Roofs are actually designed to shed water, so all the rain water striking the roof surface winds up at the drains. This is why it is so important to make sure roof drain screens are clear and allow free flow.
Water weighs over eight pounds per gallon, so ponding water can allow weight to accumulate quickly which can lead to subsidence with increasing volumes of water gathering during subsequent rain.
Most commercial roof designs include an overflow drain as a safety valve. These are often a raised scupper drain outlet, or a second sump drain with a collar surround to prevent the water level from rising above that height. Still, it is best to avoid the height of standing water prior to reaching the over flow.
Drains are frequently located in the corner of the roof area and form a natural place for wind blown debris to collect, so be sure to have the roof drains cleared annually as part of routine maintenance, and definitely check the dome strainers for blockage prior to large rain events.
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