How to install a crawl space sump pump
Same machine as a basement install, completely different job. You are digging a 22-inch hole while lying on your side, and the sequence matters.
To install a crawl space sump pump: find the low point, dig about 26 inches down for a 22-inch basin on a gravel bed, set and backfill the basin, fit the pump with a check valve, run a supported discharge line well clear of the foundation, bolt on a grommeted lid, then seal the vapor barrier to the basin rim. Allow a full day. The hard parts are the digging and the discharge route, not the pump.
Before you start, be sure a pump is what the space needs. A crawl space that is merely damp needs a sealed vapor barrier and a dehumidifier; a sump is for liquid water that arrives and collects. Does a crawl space need a sump pump is the ten-minute version of that decision.
What you need before you go under the house
- The basin, the pump, a check valve, discharge hose or pipe with adapters and clamps, and a grommeted lid that matches the basin rim
- Two to three bags of washed gravel for the bed and the backfill around the basin
- A short spade or a post-hole digger — a full-length shovel is unusable lying down
- Buckets and a helper at the hatch, because every bucket of spoil has to come out by hand
- A work light, knee pads, eye protection and a respirator rated for dust and mold spores
- Vapor barrier tape and sealant for tying the barrier to the basin rim
- A water alarm, to leave beside the finished basin
The installation, step by step
- Find the real low pointGo under on a wet day if you can, and look for where water actually stands or where the soil is darkest and softest. If the space is currently dry, run a hose into the crawl space at the perimeter and watch where it goes. Do not guess from the shape of the floor above — crawl-space grade rarely matches it. Keep clear of piers and footings.
- Check clearances before committingYou need the basin's diameter plus working room, and vertical clearance above the basin to lift the pump out later. If a pier blocks the natural low point, a pump rated for a 10 inch minimum pit opens up locations a wider-pit pump cannot use.
- Dig the hole deeper than the basinFor an 18 by 22 inch basin, excavate roughly 26 inches deep and a couple of inches wider than the basin all around. The extra depth is a four-inch washed-gravel bed, which keeps the basin off bare soil and stops silt entering from below. Bucket the spoil out as you go rather than piling it beside you.
- Bed and set the basinLevel the gravel, set the basin, and check that the rim finishes level with or slightly above the surrounding ground — never below it, or the barrier will drain surface water into the cover joint. Backfill around the sides with gravel, not the soil you dug out, firming it in stages so the basin does not float or tilt.
- Fit the pump and the check valveStand the pump in the basin, confirm the float has clear swing room on all sides, and plumb the discharge with a check valve on the vertical section above the pump. Without the valve, the water standing in the pipe falls back into the basin every time the pump stops and the pump restarts on its own backwash.
- Route the discharge out, and keep goingTake the line out through a rim-joist or foundation penetration, sealed on both faces. Then keep running it: the destination needs to be well clear of the foundation and sloping away, or the water returns to the ground you just pumped it from. Support flexible hose every few feet so it cannot sag into a water-holding low spot.
- Power it correctlyA grounded receptacle near the pit, with the cord reaching it without strain. Do not use a long, light-gauge extension cord: pumps start under load and inrush current is well above the nameplate figure, so a thin cord drops voltage at exactly the wrong moment. If there is no suitable outlet nearby, that is an electrician's job, not an extension-cord job.
- Test with real water before you close upPour water in until the float lifts. Watch the pump start, watch the level fall, watch it stop, and then watch for thirty seconds to confirm it does not immediately restart — if it does, the check valve is missing, backwards or failed. Go outside and confirm water is actually arriving where you intended.
- Seal the lid and tie in the barrierBolt the grommeted cover to the rim inserts with the discharge pipe and the cord through their grommets. Then tape and seal the vapor barrier to the basin rim so the barrier is continuous. An unsealed basin inside an encapsulated crawl space is an open hole in the envelope.
- Leave an alarm behindSet a water alarm on the ground beside the basin, not in it, so it only sounds once the basin has already lost. This is the cheapest part of the whole project and the only thing that will tell you a pump you cannot see has stopped.
Jackel SF20 sump basin (18 x 22 in)
18 by 22 inches, 20 gallons, one-piece structural foam, with a rim that takes a bolted cover.
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| Size | 18 in diameter x 22 in deep |
|---|---|
| Capacity | 20 gallons |
| Material | Injection-molded structural foam polyethylene, one piece |
| Rim | Pre-drilled holes with metal inserts to accept a cover |
| Sealing | Gas and radon tight when fitted with a sealed cover |
| Weight | 11 lb |
Buy the basin before you start digging, and dig to it rather than digging a hole and hoping. The depth you need is the basin depth plus about four inches of gravel underneath, which for this basin means roughly 26 inches of excavation.
What it gets right
- 20 gallons, so the pump cycles about half as often as it would in a 10 gallon basin
- One-piece molding — no seam to open up in wet soil
- Pre-drilled rim inserts for a cover that genuinely seals
- 11 lb, so it goes through a crawl-space hatch
Where it falls short
- 22 in deep is a serious dig on your knees
- Needs 18 in of clearance between piers
Specifications from 18 x 22 in radon sump basin distributor listing.
