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Why Do It
A roof dripline filter slows down roof runoff by sending it through a filter media at the roof dripline. This process reduces flooding and provides filtration to minimize pollutants to storm drains and streams. The dripline filter is integrated into a building's foundation backfill, and so takes up a small amount of space.
In South Portland, dripline filters are a recommended approach to meet Drainage Plan permit requirements.
The roof dripline filter is part of a building's foundation backfill. Runoff from a peaked roof without gutters falls onto the dripline filter, preventing runoff into driveways and streets, and providing filtration through the sandy foundation backfill. Water which does not infiltrate into adjacent soils is then discharged via a foundation drain or equivalent.
Buildings with footers above the seasonal high groundwater level are most appropriate for dripline filters. Peaked roofs without gutters are ideal. The drip line trench needs to extend the length of the building or area of roof to be treated.
Design and Installation
The filter bed should be considered as a part of the foundation backfill, and should be designed with input from an experienced builder. An underdrain pipe system is installed to drain the infiltrated water and can also serve the purpose of underdraining the foundation. An impermeable membrane on the building side of the dripline filter may be required to avoid flooding basements.
A dripline filter bed needs to be maintained like any other filter or drainage system. The footer drain should be checked periodically (preferably in the spring) via the cleanout to determine the longevity of ponding within the drain. If standing water is present within the system for greater than 24 hours then clogging may be occurring in the footer drain. Visual or camera inspection may be required.