Where Basement Water Comes From

Start outside

About half of homeowners with basements have some kind of moisture problem, but a wet basement isn’t a law of nature: concrete and stone-and-mortar foundations can be watertight ¹. The key fact is where the water comes from. Most foundation leaks aren’t from a high water table; they come from soil soaked right next to the foundation by snowmelt, rain, or watering. So the first rule is drainage ¹.

Handling roof and surface runoff is one of the most important things you can do for an old foundation. A heavy rain of 2 inches an hour can send about 200 gallons down a house’s downspouts in a single hour ². Dump that at the wall and it soaks the soil, wets the footings, and finds every crack ².

The checklist

1. Gutters and downspouts. Clear them, fix leaks and loose joints, and make every downspout empty well away from the foundation through an extension pipe or a drain that actually carries water off ². Watch them in a hard rain.
2. Grade. The ground should fall away from the house: try for 6 inches of fall in the first 10 feet ². Flower beds, settled soil, and patios that tip toward the wall all undo it.
3. Window wells. Cover open ones ^{2 1}.
4. Pipe holes and cracks. Where water, sewer, and gas lines pass through the foundation, seal the openings; they’re direct paths for ground water ².
5. Sprinklers. Irrigation set too close to the house adds a tremendous amount of water at the foundation ².

Around an old foundation, builders left a working trench and backfilled it, often loosely or with rubble. That builder’s trench can act like a reservoir holding water against the wall, and a leaking downspout drain can feed it far from where the water shows up inside ².

Digging out the foundation, dampproofing it, and adding footing drains are for after these steps have been tried ².

Condensation or seepage?

Not all basement water comes through the wall. Summer humidity condenses on cool walls, floors, and pipes, and clothes drying, showers, and open windows in humid weather add more ¹.

To tell which you have, tape a square of plastic, about a foot on a side, tight to the wall or floor on all four edges and leave it for a day or two. A taped plastic sheet is a standard way to show moisture coming through concrete ³. If the room side of the plastic is wet, the water came from humid air: that’s condensation, and the cure is drying the air (lesson 3) ⁴. If it’s wet underneath, water is coming through the masonry, and the cure is outside.

Don’t seal it in

It’s tempting to paint a damp stone wall with waterproofing or plaster it with cement. Don’t: a waterproof coating such as cement parging or vinyl on the inside of a damp wall just raises the pressure and drives capillary moisture higher up the wall, along with its damage ². Old stone foundations need to dry to the inside; lesson 2 covers repointing them with mortar that lets them.

Judging a salesperson

Basement waterproofing draws exaggerated claims. Be wary of anyone who ¹:

• says you have a water-table or soil problem without independent soil borings;
• claims one system fixes any moisture problem in any foundation;
• offers a discount for signing now;
• warns your foundation will collapse soon without their system;
• doesn’t mention condensation, grading, or your gutters and downspouts.

Try it

Next hard rain, put on a coat and walk around the house. Watch where each downspout empties, where water pools, and which way it runs. Then go to the basement and see where it’s wet. Most of the time the two maps line up.