Roof Drainage and Adjoining Properties

Stormwater Runoff

• Building construction changes the drainage patterns of a site; it adds impervious surfaces, increasing the volume and velocity of stormwater runoff. Runoff that's not channeled can cause erosion and leaking foundations at adjoining properties as well as at the property where the runoff originates. The roof drainage system generates much of the stormwater on the site. Improper installation and maintenance of gutters and downspouts and the discharge can create significant problems for house foundations.

Property Drainage

• Drainage needs to be managed on the property. Runoff should be channeled and directed to an approved drainage discharge point, such as an existing storm drain or open vegetated space. Allowing water to evaporate or infiltrate into the ground is an effective way of managing runoff. Grassed swales -- shallow depressions -- are commonly used to channel stormwater on residential sites. Water can infiltrate into the grass, reducing the amount of stormwater that leaves the site.

Roof Gutters and Downspouts

• Roof drainage can't discharge to adjoining properties. The downspout should discharge water so that it doesn't erode soil; a splash pad can be installed, or the downspout can be attached to a solid pipe that carries the stormwater to a safe discharge point. Stormwater from the roof should be directed well away from the foundation of both the owner's and the adjacent property so that the ground doesn't become saturated..

Grading

• Grading properly from the foundation ensures that stormwater is channeled away from the owner's house and the adjoining properties. A minimum slope of 2 percent, or 2 feet for every 100 feet, is needed for water to drain from unpaved surfaces. If the minimum slope can't be met, subsurface drainage may be necessary. Grading should meet the existing grades at the property line, so that drainage on adjoining properties isn't affected. In some places, that's a code violation.