Downspout disconnection (sometimes called roof leader disconnection) represents a cost-effective on-site alternative for reducing the volume and cost of stormwater that requires public management. Runoff from residential rooftops is collected by eaves troughs, which are installed along the edge of the roofline. Water collected in the eaves trough is conveyed to ground level by one or more downspouts. Downspouts may then connect directly into the storm sewer system or in older neighbourhoods into a combined storm and sanitary sewer system.
Disconnecting downspouts brings a number of economic and environmental benefits to the municipality and the homeowner:
- in combined sewer areas, disconnection reduces the amount of combined flow requiring treatment and reduces the threat of CSOs;
- in separated sewer areas, the diverted stormwater reduces volumes of flows conveyed and resulting loads to watercourses;
- downspout disconnection can reduce basement flooding from sanitary sewer backups and leaking downspout connections; and
- environmental benefits can result in terms of cleaner watercourses, groundwater recharge, and availability of "recycled" rainwater.
Some Canadian municipalities already have voluntary, incentive-driven, or mandatory downspout disconnection programs in place. In these cases, downspouts on existing homes are disconnected from the sewer system, and downspouts on new homes are built without connections to the system.
Downspout disconnection is focused on stormwater quantity management. By disconnecting downspouts, less conveyance and treatment infrastructure is needed. In addition, major environmental benefits emerge as the volume of stormwater direct discharged to watercourses is reduced, and the frequency and severity of combined sewer overflows (CSOs) can be reduced.
Downspout disconnection reduces the amount of stormwater that is either:
- conveyed along a public separated storm sewer system, and ultimately direct discharged to a watercourse; or
- conveyed along a public combined sanitary / storm sewer system, and ultimately treated at a treatment plant.
The amount of stormwater runoff diverted depends on the amount of rainwater intercepted by rooftops (annual rainfall and roof area) as well as the number of downspouts that can be disconnected.
The City of Toronto investigated the effectiveness of a residential rooftop downspout disconnection program. Evidence from a primarily residential area of the City of Toronto suggests that rooftops cover approximately 20% of surface area, indicating the amount of rainwater that could diverted [J.F. Sabourin and Associates Inc. (1999), Implementation Plan Overview Moore Park / North Rosedale Demonstration Area].
Assuming an annual rainfall of about 700 mm (typical of Toronto, excluding snowfall), disconnection of an average home with a roof area of 140 m2 would result in diversion of nearly 100,000 litres of stormwater from the sewer system each year. In reality, downspout disconnection usually can be done on about 3/4 of a property's downspouts (without resorting to more complex and costly disconnection schemes) in average density urban areas.
Disconnection is expected to have a small impact on the quality of stormwater runoff conveyed through a combined or separate sewer system. A major quality benefit, however, results to receiving watercourses through avoided CSO incidents. The Toronto study indicates that some parts of the combined sewer system overflowed as many as 15 times per year, resulting in a discharge of stormwater and wastewater to receiving watercourses. It was estimated that disconnecting one quarter of the downspouts in the study area would result in a 50% decrease in the number of CSOs. Disconnecting two thirds of the downspouts would nearly eliminate CSOs. The number of CSOs avoided elsewhere depends on specific features of the sewer system.
The costs of downspout disconnection vary depending on whether a simple or complex disconnection occurs. The costs of a simple disconnection are quite small relative to other lot-level stormwater management alternatives. If a connection already exists, the costs of disconnection have been shown to be as low as:
- labour cost (per house) $4.00
- material cost (per house) $6.00
- total cost (per house) $10.00
At the high-end, some municipal governments have made available a subsidy of about $100 to homeowners to disconnect downspouts. This subsidy was estimated to be sufficient to cover the full time and materials costs of a complete disconnection. Materials costs are for downspout extensions, elbows, splash pads, and possibly rain barrels. Note that it may be necessary to deploy rain barrels to manage runoff at some properties, which will cost homeowners about $100 - $200 per barrel depending on volume and make.
For some properties, notably those in high density neighbourhoods or with low soil permeability, more complex disconnection techniques may be required, for example using soakaway pits. These alternatives would have additional construction and on-going maintenance costs.
For new home builders, cost savings may actually result from not initially connecting downspouts to the sewer system. Additional savings may be realized because the reduced flows may allow smaller sewers to be built. No estimates of these cost savings are available.
A great deal of experience exists with downspout disconnection programs. However, no firm technical guidelines exist identifying when a disconnection can or cannot be attempted. Rather, a range of property characteristics must be assessed subjectively. The three most important considerations are:
- Lot Size - The lot size should be sufficient to provide an area for the diverted runoff to infiltrate. Runoff should not pool significantly, or run across the surface onto a neighbour's property. Pooling and cross-property runoff raise a number of safety and legal issues.
- Soil Perviousness - Runoff should be redirected to soft landscaped surfaces such as lawns, gardens, or swales to allow infiltration. If directed to hard landscaped surfaces such as driveways, runoff will flow to the street and sewer system, eliminating potential benefits.
- Property Grade - Downspout disconnection works well on properties with small grades. The grades help avoid significant pooling of runoff. Higher grades (for example greater than a few degrees) are too steep to allow infiltration.
- Proximity to Buildings - Runoff should not be discharged immediately beside buildings or on a grade which would direct flows to buildings. Runoff reaching a building could cause foundation damage or basement flooding.
As such, a number of safety and legal concerns can arise in consideration of downspout disconnection programs. Safety concerns relate primarily to the threats posed by pooled runoff and ice formation on walkways in the winter months. Legal concerns may arise in respect of either of these, or from foundation damage or basement flooding. Discussions with managers of municipal downspout disconnection programs indicate that these concerns can be mitigated through the use of prudent planning (i.e., ensuring that the disconnection is performed properly, and only disconnecting downspouts where the right lot conditions exist).