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Sump Pit Cover Basin
Quick answer: A sump basin is the liner that holds water for the pump; its diameter sets how many gallons sit in each inch and how long each cycle lasts. Common sizes run 18 to 30 inches across. The cover keeps out debris, children and pets, cuts odour and humidity, and, when sealed, helps radon control.
What the basin does
The basin, or pit liner, is a plastic, polyethylene or fibreglass container set into the slab. Drain tile enters through openings in its side, and gravel surrounding it helps water flow in while keeping soil out. The basin's shape controls more than people expect. A wider basin stores more water per inch of depth, so a given pump runs longer, less frequent cycles. A deeper basin keeps the pump submerged and gives room for a backup pump stacked above the primary.
Basin sizes and volume
| Diameter × depth | Gallons per inch (approx.) | Total gallons if full (approx.) |
|---|---|---|
| 18 × 22 in | 1.10 | 24 |
| 18 × 24 in | 1.10 | 26 |
| 18 × 30 in | 1.10 | 33 |
| 24 × 24 in | 1.96 | 47 |
| 24 × 30 in | 1.96 | 59 |
| 30 × 36 in | 3.06 | 110 |
Values use the volume of a cylinder divided by 231 cubic inches per gallon and ignore the space the pump occupies. Many basins taper slightly, so real volume is a little lower. Use the pit volume calculator for your own measurements.
Solid versus perforated basins
Perforated basins have holes throughout the wall so groundwater can enter from the surrounding gravel even without drain tile. Solid basins accept water only through specific inlets, which keeps more soil out and is often preferred where drain tile is connected or radon sealing matters. Some installers wrap perforated basins in filter fabric to reduce silt. The choice depends on soil, drainage design and local practice; a plumber or waterproofing contractor will usually recommend one.
Worked example: upgrading a small pit
A homeowner's original 14-inch clay-tile pit holds about 0.67 gallons per inch. Their 1/3 HP pump moves about 30 GPM at their head and empties a 6-inch drawdown (about 4 gallons) in 8 seconds, cycling up to 60 times an hour in spring. A contractor proposes replacing it with a 24 × 30-inch basin. At 1.96 gallons per inch, the same 6-inch drawdown now holds about 11.8 gallons; the pump runs about 24 seconds per cycle and, at the same inflow, starts roughly one-third as often. Fewer starts should mean less switch wear and space for a backup pump.
Illustrative note (2026-10-29): This example uses hypothetical inflow and pump figures prepared on 2026-10-29. It illustrates how basin diameter affects cycle frequency, not a guaranteed result.
Types of cover
- Basic snap-on lid. Keeps out debris and small hands. Often has cut-outs for the discharge and cord.
- Sealed or airtight lid. Gasketed lid bolted to the basin with sealed grommets for pipes and cords; reduces humidity, odour and soil gas.
- Radon-rated lid. Designed as part of a radon mitigation system, often clear so you can see the pump.
- Clear lid. Lets you inspect the water level and float without removing the cover.
- Lids with inspection ports. Small openings for checking water level or adding test water.
Why seal a pit?
An open pit lets moist air into the basement and can be a pathway for soil gas. In radon-prone areas, sealing the sump is often part of mitigation. Health Canada and the U.S. EPA publish radon testing and mitigation guidance; a certified radon professional can design a system. A sealed lid also keeps odours down and stops debris falling in.
Fitting a cover
- Measure the basin's inside diameter and rim style; lids are made for specific basin sizes and lip profiles.
- Count penetrations: primary discharge, backup discharge, cords, alarm cable, radon pipe.
- Use grommets for cords and seal pipe penetrations with rubber gaskets or appropriate sealant.
- Leave a way to access the pump for testing without cutting the seal.
- Ensure the lid can bear the weight of someone accidentally stepping on it.
Replacing a damaged basin
Cracked or collapsing basins let soil and gravel into the pit, wearing impellers and clogging floats. Replacement involves removing the pump and discharge, breaking out the slab around the basin, setting a new liner, re-bedding with gravel and patching concrete. It is a natural moment to upsize the basin and add a backup. See installation cost for factors that affect price.
Safety considerations
Open pits are a drowning and injury hazard for young children and a trap for pets. Keep a secure lid in place at all times except during inspection. Never stand on a lid not designed for load. Keep electrical connections above the lid and out of the pit.
Choosing a basin size for a new pit
For new installations, many contractors favour a basin of at least 24 inches across and about 30 inches deep, because it allows longer pump cycles, room for a backup pump on a stand above the primary, and space for a float to swing freely. Smaller basins are common in older homes and tight spaces, and they can work well with vertical floats or pedestal pumps. The right size depends on inflow, pump choice and the space available under the slab, so treat these preferences as a starting point for a conversation with your plumber.
Living with an existing small pit
If enlarging the basin is not practical, you can still reduce cycling: choose a pump with a vertical or adjustable switch that allows the widest safe drawdown, make sure the check valve is working so drain-back does not refill the pit, and keep the basin clean so its full depth is usable. Those steps cost far less than breaking concrete.
Frequently asked questions
Can I put any lid on my basin?
Lids are sized to basin diameter and rim style. Measure carefully and check penetrations.
Does a sealed lid make the pump run hotter?
Submersible pumps are cooled by pit water, not air. A sealed lid should not affect cooling.
Should a backup pump fit in the same basin?
Ideally yes, which is why many installers prefer 24-inch or larger basins.