what is a Flat roof

A flat roof is a rooftop that is almost horizontal, with only a slight slope of between 1/4″ to 1/2″ per foot (up to 10 degrees) – enough to drain water if fitted properly.

However, flat roofs can hold snow and water for longer durations compared to the standard pitched roofs hence they require special type of materials to remain waterproof. https://en.wikipedia.org/wiki/Flat_roof

Different Types of Flat Roof Material Options.

Built-Up Roofing

Going back about 120 years, the grandfather of flat roofs is the built-up roof or BUR. Built-up roofs have solved the low pitch roof problem for ages, a challenge that asphalt shingles could never handle.

Built-up roofs are installed using several layers of a special type of roofing felt that has been asphalt impregnated and embedded in bitumen applied with a hot mop.
It consists of several layers of material: a bottom layer of insulation board, several middle layers of bitumen alternated with layers of reinforced fabrics (roofing felt, or ply sheets), and a top layer of gravel.https://www.thespruce.com/types-of-flat-roof-material-options-1824707

Single Layer Membrane Roof
The membrane roof is the latest roofing technology and is also the roof of choice in commercial construction.Membrane roofs are attached in a variety of ways. They can be partially loose laid and held down with river rock ballast (requires roof structure to support the weight) or fully adhered to the roof deck. EPDM and Neoprene roof seams are fastened by adhesive and PVC roof seams are heat or solvent welded.

Modified Bitumen Roof
Modified bitumen roofing (MBR) is asphalt-based sheet roofing that was developed in Europe in mid 1960s as a lighter alternative to BUR. It is designed for buildings with low-slope roof structures.

It is designed using the built-up roofing technology, with manufacturers adding polymer reinforced roof wear layers or rolled sheets to give builders a wider array of options compared to the BUR system