By Mary Ellen Polson
Basements in older homes are desirable and undesirable in roughly equal measure. On one hand, many of these subgrade spaces offer room for storage, mechanical systems, and potential living space. On the other, any hole in the ground tends to fill with water. No matter what age the house, trying to have the benefits of the former without the problems of the latter is a continuing challenge.
Basements are common in some areas and rare in others, depending on when and where the house was built, the climate, and the difficulty of excavation. Types of below-grade space vary as well, from crawl spaces with four feet of headroom or less, to full-height “daylight” basements built at grade. Whatever type of basement you have, if the water that wants to collect there isn’t vented or drained away, it will inevitably make its impact felt in one undesirable way or another.
Even if you’re not interested in adding a man cave or media room, dampness in the basement is not harmless. It creates an optimum environment for the growth of harmful mold, water penetration that can ultimately undermine the foundation, and wood-boring insects that can eat the house from within.
The Big Five
There are five leading causes of moisture problems in basements and crawl spaces. Solving a moisture problem in the basement may be as straightforward as repairing a few leaky gutters, or as complex and costly as installing a subsurface drainage system. The best approach is to remove or control the source of the moisture, not to try to stop it at the last line of defense. If obvious fixes don’t completely solve the water problem, get a professional assessment from a well-regarded masonry or waterproofing company in your area.
1) Missing Gutters
A well-designed and maintained gutter system directs water away from the house when it rains. When gutters or downspouts are missing, plugged, or improperly hung, rainwater follows the path of least resistance, often percolating—or even flowing directly—into the basement.
The Fix Repair or replace missing or damaged gutters. Make sure they’re installed on a slight slope that encourages the water to flow toward the downspouts, which should be placed at corners and at least every 50' of run. Position the downspouts so that they direct water away from the house, either with extenders or splash blocks. Extensions should discharge water at least four feet beyond the foundation wall.
When condensation appears on cold water pipes or other basement surfaces only when it’s warm, that’s a sign that the basement needs more ventilation.
The Fix Open up any obstructed vents and make sure they’re screened to keep out pests. Add new vents as needed to increase air circulation. It’s especially important that fresh air reaches areas where wood sills are close to the soil, such as corners. To discourage insect infestations, the moisture content of exposed timbers should be 20 percent or less.
3) Structural Cracks
Masonry is porous—that’s one reason there’s such a big market for waterproofing sealers, which create an impervious barrier on the surface of a brick, concrete, or stone wall. If there’s enough water making its way through the wall, however, even the thickest waterproofing material may ultimately allow water to pass into the basement. In some cases, sealants that stop up pores in the masonry can actually cause water pressure to build up behind the foundation wall, leading to cracking and the flaking or pitting known as spalling. Shifting soil, frost heaves, or the expansion of water-saturated earth can cause movement in a foundation wall, especially old ones constructed without footings. All of these forces can lead to cracks in the wall.
The Fix The best way to stop water from entering through cracks or fissures in the wall is to plug them. To patch voids less than ½" wide, use a high-performance exterior masonry caulk. The material should be injected into the crack with a grout gun. If the crack is deeper than it is wide, pack it with epoxy mortar or hydraulic cement, which cures even when wet. Patching small cracks may fix the problem permanently, for a short while, or not at all; water may simply enter through another weak point in the wall.
For larger cracks or failing mortar, repointing is the next logical step. To repoint a damaged area, remove the old mortar to a depth of 1" to permit adequate bonding between the old mortar and the new. Where possible, use a hammer and chisel to remove old mortar; power tools can easily damage the edges of masonry, especially old brick.
If the foundation is more than 100 years old, use a softer, lime-rich mortar that closely approximates the proportions and chemical balance of the original, available from specialists in historic and material-specific mortars. Off-the-shelf premixed mortars are higher in Portland cement than historical mortars and are too hard for historic repair work.
4) Poor Grading
When the ground around a foundation slopes even slightly toward the house, water is directed toward the lowest possible point. Proper drainage is especially important around entry stoops and bulkheads. In older houses, the soil below concrete or brick steps has often settled or eroded away, creating an opportunity for water to enter the basement.
The Fix Correcting grading problems may require anything from minor landscaping to full-scale reshaping with a backhoe. The goal is to create a gentle grade that slopes gradually away from the house.
Sometimes all that’s needed is a bit of contouring of the soil at grade to create the necessary relief, along with the creation of swales—shallow depressions in the landscape—to accommodate larger amounts of water from heavy rains. A swale with as little as 2" of drop across a distance of 5' can channel large amounts of surface water effectively.
Where areas under steps have eroded or compacted, back-filling or even reconstruction of the stoop may be required to meet modern codes. Bulkheads and hatchways should be in good, airtight condition. Make sure the doors are out of the path of water runoff, and adequately flashed to divert rainwater.
5) Poor Subsurface Drainage
A house that still has water in the basement after all the obvious remedies have been explored may be a candidate for a subsurface drainage system. If there’s a history of water in the basement, you may find evidence of an old clay pipe drainage system; often these have failed because they’re broken or become filled with dirt or roots.
