Sump pumps: what they do and whether you need one

1. Perimeter drain in gravel beside the footing
2. Sump pit
3. Pump
4. Float switch
5. Check valve
6. Discharge line, out through the wall above grade
7. Outlet well away from the house, where the ground slopes away

SoilConcrete / masonryWoodInsulationWater

How a sump pump works

The first thing I tell people is that the pump is only one part of the system. The water starts in the drainage around or under the foundation and flows into the sump pit. As the level rises, the float switch turns the pump on. The pump pushes the water up the discharge pipe, and the check valve keeps the water already in the pipe from falling back into the pit when the pump shuts off. Then the discharge has to carry that water somewhere it can move away from the house.

If any of those parts fails, you can have a perfectly good pump and a system that doesn’t do its job.

Do you need a sump pump?

I would think about a sump pump when:

• groundwater comes in under the slab;
• water shows up at the joint between the wall and the floor;
• a footing drain needs a place to discharge;
• the basement sits below the level it could drain to by gravity;
• the basement has already flooded because of groundwater;
• the water table rises around the foundation during long rains.

I wouldn’t install one just because a basement is damp. If the problem is condensation, a leaking pipe or a downspout emptying next to a window well, a sump pump isn’t the main fix. And if the drainage can leave reliably by gravity, I’d rather not depend on a pump and on electricity when there’s no need. To track down where the water is coming from first, see Water in the basement.

Where should sump pump water go?

I want the discharge to end well away from the foundation, somewhere the ground can take the water without sending it back toward the house. I never assume that a sanitary sewer and a storm sewer are interchangeable. If there’s a storm sewer and the locality allows a connection, that can be an option.

What I don’t want to see:

• a discharge that ends right next to the wall;
• water running across a sidewalk;
• erosion;
• ponding;
• water sent toward the neighbor’s property;
• groundwater connected to a sanitary sewer without a reason.

Local rules matter, and they vary. Two examples from Northern Virginia:

• Alexandria: Ordinance No. 3998 (1998) prohibits connecting new sump pump discharges to the sanitary sewer lines that drain to the city’s combined sewer system.
• Fairfax County: Technical Bulletin 20-07, in effect since January 28, 2021, applies to new homes built under a grading plan. The discharge has to end at least 10 feet from the foundation and, where conditions allow, at least 10 feet from the property line, without causing ponding, erosion or nuisance flooding. It doesn’t automatically apply to every existing home in Virginia.

Check with your city or county before you change where a discharge ends.

Submersible or pedestal

I see far more submersible pumps. They sit inside the pit, they’re usually quieter, and I like them for most basements. A pedestal pump keeps the motor above the water, which can help in a small pit or when you want easy access to the motor and the float. I wouldn’t choose by horsepower alone: I look at flow, how high the pump has to lift the water, the size of the pit and how much water the system actually gets.

How long a sump pump lasts and basic upkeep

From experience, I start watching a pump more closely once it reaches seven to ten years. That’s field experience, not a rule or a guaranteed lifespan. I’ve seen pumps last much longer, and others die much sooner because they ran constantly or the pit was full of debris.

Basic upkeep:

• test it three or four times a year, especially before storm season (my maintenance recommendation, not a code requirement);
• keep the pit clean;
• check that the float moves freely;
• listen for changes in the sound;
• check the check valve;
• look at the discharge outside;
• check the backup and the alarm.

Warning signs