Flat roof homes have particular requirements. Steep sloped roofs rely on gravity to shed water, but flat roofs depend on drains and a waterproof membrane to protect the property below it. The term ‘flat’ is a little bit misleading because no roof is completely flat. It it were, you’d be living under a small lake on your roof. “Gradual slope” is a better description for what’s commonly called a flat roof. Gradual slope roofs have at least a 1/4 inch of slope per foot (or more) so water can flow downhill to drains that direct it to the ground.