How your house is sited—where it sits on its lot and how the land slopes around it—is important in keeping basements dry. Improper grading—that is, ground that doesn't correctly slope away from the foundation—can allow excessive amounts of groundwater to collect in the soil around basement walls. The ground should slope away from the foundation of your house for several feet at a grade of at least 1 inch per foot. It is improperly sloped if it is level or angles toward the house. Unfortunately, all of the following can easily affect grade:
A level (or spirit level) is used to determine whether a surface is flat or at an angle. It contains a sealed, liquid-filled tube with an air bubble. The bubble moves to the center of the tube when the instrument is on an even plane.
Incorrectly installed landscaping. Foundation beds should follow the original grade of site, not slope toward the house.
Soil compaction. The soil used to back-fill around foundations settles over the years. As it does, grading can flatten or even begin to slope toward the foundation.
Soil erosion. Wind and water can cause soil erosion, also resulting in flat or negative grades.
It's easy to check the grade around your house. You'll need a level and a flat, straight piece of thin plywood or metal that won't sag. Here's what you do:
Put the piece of wood or metal on the ground next to the house. Remove any decorative rock or mulch—you want to take the measurement on bare ground, as water flows right through rock or mulch.
Place the level on the wood or metal, perpendicular to the house. Raise the lower end of the level until the bubble centers on the indicator.
Measure the height between the end of the level furthest from the house and the ground. This will tell you how many inches of drop there is from the house.
As mentioned, the ground should slope away from the house for several feet at a grade of at least 1 inch per foot. As an example, if you're using a 3-foot level, there should be a 3-inch difference between the two ends of the level. This may not seem like much, but it's all you need to keep water away from your house.