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An environmentally friendly roof
By STEVE HARDY
Impervious surfaces in urban and suburban areas can have a significant negative effect on our environment, specifically our fresh water supply, via rivers, streams and aquifer. Impervious surfaces are sidewalks, paved parking lots, roads, roofs, and any items that are impervious' to water.
Some studies have shown that pollution entering Elliott Bay from run-off occurring across impervious surfaces equals approximately 250,000 gallons of motor oil per year.
In urban areas, roof coverings often create the major impervious surfaces. The water discharge from these roof areas contains contaminants and pollutants, such as oils leaching from roof bitumens, contaminants leaching from membrane adhesives and polymeric sheet membranes, and concentrated amounts of dust and dirt accumulation, often containing animal (bird) refuse.
In commercial buildings and urban areas, water is usually directed from roofs via roof drains, gutters and downspouts, while rain water from roads and sidewalks is directed into gutters. In either case the water is further directed into the city storm sewer system. Once in the storm sewer, the water eventually reaches our bays, rivers or streams, with perhaps only one catch basin system encountered along the way.
A catch basin is an underground concrete tank system, used for collecting water from multiple source pipes, and then re-directing the water toward the dump site. The water discharged is not fresh, and is not environmentally friendly, and can if abundant enough for extended periods of time destroy the aquatic eco-system of a lake, river, stream or bay.
There are positive and economical solutions to this problem. According to Monte McDaniels, the surface water quality investigator for the city of Seattle, the main aspect of making impervious surface water environmentally friendly is allowing the water to settle and deposit pollutants to the bottom of a catch basin or holding tank. The water can then be used for irrigating landscape vegetation, or with other minor treatments, as non-potable water for various building functions. This is demonstrated by King County's new office building, King Street Center, where roof water is used for toilet flushing. The rainwater will supply 60-80 percent of the 2.2 million gallons of water needed annually to flush toilets in that building.
Most commercial buildings in urban Seattle, and specifically those in the downtown area, are already required to have a holding tank, but these tanks are located below ground. A more economical solution for building owners, and a more positive environmental design practice, would be to equip a building with two holding tanks, having the combined capacity of the single tank normally used, with one located within or adjacent to the building.
This accomplishes two significant tasks.
One is economics. According to McDaniels, water does not cost the building owner once, but four times one for supply to the building, and three times for disposal to either the storm sewer system or to the sanitary sewer system. As we all know, the cost for all four items is included once in the monthly bill from the city. With a two-stage holding tank system, the settled roof drainage water from the tank located within or adjacent to the building can be used for non-potable water or for irrigation of landscape. Water used for landscape is naturally filtered back into our aquifer, thereby bypassing the storm sewer system. Recycling of the roof drainage water will save money, eventually paying for the tank system itself.
The second significant task such tanks would perform is that they would allow for better settling and natural filtration of the water, resulting in cleaner water being delivered to our estuaries and bays.
The two-stage system also provides a buffer during heavy rains, and will reduce water surge through the storm sewer system that dislodges contaminants from the catch basing and storm sewer system. If the flushing' action within the storm sewer system can be lessened, cleaner water will be delivered to our fresh water streams and bays.
Another more exotic option is sometimes available, with associated aesthetic benefits as well. This is still a two-stage holding tank system, but the first tank' is a slit trench dug into the grounds surrounding a building. The trench must be designed for size/capacity and percolation or lack of it by a soils engineer. The trench tank will hold water and allow it to settle before being transferred to the secondary, underground tank, which holds the water before it is moved to the storm sewer system.
The trench tank is often filled with gravel aggregates, and specific aquatic flowers and plant life grows from the tank, hiding the aggregate and further cleansing the water.
Steve Hardy is managing principal for Moisture-Tech N.W., a waterproofing and roofing consulting and design firm.
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