Proper Grading and Drainage
By John Nolan On March 22, 2017
It's a narrative I experience far too often: I show up on inspection day and one of the first things I notice is poor drainage: Overflowing or (worse) missing gutters, disconnected downspouts or downspouts that deposit water on the ground right next to the house. As I take my first walk around the perimeter I see drip troughs, sunken areas, and puddles of water/ice next to the home, moss growing on the concrete, and earth that slopes toward the house (known as negative grading), all of which indicate water trapped against the foundation. I haven't gone inside yet and I can already see the damage from excess moisture within.
Capillary Action
Water that is held against the foundation gets absorbed through the concrete via capillary action and can actually flow upward much like a paper towel dipped into a bowl of water (see photo, credit Wikimedia Commons). This moisture can reach the wood framing members and basement window sills, causing rot and mold growth. Nearby wood destroying insects such as termites and carpenter ants are attracted to soft, deteriorated wood and will often gain a foothold in these vulnerable areas.
Water Intrusion
Most concrete foundations settle, resulting in hairline cracks that are seldom a structural concern. However, these cracks can allow water that has been trapped against the exterior foundation walls to easily get through to the inside. Even if the foundation hasn't cracked, hydrostatic pressure created from the buildup of water on the outside can force moisture through the porous concrete walls. The resulting efflorescence (white, salty deposits, see photo) can be seen on the inside of the foundation and is an indication that moisture has gotten through. Standing (liquid) water can cause serious issues: Corrosion of the vertical support columns, bacterial growth, electrical shock hazards, etc. and both liquid water and water vapor can contribute several gallons of moisture to the whole envelope, raising humidity levels as it migrates throughout the entire home, and can even result in attic mold growth.
Carry the Water Away!
Alright, so we know lot grading and drainage have a significant impact on the building, simply because of the direct and indirect damage that moisture can have on the foundation. 90+% of foundation leakage problems are caused by surface water (rain or snow), not underground water (streams or high water tables). It is very important, therefore, that surface runoff water be adequately diverted away from the foundation via proper drainage and grading. So what does proper grading and drainage look like?
Gutters and Downspouts
There is an old adage: An ounce of prevention is worth a pound of cure. No foundation wall system is waterproof and preventing water from getting near the foundation is the simplest, cheapest, and often the most effective way to prevent moisture issues within the home. Aluminum gutters cost around $7 per linear foot so a full installation is not going to break the bank and could save thousands in the long term. Gutters should be slightly sloped toward the downspout(s) and downspout extensions should be present to carry the water away from the home, ideally 6 to 10 feet. Be aware that downspouts that discharge onto driveways, walkways, and patios can result in ice hazards. If there is any tree cover around the house, you can expect to have to clean your gutters every spring and fall. A gutter clogged with leaves and debris is likely to overflow, putting water next to the foundation and resulting in unsightly drip troughs below. Gutter guards can help keep the larger leaves and twigs out so you will not have to clean them as often.
Grading
The earth around the home should carry water away from, not toward, the foundation. A slope of 1/2 inch downward per foot of horizontal run should be maintained for the first 10 feet. If this slope is not present, backfill may be added on the foundation side and tamped down until the proper slope is present. Do not use gravel or sand as water tends to flow downward through these materials rather than on top. Be advised not to fill too close to the siding - leave at least 6 inches of foundation showing to dissuade wood destroying insects and to allow the exterior wall to breathe.
If you have a nearby hill or other land feature that directs water toward the house, you can create what's called a swale (see photo) to direct water around the home.
