First, all roofs must have a slope. Sometimes the word “flat roof” is used in commercial roofing but by code and for practical purposes, it must have a slope. It can never be completely flat.
The slope can result from the structure and framing underneath. Or if it is nearly completely flat, you can add slope with the insulation you use.
The roof’s slope must at least be 1/8 of an inch per foot. Without that, you will have standing water, which will cause major issues. Most manufactures like Carlisle make 4×4’ or 4×8’ insulation board with a 1/8” slope (labeled AA, A, B, C, D, E, F, FF) that can be installed over 2” or 4” ISO to create a gradual slope throughout the commercial roof.