excellent for factories and commercial buildings that abut boundaries. In domestic designs they are flawed however, by simply being there.
Think of a box gutter as being a very long bath tub. It fills up with roof water and needs to then be let out.
You can see from the picture below that the box gutter fills with water which thus must exit. But if it blocks at the drain, or gutter ends there can be problems. The water can go up under the flashing on the parapet wall.
So, buyers need to be aware. That is all you can do. You can get a specialist roof plumber to look at your roof all over. It is recommended, as well as a building inspector.