Yes, rainwater can still be collected from a flat roof.

50 gallons of rainwater can go a long way to improve your plants life. Most tap water contains disolved salts and minerals. These salts can build up in the soil over time. This build up can cause your plants roots to be inefficient or reduce their depth. Using rainwater periodically will help to push the salts farther down into the soil, allowing your pants roots to grow deeper and be more effective absorbing nutrients.

Most people use rainwater to supplement tap water for outdoor irrigation. Since rainwater is naturally soft water, it helps to push salts in the soil further down, away from plant roots. Rainwater can also be used indoor for flushing toilets, doing laundry, etc. However, indoor use requires additional filtering and plumbing.

Water is directed to the tanks with either a Dry or Wet system. In a Dry system, the water from your roof is directed by gutters into the top of the tank. Typically, this means that the tanks are located next to the home and near the downspout. In a Wet system, the water from your roof is directed by gutters to the downspout. The downspout then runs underground to the tank. The tanks in a Wet system can be located farther away from the house.

There are two types of rainwater harvesting, active and passive. Active rainwater harvesting is the collection and storage of rainwater from your roof into tanks for later use. Passive rainwater harvesting directs ground runoff with earthworks to slow water and allow it to soak into the ground on site.