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Site drainage issues

The Old House Web

Editor's note: This story is adapted from theU.S. Department of Housing and Urban Development's Residential RehabilitationInspection Guide, 2000. Clickhere for other stories in this series.


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Poor site drainage leads to a variety of problems, in this case a wet basement.

Observe the drainage pattern of the entire property, as well as that of adjacentproperties.

The ground should slope away from all sides of the building. Downspouts, surfacegutters, and drains should direct water away from the foundation. Check the planting bedsadjacent to the foundations.

Plantings are often mounded in a way that traps water and edging around planting bedsacts like a dam to trap water. Most problems with moisture in basements are caused by poorsite drainage. The ground also should slope away from window wells, outside basementstairs, and other areaways.

The bottom of each of these should be sloped to a drain. Each drain should have pipingthat connects it to a storm water drainage system, if there is one, or that drains toeither a discharge at a lower grade or into a sump pit that collects and disperses wateraway from the building. Drains and piping should be open and clear of leaves, earth, anddebris. A garden hose can beused to test water flow, although its discharge cannotapproximate storm conditions.

Where a building is situated on a hillside, it is more difficult to slope the groundaway from the building on all sides. On the high ground side of the building, the slope ofthe ground toward the building should be interrupted by a surface drainage system thatcollects and disposes of rainwater runoff.

There are two general types of surface drainage systems:

  • an open system consisting of a swale (often referred to as a ditch), sometimes with a culvert at its end to collect and channel water away,
  • and a closed system consisting of gutters with catch basins.

Combinations of the two are often used. The locations and layout of culverts, gutters,drains, and catch basins should be such that if they became blocked and overflowed nosignificant damage will occur and that any resultant ice conditions will not pose a dangerto pedestrians or vehicles.

The design of surface drainage systems is based on the intensity and duration of rainstorms and on allowable runoff. These conditions are usually regulated by the localbuilding code, which can be used to check the adequacy of an existing surface drainagesystem.

In some locations, especially where slopes lack vegetation to slow water flow, it maybe possible to reduce rehabilitation costs by diverting rainwater into a swale at or nearthe top of the slope and thereby reduce the amount of rainwater runoff handled by asurface drainage system.

This swale, of course, must be within the property on which the building is located.The ground beneath porches and other parts of a building that are supported on piersshould be examined carefully. It should have no low areas and be sloped so that water willnot collect there.

Water from the roof reaches the ground through gutters and downspouts or by flowingdirectly off roof edges. Because downspouts create concentrated sources of water in thelandscape, where they discharge is important. Downspouts should not discharge where waterwill flow directly on or over a walk, drive, or stairs. The downspouts on a hillsidebuilding should discharge on the downhill side of the building. The force of water leavinga downspout is sometimes great enough to damage the adjacent ground, so some protection atgrade such as a splash pan or a paved drainage chute is needed.

In urban areas, it is better to drain downspouts to an underground storm water drainagesystem, if there is one, or underground to discharge at a lower grade away from buildings.Water that flows directly off a roof lacking gutters and downspouts can cause damagebelow.

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Settled backfill allows water to collect next to thefoundation wall and penetrates into the basement.

Accordingly, some provision in the landscaping may be needed, such as a gravel bed orpaved drainage way.

When a sump pump is used to keep a building interior dry, the discharge onto the siteshould be located so that the discharge drains away from the building and does not add tothe subsurface water condition the sump pump is meant to control.

The site should be examined overall for the presence of springs, standing water,saturated or boggy ground, a high water table, and dry creeks or other seasonal drainageways, all of which may affect surface drainage. It is especially important to inspect theground at and around a septic system seepage bed, seepage pit, or absorption trenches.

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Where a building is situated on a hillside, swales can beused to direct surface water away from the foundation.

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