Liberty Pumps 257
1/3 HP cast iron, rated for a 10 inch minimum pit, 5.2 A at 115 V, 23 ft maximum head.
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| Type | Submersible, Class 25 cast iron |
|---|---|
| Horsepower | 1/3 HP |
| Electrical | 115 V, 5.2 A |
| Discharge | 1-1/2" NPT vertical female |
| Impeller | Thermoplastic vortex |
| Solids handling | 3/8" |
| Minimum pit | 10 in diameter |
| Maximum head | 23 ft (rated head 5 ft) |
| Flow, by lift | 2,640 GPH at 0 ft |
| Warranty | Three years |
The quick-disconnect cord is worth noticing on this particular install. A crawl-space pump is hard to reach, and the ability to change a failed float without breaking the discharge joint is the difference between a half-hour job and an afternoon.
What it gets right
- 10 in minimum pit diameter — the most forgiving in our registry
- 5.2 A at 115 V, which is gentle on a long branch circuit
- Vertical magnetic float with a quick-disconnect cord, so the switch is serviceable without unplumbing
- 23 ft maximum head for a long horizontal run
Where it falls short
- 3/8" solids handling in a silty environment
- No published flow figure at ten feet
Specifications from Liberty 257 specification listing, Liberty 250-series distributor spec page.
Sump pump discharge hose kit
Flexible hose with adapters and clamps, sized to the pump outlet.
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| Type | Flexible discharge hose with adapters and clamps |
|---|---|
| Typical size | 1-1/4" or 1-1/2" to match the pump outlet |
| Use | Temporary or above-ground runs, and seasonal re-routing |
Support the run every few feet. An unsupported flexible line finds a low spot, holds water, and in a cold crawl space that low spot is where it freezes.
What it gets right
- Adapters for 1-1/4" and 1-1/2" outlets
- No solvent cement in an unventilated space
- Lets you re-route seasonally where the line can freeze
Where it falls short
- Kinks if unsupported
- Needs a separate check valve
Specifications from Amazon listing (search).
How long it takes, and when to hire it out
A capable DIYer with a helper should budget a full day: most of it digging and bucketing spoil, and a surprising amount of it on the discharge route. The pump itself takes under an hour.
Hire it out if the crawl space has under about 24 inches of clearance, if you need a new electrical circuit, if the discharge has to cross a driveway or a finished area, or if there is standing water you cannot account for. That last one is important: water with no obvious source may be a plumbing leak or a failed footing drain, and a sump will mask the symptom while the cause continues.
Mistakes that cause callbacks
- Discharging too close to the house. The most common failure by a wide margin, and it presents as a pump that never stops. Where to discharge sump pump water has the distances and the options.
- Setting the basin rim below grade. Surface water then runs toward the cover joint instead of away from it.
- Skipping the gravel bed. The basin silts up from underneath and the pump ingests grit for the rest of its life.
- Leaving the basin uncovered because the lid is 'for later'. Later rarely arrives, and under a sealed crawl space it is a real loss.
- No check valve. The pump short cycles forever and wears out years early.
- No alarm. A crawl-space pump can fail in spring and be found in summer.
Questions people ask
How deep does a crawl space sump pit need to be?
Deep enough for the basin plus about four inches of washed gravel beneath it. For a common 18 by 22 inch basin that means excavating roughly 26 inches. The basin rim should finish level with or slightly above the surrounding ground, never below it.
Can I install a crawl space sump pump myself?
Yes, if the space has reasonable clearance and there is already a suitable grounded outlet near the pit. Budget a full day with a helper; most of it is digging and bucketing spoil out through the hatch. Hire it out if clearance is under about 24 inches, if a new circuit is needed, or if the discharge has to cross a driveway.
Where should a crawl space sump pump discharge to?
Well clear of the foundation and onto ground that slopes away, so the water keeps going instead of soaking back into the soil you pumped it from. A line that dumps a few feet from the house is the single most common reason a crawl-space sump appears to run constantly.
Does a crawl space sump pump need a check valve?
Yes. Without one, the column of water standing in the vertical discharge falls back into the basin the moment the pump stops, which refills the basin and restarts the pump. That cycle repeats indefinitely and is one of the fastest ways to wear a pump out.
Should I install the vapor barrier before or after the sump?
Install the sump first. That way the basin is already in the ground at its final level and the barrier can be taped and sealed directly to the rim, giving a continuous envelope. Doing the barrier first means cutting and patching work you have already finished.
Sources
Read next
The best crawl space sump pump kits
Nobody sells a single box that does this job properly. Four parts do, and three of them are the ones people skip.
Does a crawl space need a sump pump?
Damp is a barrier-and-dehumidifier problem. Standing water is a pump problem, and sealing over it makes things worse.
How to install a crawl space vapor barrier
An unpleasant day's work that pays back more than almost anything else you can do to a house. Here is how to do it once.
How to install a sump pump
A like-for-like replacement in an existing pit is a two-hour job. Cutting a new pit into a slab is not, and this page is honest about which is which.
Where to discharge sump pump water
The most expensive mistake in this whole category costs nothing to avoid: putting the water back where it came from.