French Drains, Sump Pumps & Grading: A Homeowner's Guide to Crawl Space Drainage

Start With the Right Order

Good drainage work follows a simple order, with the cheapest and most effective fixes first.

1. 1Keep roof water away - clean gutters, and downspouts that discharge well away from the foundation.
2. 2Fix the grade - soil and hardscape should slope away from the house.
3. 3Intercept what is left - a French drain collects water that still reaches the foundation or crawl space.
4. 4Pump it out - a sump pump removes collected water when gravity alone cannot carry it away.
5. 5Seal the space - once liquid water is handled, a vapor barrier or full encapsulation controls ground moisture and humidity.

Yard Grading: The First Line of Defense

Grading is the slope of the ground around your home. When the soil is flat or tilts toward the house (called negative grade), every storm sends water straight to the foundation. The standard building-code target is for the ground to fall about 6 inches over the first 10 feet away from the foundation, roughly a 5% slope. Paved surfaces next to the house, like patios and walkways, should slope away at about 2%, which is a quarter inch per foot.

Common grading problems we see:

• Soil that has settled along the foundation over the years
• Planters and flower beds built up against the house that trap water
• Patios, walkways or driveways that tilt toward the home
• Sprinklers soaking the soil right at the foundation
• Soil or mulch piled too high against siding or over crawl space vents

How it is corrected: low spots are filled and compacted, the surface is reshaped to fall away from the house, and where a property line, wall or slope leaves no room for a proper grade, a shallow swale or an area drain carries the water around the home instead. Soil should stay a few inches below siding and wood framing so the fix does not create a termite or rot problem.

Gutters and Downspouts

One inch of rain on a 1,500 square foot roof is more than 900 gallons of water. If the downspouts drop it at the corners of the house, much of it ends up in the crawl space. Keep gutters clean, make sure every downspout is connected, and discharge the water at least 5 feet from the foundation using extensions or a solid (non-perforated) drain line that carries it to a safe discharge point. Never connect downspouts to a perforated French drain pipe - that pushes roof water into the very soil you are trying to dry out.

What Is a French Drain?

A French drain is a gravel-filled trench with a perforated pipe at the bottom. Water in the soil takes the easy path into the gravel, drops into the pipe and flows downhill to a discharge point or a sump basin. It does not block water; it gives water a faster place to go than your crawl space.

How a French Drain Is Installed

Sump Pumps: When Gravity Is Not Enough

A crawl space is often the lowest point on the property, so collected water has nowhere to drain by gravity. A sump pump solves that. A basin is set into the ground at the low point of the crawl space, the drains empty into it, and when the water rises a float switch turns the pump on and sends the water out through a discharge pipe.

Parts of a sump pump system:

• Sump basin - a pit set flush with the ground, with a sealed lid to keep out debris, moisture and odors
• Submersible pump - sized to the amount of water expected. A 1/3 horsepower pump handles most homes; larger pumps are used for high water volume or long, high discharge runs
• Float switch - turns the pump on and off automatically
• Check valve - stops water in the discharge pipe from flowing back into the basin when the pump shuts off
• Discharge line - solid pipe that carries water well away from the foundation to an approved discharge point
• Dedicated GFCI-protected outlet - safe power for the pump
• High-water alarm - warns you if water rises higher than it should
• Battery backup - keeps the system pumping when the power goes out, which is most likely during the same storm that is filling the basin

Where Does the Water Go?

Collecting water is only half the job. It has to be released somewhere that will not send it back to your foundation, onto a neighbor's property or onto an unstable slope. Acceptable discharge points and permit requirements vary by city and are stricter on hillside lots, so the discharge plan should be confirmed for your address before work begins. We go over this during the free inspection.

How Drainage and Encapsulation Work Together

Drainage handles liquid water. Encapsulation handles water vapor and humidity. In a crawl space that gets wet, the drain and sump pump are installed first, then the vapor barrier is laid over the floor and sealed to the walls and piers, and a dehumidifier is added when humidity stays high. Installing a vapor barrier over a crawl space that still takes on water just hides the problem.

Drainage Maintenance Checklist

• Clean gutters and check downspout extensions every fall, before the rainy season
• Walk around the house during a heavy rain and watch where the water goes
• Test the sump pump before the rainy season by pouring a few gallons of water into the basin - the pump should start, empty the basin and shut off
• Check that the discharge outlet is clear and not buried or blocked
• Test the battery backup and replace the battery on the manufacturer's schedule
• Keep soil, mulch and planters from building up against the foundation
• Adjust sprinklers so they do not spray the house or saturate the soil beside it
• Look in the crawl space after major storms for standing water, damp soil or new stains
• Plan on replacing a sump pump about every 7 to 10 years, sooner if it runs often

Drainage Questions & Answers