Going to Great Lengths for the Good of Your Basement

By Jeanne Huber
Special to The Washington Post
Thursday, August 11, 2005

Q I've read that one of the best ways to prevent basement flooding is to extend gutters some distance from the house. How far should they extend, and are there any clever ways to conceal them?

A Soil conditions and the slope of your lot dictate how far your gutters should extend. If you have sandy soil or a flat lot, you'll need to get the water farther from your house than you will if you have clay soil or a lot that slopes away from the house.

But whatever it takes, it's worth the hassle. If you have a chronically damp basement or crawl space, extending the gutters may fix the problem -- at far less cost than a fancy waterproofing system. This simple fix can also rid a house of mildew smells. It can prevent carpenter ant or termite problems. It can even extend the life of your roof, because excess moisture moving up from underground can condense on the underside of the roof sheeting and rot it out from underneath.

To determine how far you need to extend your gutters, begin by inspecting what you inherited from previous owners or your builder. Many homes have looser soil next to the foundation where the builders filled in after the structure was done. If the surface slopes toward the house, water flows easily through the loose soil and seeps into the basement or crawl space; the water can even cause the building to settle unevenly. Smart builders sculpt the land before they build so that each house sits on a crown of soil that drops at least six inches in the first 10 feet away from the structure. Ideally, this crown includes a cap of clay-rich soil that extends out at least six feet to encourage whatever rainwater lands there to flow away.

The easiest way to check the situation at your house is to go outside during a heavy rain and watch what happens. If the runoff flows away from the walls, you're in luck. A simple concrete or plastic splash block may be all you need under the gutter downspouts. Splash blocks absorb the force of water cascading down from the roof and spread out the flow so it doesn't erode a hole into the soil next to the house. The blocks also direct the water farther from the foundation.

A manual published by ASTM International, originally known as the American Society for Testing and Materials, recommends using splash blocks that get the water at least two feet and preferably three feet from the building if there is a perimeter foundation. Where there are basement walls, it recommends a distance of at least three feet and preferably six feet. The National Association of Waterproofing and Structural Repair Contractors has similar advice but modifies it based on soil type. On nicely sloped, clay-rich soil, it says that downspouts can end as little as three feet away, although five feet is safer. On flat land, it recommends 10 feet unless the soil is very sandy, in which case you might need to go much farther.

If the soil slopes toward your house, extending the gutters won't be enough to keep water away. You'll also need to build up the soil next to the house. However, to avert rot and termite damage you must keep all wood materials, such as siding, at least four inches above the ground; a wider band is even better. To create the slope, add soil rich in clay, not sandy soil, because clay helps seal out water, while sand lets it flow almost straight through. If you can't build up the soil without getting too close to wood, re-contour your lot so that it slants away from the house for about 10 feet out, then rises gently back to the original grade.

As to ways of concealing gutter extenders, you have options. A length of gutter can be attached directly to the elbow at the bottom of the downspout, but an extension installed this way looks ugly and can easily be tripped over by feet or damaged by a lawn mower. So it's probably no surprise that clever people have been busy inventing different approaches.

At you can find a hinge fitting that allows you to flip the extension up against the house when you want to mow or walk nearby. Ace Hardware ( ) also carries Rain Drains, roll-up vinyl attachments that extend automatically when water is flowing. Although they probably work fine initially, they seem prone to damage.

A better solution would be to adapt the splash block concept and build a longer but still shallow channel of bricks or concrete to direct water farther from your house.

Or you can connect the gutter to an underground pipe. On a sloped lot, the pipe can empty the water a short distance downhill, onto a splash block in a planting bed. Just be sure you're not channeling the water where it will cause problems downstream, or for your neighbors. On a flat lot, the pipe can lead to a dry well: a hole in the ground that's lined with filter fabric (also known as weed cloth or landscape fabric) and filled to within a foot of the surface with large rocks, topped by a layer of soil and plants. A dry well gives water a place to pool during heavy rainstorms so that it can then slowly percolate back into the soil. However, if spaces between the rocks fill with soil, the dry well stops functioning. Because it's underground, you might not notice when this has happened.

The most elegant solution is to connect the gutter to a pipe that leads to a rain garden, a shallow trench filled with plants that thrive when their roots are periodically submerged.

Next week: How to build a rain garden.

© 2005 The Washington Post Company