The answer to 'how to install a flat roof' is to use a built-up roofing system or a single-ply membrane system. These systems involve multiple layers of waterproof materials like asphalt, tar, or rubber that are installed over a flat or low-sloped roof deck. The process typically includes preparing the roof surface, applying a base layer, adding insulation, and then installing the top waterproof membrane.
Proper installation is crucial to ensure the roof is durable, watertight, and energy-efficient. It's generally recommended to hire a professional roofing contractor with experience in flat roof systems to ensure the work is done correctly and safely.