The Fix The purpose of a subsurface drainage system is to collect and channel water out of the basement, usually by means of a pump connected to one or more drains. In interior systems, the drains are recessed below the basement floor, usually near the perimeter. In exterior systems, the drains are embedded in a trench around the perimeter of the house and back-filled with a layer of gravel and sand.
Drop a marble to find the lowest point on the floor. Place the sump pump there. Dig shallow trenches (about 12" deep) along the walls leading to the pump, allowing several inches of clearance between the foundation wall and the trench.
Paint a waterproofing membrane on the basement wall from top to bottom. The membrane will create a moisture barrier to help prevent seepage. Then install dimpled membrane sheeting at the base of the wall to allow any water that collects to flow into the drainage system.
Line the bottom of the trench with 2" of gravel, sloping it toward the sump pump. Cut and lay sections of drain pipe to run the length of the trench. Wrap them in landscaping fabric as you work. For later access, install a PVC elbow at the end of the trench furthest from the sump pump. Cover the drains with more gravel until the trench is flush with the floor.
Dig a trench along the outside of the foundation. The trench should be a minimum of 2' wide, and ideally as deep as the basement (less for a crawl space). Lay perforated drainage pipes over a layer of clean soil at the base of the trench. Options include rigid PVC with predrilled holes, or flexible drain pipe cut with slits. For extra protection against clogs, line the trench with a geotextile liner and add a soil particle sock to the pipe.
To get a proper gravity feed, the pipe should slope from a higher point to a point at lower elevation—at least 1" for every 8' of pipe length. Cover the pipe with at least 12" of washed stone, then layer with permeable geotextile landscaping fabric to keep weeds or soil from clogging the pipes. Back-fill the trench with top soil or pea gravel to bring it back to grade height.
Where the Water Comes From
Water can enter the basement from above, below, beside, or even from inside the house. Even if the roof is sound, rainwater that isn’t channeled away from the foundation can eventually trickle or seep into and through basement walls. Water can enter from the ground itself, either from deep below (“rising damp”) or laterally. Moisture produced within the house—from clothes dryers, showers and running water, or steam produced by cooking—can make its way to the basement if the area is not properly vented. Last but not least, heavy, humid air in warm months may drop to the basement and condense on cooler surfaces.
Basement The lowest storey of a building, usually at least partially above grade, with windows and exterior ventilation.
Cellar A traditional term for underground spaces used to store food (as in root cellars) or coal, or a below-grade space with a dirt floor; used regionally as a synonym for basement.
Crawl space An unfinished space below the first floor that’s less than a full storey in height, normally enclosed by the foundation wall, often for plumbing or mechanical systems.
Efflorescence Encrustations of soluble salts deposited on masonry, usually white and typically caused by alkalies leached from mortar as moisture moves through it.
French drain Popularized by Henry Flagg French in the mid-1800s, a French drain is a trench filled with loose stones containing a perforated pipe that directs surface and ground water away from a structure.
Geotextile Permeable fabric designed to separate, filter, reinforce, and drain when in contact with soil.
Raised basement Sometimes called an English basement, this is properly the ground floor of a house, used for service and utility areas.
Spalling The result of water entering brick, concrete, or stone and forcing the surface to peel, pop out, or flake off.
Sump pump A pit, tank, or receptacle installed below grade that receives water or waste, and equipped with a pump to empty the water mechanically.
What’s French About Drains?
Also called a perimeter drain, a French drain is an ingenious method of rerouting subsurface water. The name likely comes from its 19th century proponent, Henry Flagg French, who popularized an underground drainage system in his 1859 book Farm Drainage. French made his drains from ordinary clay roofing tile laid with a 1⁄8" gap between the sections to allow water to percolate.
Perforated drain tiles and pipes soon followed. Since the perforations could easily clog, the pipes had to be embedded in coarse gravel or rock, with finer materials on the outer edges of the trench to act as filters. Today, most French drains are installed with liners made of geotextiles that effectively block the intrusion of soil or particulates, while still allowing water to percolate through to the drainage pipes.
The newest piping materials offer a much greater flow capacity and are made of two 21st century materials: a geotextile drainage fabric and a molded plastic core, or “dimple board.”
Radon is an invisible, odorless radioactive gas that occurs naturally in soil. It’s found all over the United States and Canada, especially along the northern tier of states and most of the West. The radioactive particles from radon filter up through ground sources such as unfinished basements and cracks in the foundation and walls. When inhaled, these particles can damage the cells that line the lungs. Long-term exposure to radon can lead to lung cancer. If your house is in a radon-prone area but hasn’t been checked, buy a test kit. Radon levels in excess of 4 picoCuries per liter are considered potentially dangerous.
Add a dehumidifier While not a solution for a chronically damp basement, a dehumidifier that plugs into an electrical outlet can help clear dampness after a heavy rain.
Vent the dryer & bathroom If your home lacks vents in the bathroom or laundry room, call a plumber to install them. Vents in these areas are required by most local building codes.
Turn on the a/c If condensation appears on pipes, joists, or other basement surfaces when it’s warm and humid outside, close off the vents and turn on the air conditioning. HVAC systems are designed to cool by removing excess water from the air.
Reverse the polarity Using a device that’s smaller than a household dehumidifier, a proprietary system from Aquapol utilizes natural energy forces to reverse the electrical polarity of water molecules in old walls, driving the water back where it came from.