How Much Slope for Good Site Drainage?

Keith writes: Is there any recommendation to how high the grade of a home needs to be above an existing drainage ditch.  What is the typical elevation above the ditch needed to get water flowing?

Steve Bliss of BuildingAdvisor.com responds: Many basement water problems result from rain and snow melt that runs of the roof,  pools around the foundation,  and then runs down the foundation wall into the basement or crawlspace. The best solution is to slope the finish grade away from the house so surface water drains safely away. The general rule of thumb is to provide a grade of 1/2 in. to 1 in. per foot for a minimum of 6 to 10 feet on all sides of the house. That’s a drop of 5 to 10 in. over 10 ft. This slope should be adequate to get the water safely away from the foundation into a drainage ditch, swale, storm sewer, or level lawn area. When evaluating a site, too little or too much slope can drive up costs.

While it is a good idea to backfill around the foundation with granular fill such a gravel or crushed stone, the top foot or so should be a dense, non-permeable soil such as clay (with 6 in. or so of topsoil above for plantings). This will help move the water quickly away from the foundation.

 

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