Flat roofing has come along way in the last then years since the days of them just being a cheap alternative to a pitch roof and nearly always leaking. That has changed now and thanks to the development of better materials.
We live in a climate of consistent damp and wet and all roofing systems need to be able to cope with that certainty not to mention driving winds and unpredictable conditions. Our wind driven rain will test any roofer worth his salt. “Modern materials, design and ventilation have completely changed flat roofing” see more here: https://www.irishtimes.com/
Flat roofs which should not not be totally flat as its needs to give a small fall in order to direct water using 1 degree fall to direct water to the gutter. Flat roofs are popular usually on rear or side extensions avoiding the need for an apex or pitch. With some buildings depending on where the windows are giving better views and vista.
Torch-on felt is what covers most flat roofs and has come a long way in the last ten years. This is the go-to option, cheap and readily available, it’s usually applied in two or three layers. They can either have a granular finish or a smooth finish and this can be then painted with solar reflective paint. It usually comes with a materials guarantee of 10-15 years and costs about €55 per sqm.