Q. I get water in part of my unfinished basement. I can’t determine whether it is coming in from the outside or up from the water table. I installed a dehumidifier, but I’ve been told that this actually draws in more water if the problem is the water table. What are your thoughts?
SAM KAFRISSEN, Arlington
A. You may be getting water in your basement for any of the following reasons: lack of gutters, improper ground sloping, clogged perimeter pipes, or hydrostatic pressure.
Your dehumidifier is not causing the water table to rise. Water is seeping in at the wall and floor joint, and oftentimes that can mean hydrostatic water pressure is present. During wet weather, the soil outside your basement becomes saturated. Exterior footing drains aren’t very effective at relieving water pressure, because they usually become clogged with silt or plant roots. With no place to go, hydrostatic pressure continues to build up, and the water will eventually seep in. You have options:
If you do not have gutters, consider getting them. If you do have them, make sure they are not clogged and draining 10 feet away from your foundation. Most folks I know do not like them extending out that far, but it’s necessary.
Make sure the ground slopes away from the foundation at a half-inch per foot or 5 inches over a 10-foot distance. If the property lines are too close, consider increasing this slope and use swales or underground drain piping. If there is pavement in this area, make sure it slopes away properly. Gutters can also be piped to dry wells a minimum of 10 feet away.
Install a swale
Sometimes adjacent properties can drain water toward your house. A swale can easily solve standing water, flooding, and yard-drainage problems. It is simply a shallow ditch that carries off water and uses gravity to drain. Swales are often wide but so shallow that you do not notice them in a lawn. I like them because they are maintenance and electricity free, with only one negative aspect. The longer a swale gets, the deeper it needs to be.
Tip: For proper drainage, I install my swales 1 inch deep for every 10 feet.
My neighbor has a hill that dumps a lot of water into my yard when the ground is frozen. I installed a swale that channels it around my house and into a drain basin.
Put in a dry well
Dry wells give downspout water a place to go if the existing grading does not allow for runoff or if there are hardscape issues (patios, driveways, etc.). If installed correctly, they will also work in freezing temperatures. Installing a dry well is not hard, but it is labor intensive if you do not have a backhoe or other digging machine.
A dry well is a passive structure that gives water a place to percolate into the ground away from the foundation. Water flows through pitched pipes and into the well thanks to gravity. When a dry well is installed above the water table, most of the time it will contain air. This way the dry well can accept an initial rush of rainwater very quickly, until the air inside it is displaced. After that, the dry well can accept water only as fast as it can dissipate.
Chose a location 10 to 20 feet away from your foundation and make the dry well pit at least 6 feet deep. This will ensure that you are below the frost line and it will drain. Consider putting your drain lines deeper, too.
Assuming you have ruled out gutters, grading, and ground-water issues, your home’s perimeter drain may be clogged. Fixing this means major excavation, and I often tell my clients to consider other measures.
Tuck-pointing an old mortar basement wall can also help keep moisture from weeping into the basement. Often the best solution to your water problem is to install drainage underneath the floor slab that empties into an integrated sump pump pit.
Most full-time basement waterproofing companies offer and install “interior’’ drainage channels with a special profile that includes a wall flange with spacers for collecting wall seepage, channel holes that allow ground water to enter the drain line, and a large-diameter channel that slopes to the sump pump pit. While this is costly, the repair works.
Rob Robillard is a general contractor, carpenter, editor of AConcordCarpenter.com, and principal of a carpentry and renovation business. Send your questions to email@example.com or tweet them to @robertrobillard. Subscribe to our free real estate newsletter at pages.email.bostonglobe.com/AddressSignUp.