Water Under a House
By John R. Schneider
Mar 10, 2008, 19:11
Q. Over the past few weeks, I’ve noticed a strong musty smell in the living room and dining room. I looked all over for signs of moisture or leakage but did not find any. However, when I looked into the sub area the smell got worse, and I noticed standing water under my house. The water appeared about 2” to 3” deep, and covered most of area that I could see. I’m very concerned, and do not know where the water is coming from. Is it possible that this is from a plumbing leak? How do I find out, and what do I need to do? D.S., Dublin
A. A little standing water under a house is not uncommon especially after all of the rains we have been having lately. Once the soil around the house gets saturated with water, water will tend to seep under it and often go unnoticed. However, too much standing water can cause structural damage to the foundation, corrosion to furnace ductwork, wood rot to the framing members, and cause mold and mildew to grow. The amount of water necessary to cause this damage depends a lot on the type of soil under a house, the amount of underfloor ventilation, and the clearances between the earth and the components of the house.
Standing water under a house is usually the result of only two things; either a leaking water line, or from surface water drainage around the house. If it was from a leaking water line, the leak would have to be substantial, and you would hear running water in the house. It would take a few days of leaking to create the amount of water you described, and the leak would continue until the water to the house was shut off.
I suspect that the water under the house is actually the result of poor surface water drainage around your home. It is important for all homes, whether they are built with a raised foundation or on a slab, to be constructed so that the water from rain and irrigation drains away from the house. This is why most homes built within the past 20 years have all of their downspouts connected to a drainage pipe to carry the water to the curb and gutter, and why walks, patios, and planter areas slope away from the house. Builders understand the importance of keeping water away from the house’s foundation.
To determine if the amount of water under your house is serious, look into the sub area from the access opening with a flashlight. Usually, if there is enough height to the sub area, standing water is not necessarily a concern. If the water only covers a small portion of the sub area, and other areas are dry, it is not significant, and will probably drain away after a few days.
However, if the water covers a large area and is in contact with the base of any wood supports, or the furnace ductwork, then the water should be removed. The simplest way to remove water from a sub area is to use a sump pump to pump the water out. The pump must be installed at the deepest area of water to be efficient, and the water needs to be discharged away from the house.
If it turns out that there is a lot of water under the house, then consideration must be given to improving the drainage around the perimeter. A simple investigation can often determine where the water is coming from. Water can drain into a sub area from planter areas next to a house, or low points in the landscaping. Walk around the outside of your house and see if you can spot where the low points might be. If most of the water is under the rear of the house, there is probable a low point at the rear yard. Next, check to make sure that all downspouts drain away from the building by using either splash blocks, or corrugated piping to divert the water to a safe location.
Sometimes correcting the low points of drainage and adjusting the downspouts is not enough to prevent the water from infiltrating into the foundation, and other measures must be considered. The best solution for keeping water from getting into the sub area is to install a French drain on the sides of the house where the water appears to be coming in. A French drain is basically a 12” wide trench dug around a building down to the base of the foundation, and then filled with gravel and a perforated pipe sloped to collect and divert the water to the street.
Although, basic in design, French drains often require the expertise of a foundation or drainage contractor, or landscaper to install so that they operate properly. If the drain is installed too far away from the house, does not have the proper slope, or has not be installed with a filter fabric (to keep the soil from clogging the trench), a French drain will not work. Installing a French drain can cost thousands of dollars, therefore before you consider one, make sure that it is necessary.
My best advice for homeowners is to check your sub areas at least twice during the rainy season for signs of standing water. If standing water has been under the house in the past, it will leave a mud like (bathtub) ring around the inside face of the foundation, the interior pier supports, and any furnace ducting or pipes that have been in contact with the water. Find out if there are drainage issues before they cause a problem, because it is much easier to fix them when it is not pouring rain and the ground is real muddy.
John R. Schneider is a licensed general building contractor and an ICBO certified residential code specialist. He is president of All About Homes, a residential inspection company, and has been performing code and construction consultations since 1985. Readers may address their comments to John Schneider, 24326 Mission Blvd., Suite 7, Hayward, CA 94544, Fax number: 510-537-8666, or on the web at www.allabouthomes.com . Schneider will answer questions of general interest in the paper. He reserves the right to edit the letter for brevity and clarity. Readers are encouraged to contact a competent contractor or code consultant for specific information regarding questions they may have about their home.
Copyright 2005, John R. Schneider, all rights reserved.
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