How the two differ in kind
Water gets into a basement because there is water in the ground beside it under pressure. Both options on this page reduce that pressure; they do it at different points and by different means. A French drain sits at or below the footing outside, intercepts groundwater as it moves toward the wall and carries it away by GRAVITY. A sump system collects water that has already reached the structure — usually via a perimeter drain inside the slab edge — into a pit, and lifts it out with a pump.
The constraint that usually settles it is elevation. A gravity drain has to discharge somewhere lower than its invert: a slope, a ditch, a watercourse, a storm connection where one is permitted. A house at the bottom of its plot with no fall available has nowhere for a French drain to go, and no amount of trench solves that. A pump is indifferent to elevation — it manufactures the head — which is exactly why it exists and exactly what it costs to run and maintain.
Before either, there is a cheaper question worth answering. A large share of wet basements are not a groundwater problem at all: they are a downpipe discharging at the wall, a gully that has silted up, or ground that falls towards the house instead of away from it. Those are a morning's work and they remove the load that both of these systems exist to handle. Excavating a perimeter drain to deal with water a gutter was delivering is an expensive way to fix a gutter.
The factors that actually differ
Show
| French drain, gravity outfall | Sump pit and pump | |
|---|---|---|
| What it needs from the site | A fall to somewhere legitimate to discharge. Without it the drain fills and stays full, which is worse than not having one. | Electricity, and a discharge point that will not simply return the water to the same ground. |
| Where the water is when it is dealt with | Outside the structure, before it loads the wall. The hydrostatic pressure never develops. | At or inside the structure, after it arrives. The wall is still wetted; the pressure is relieved rather than prevented. |
| The thing that makes it fail | Blinding. Fines migrating into the stone or through the wrong geotextile clog the void and the drain silently stops working, years later, with nothing visible. | Anything electrical or mechanical: a stuck float switch, a power cut during the storm that caused the need, a frozen or blocked discharge line. |
| Disruption to install | