Guide

Sump Pumps, Sump Pits and Radon

Why the pit is such a big opening

A sump pit is a hole through the slab into the gravel beneath it, usually connected to a perimeter drain tile loop that runs the full footprint of the house. From a soil-gas perspective that is not a crack, it is a doorway.

Everything else in a basement floor is a joint, a penetration or a hairline. The pit is an opening measured in square feet, connected to a distribution network under the entire slab. In a house with an uncovered pit, it is routinely the dominant entry path.

The pump itself is irrelevant to this. An empty pit with no pump at all behaves the same way.

What a proper lid looks like

• Rigid, and sized to the pit, not a sheet of plywood laid over it
• A gasket or a bead of flexible sealant around the full perimeter where it meets the slab
• A sealed grommet or boot where the discharge pipe passes through
• A sealed pass-through for the pump's power cable and float switch wiring
• Clear or with a viewing port, so you can see the pit without opening it
• Fastened, but removable, because the pump is a service item
• Where the system draws from the pit, a fitting for the suction pipe

The removable-but-sealed requirement is where most DIY versions fail. Either the lid is caulked down so hard the pump cannot be serviced, or it is loose enough to be useless. Both are avoidable.

Using the pit as the suction point

This is the part worth knowing before you get quotes, because it can meaningfully change the price.

The expensive part of a sub-slab system is establishing a suction field: coring the slab, excavating a pit beneath it, and hoping suction travels far enough to depressurise the whole footprint. A drain tile loop is already a distribution network running the perimeter of the slab, and the sump is already connected to it.

So a sealed sump lid with a suction pipe off it can depressurise the entire slab through infrastructure that is already installed. No new core, no new pit, often a single point instead of two or three.

If your basement has a sump connected to drain tile, ask every contractor whether they intend to use it and why or why not. See system types.

What to check on your own sump

1. Is there a lid at all, and is it sealed or just resting?
2. Can you see or feel drain tile entering the pit from the sides? That is the loop, and it is good news for the system design
3. Does the discharge line pass through a sealed grommet or an open hole?
4. Is the pit dry, damp or holding water? A permanently wet pit is a moisture question as well as a radon one
5. Does the pump actually run when you pour water in?
6. Is there a battery backup? Relevant to flooding rather than radon, but you are down there anyway

Two things that go wrong

Sealing the pit and calling it done

Sealing the largest opening sometimes produces a useful reduction and rarely produces a sufficient one, because the house is still pulling through the perimeter joint and everything else. Seal it, then retest. A fifteen dollar kit settles it. See does sealing cracks reduce radon.

A waterproofing system that opened everything back up

Interior basement waterproofing typically installs a perimeter channel deliberately open to the sub-slab gravel, because it is designed to collect water and route it to the sump. That is a large new opening. A house that tested higher after waterproofing is a common pattern, not a coincidence. See radon after waterproofing.

Radon and water in the same pit

Two things share that hole, and they are usually worth handling together. A sealed pit reduces soil gas entry and also reduces the humidity a wet open pit adds to a basement. If the pit is often running, the house has a groundwater situation worth addressing on its own terms, and correcting exterior grading, gutters and downspouts is the cheap end of that.

If the well serving the house is private, and air levels stay high after a correctly performing system, waterborne radon becomes a separate suspect. See radon in well water.

Common questions

Should I cover my sump pit?

Yes, with a proper airtight lid that has sealed pass-throughs for the pump cable and the discharge line, and that can still be removed for service. A loose board or a bin lid does nothing.

Will sealing the pit stop my pump working?

No, provided the lid is done properly. The pump still runs, the discharge line still passes through a sealed grommet, and the lid lifts off for maintenance. A lid that cannot be removed is a bad lid.

Can the sump be used as the radon suction point?

Often, and it is usually the cheapest good option. The pit already connects to the drain tile loop running the perimeter of the slab, so drawing from it depressurises a large area without coring new holes.