Green Roof Runoff

Precipitation generally runs off the roof of a building into gutters and flows into a storm sewer.

Green roof runoff.

Green roofs have a layer of plant material that absorbs water like a sponge. They capture water when it rains slowly releasing it through evaporation and plant use. Green roofs can reduce stormwater runoff via water wise gardening techniques. A green roof will typically intercept the first 5mm and more of rainfall providing interception storage.

A study presented at the green roofs for healthy cities conference in june 2004 cited by the epa found water runoff was reduced by over 75 during rainstorms. The amount will be dependent on the depth and type of substrate in the green roof system. Due to the volume reduction actual nutrient loadings from green roofs are less than asphalt roofing runoff or otherwise manageable at the downspout. In the summer a green roof can typically retain between 70 80 of the run off.

Green roofs mitigate water runoff and sewer overflows. A green roof s plants remove air particulates produce oxygen and provide shade. Green roofs can significantly reduce the amount of rain water that would otherwise run off an impervious roof surface. The depth of water falling on the green roof that is permanently held within it.

Vegetation and soil act as a sponge absorbing and filtering water that would normally plunge down gutters wash through polluted streets and over tax sewer systems. Their major drawback is the initial cost. Green roofs play a significant role in retrofitting the low impact development lid practices in urban areas. Green roof runoff does contain concentrations of some nutrients and other parameters but values are in line with other planted systems.

A green roof also helps control storm water runoff and retention. A typical extensive green roof of 3 5 inches depth will reduce the volume of runoff by about 55 in central pennsylvania and attenuate the peak runoff rates to the level of the pre development land use. Green roofs are very effective stormwater bmps. Water is stored by the roof s substrate and.

The increased urbanization of towns and cities has resulted in less green space and more impervious surfaces. Research in the uk has shown the average depression storage value to be 5 of the soil substrate depth.