Rainwater falling on steeply pitched roofs with overhangs is collected by gutters and downspouts and is carried away as surface runoff, or underground through a storm sewer. Even flat roofs have a slight pitch, and the water collects into roof drains that pass through the interior of the building. Drain leaders are pipes that run vertically within partitions to carry the water down through the structure to the storm drains. Interior drains are usually more expensive than exterior gutters and leaders.
Rainwater can be retained for use on site. Roof ponds hold water while it slowly flows off the roof, giving the ground below more time to absorb runoff. The evaporation from a roof pond also helps cool the building. Water can be collected in a cistern on the roof for later use, but the added weight increases structural requirements.
Porous pavement allows water to sink into the earth rather than run off. One type of asphalt is porous, and is used for parking lots and roadways. Low-strength porous concrete is found in Florida, but wouldn't withstand a northern freeze-thaw cycle. Incremental paving consists of small concrete or plastic paving units alternating with plants, so that rainwater can drain into the ground. Parking lots can also be made of open-celled pavers that allow grass or groundcover plants to grow in their cavities.
Sites and buildings should be designed for maximum rainfall retention. In some parts of North America, half of residential water is consumed outdoors, much of it for lawn sprinklers that lose water to evaporation and runoff. Sprinkler timing devices control the length of the watering cycle and the time when it begins, so that watering can be done at night when less water evaporates. Rain sensors shut off the system, and monitors check soil moisture content. Bubblers with very low flow rates lose less water to evaporation. With drip irrigation, which works well for individual shrubs and small trees, a plastic tube network slowly and steadily drips water onto the ground surface near a plant, soaking the plants at a rate they prefer. Recycled or reclaimed water, including graywater (wastewater that is not from toilets or urinals) and stored rain, are gradually being allowed by building codes in North America.
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