Lot Grading

Do not underestimate the importance of proper lot grading. Improperly sloped lawns are probably the single biggest contributor to wet basements.

The lawn should be sloped downwards so that it drops at least 6 inches in the first 10 feet (or about 5%) away from the foundation. A harder, impervious surface, like an asphalt driveway, can be sloped less (1.5 inch drop over 10 feet, or 1.25%). If you can establish these slopes around the entire perimeter of the home you are in good shape. Sometimes though, the lay of the land prevents achieving these slopes. In such cases a swale (small ditch) or french drain may be required close to the foundation to catch and divert water away from the home. These kind of features are necessary if your home is built into a hill or you simply have the lot sloping towards the home. Never let water accumulate/pool next to the home. It will find its way into the basement through the foundation; and if you have a finished basement this water damage can go unnoticed for a long time.

Keep siding at least 6 inches above grade

When siding is in contact with the ground it can absorb moisture. This can directly damage the siding, but even worse it can allow the wooden floor and wall structure components to become wet and decay. Again, it can take a long time for you to notice that the OSB sheathing behind your siding is rotted. If your floor joists begin to rot you may be in for expensive repairs.

To illustrate my point have a look at the next photo from one of my inspections. Surface water was not adequately diverted away from the foundation and there was virtually no clearance between the siding and grade. When I accessed this corner from inside the crawlspace my suspicions were confirmed; significant wood decay in the rim-joist, not good.

The final word …

Keep a good eye on the way your property handles water. Go outside during a heavy rainfall. Backfilled lawn material will compact and settle over time – where you once had an appropriate slope you may not anymore. Slope alone is not enough – the soil should not be porous – ideally there is some clay in your top layer of soil adjacent to the foundation to minimize how much water gets absorbed into the ground. Even if it looks like you have good slope the water may quickly filter through your top layer of soil and then it may encounter a negatively sloped impervious subsoil underneath that will direct the water right at your foundation. If you have a wet basement but good lot grading then you probably have a bit of digging to do to figure out what kind of soils you’re dealing with.

A good home inspector doesn’t simply look at the house but also looks at things that can affect the house; such as lot grading.