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To ensure good drainage and a dry basement, consider the total volume of water that you must manage or divert from the basement perimeter. For example, consider a house built on a city lot that's 75 feet wide and 125 feet deep (23 meters by 38 meters), or a total of 9,375 square feet (874 square meters). If we multiply 9,375 square feet by 144 (square inches per square foot) (or 874 square meters by 10,000 centimeters per square meter) we have 1,350,000 square inches (8,740,000 square centimeters). A rainfall of 1 inch (2.5 centimeters) would mean that we have 1,350,000 cubic inches (8,740,000 cubic centimeters) of water. If we divide 1,350,000 by 231 (the number of cubic inches in a gallon of water) 3,785 mililiters/gallon/3, 788 liters, we realize that 5,844 gallons (22,119 liters) of water is dropped by a 1-inch (2.5 centimeters) rainfall on this lot. A 2-inch (5- centimeters) rain would mean dealing with 11,688 gallons (44,138 liters) of water in one rain storm—enough water to fill several swimming pools. Water naturally flows down hill, and for water that reaches the perimeter of the foundation, downhill means into the basement. You thus must manage the water that falls on the entire lot so that it's diverted or deposited at least 5 to 10 feet (1.5 to 3 meters) from the foundation wall. If your landscaping is poorly done, much of that water can find its way into your basement.
If you make your inspection during a rain, you can see which way the water runoff flows, so it's wise to do your detective work when it's raining. Stand in your back yard. Note which way the lot slopes. If the lot is properly graded, most of the water will find its way to curbside, and be carried away by storm drains. If you live near a pond or running stream, the water runoff may be toward these bodies of water.
If standing in your back yard is like standing in an arena, with all sides of the grade sloping toward your foundation, you may have to call in an engineer to create a new landscape plan. The engineer will be able to assess the problem and order a new survey, then have an excavating contractor re-grade the property. This extensive remedy is rarely needed for existing houses, because establishing proper grade and water runoff should have been done, and usually is done, during the construction process.
Also, be aware that the most critical portion of your lawn is the 10-foot (3-meter) strip on all sides and immediately adjacent to your house. A lot may have a considerable slope or even a hill sloping toward the house, but if the grade slopes away from your basement walls so that water can't stand or puddle within 10 feet (3- meters) of the basement, the basement will remain dry.
So, to maintain a dry basement you must be especially careful to divert all the water that (1) falls on the roof of the house, as well as (2) the water that falls on the 10-foot-wide (3-meter-wide) strip of lawn immediately adjacent to the house on all of its four sides. I chose the 10-foot- (3-meter) width because any water that falls and puddles within 10 feet (3 meters) of the house perimeter may either run back toward the basement or may seep through light (sandy or loam) soil to reach the basement, where it may cause a problem.
Obviously, if even a small portion of ground water finds its way back to the basement, you can have a serious basement water problem. To avoid any problems with water or humidity in the basement, you must formulate a plan to divert all this water away from the basement walls. First, you must solve the problem of disposing of the roof water, then establish a proper slope or grade to carry away the water that falls upon the lawn.
Sump Pump: Foundation perimeter drain pipe carries water into the sump; pump directs water to disposal.
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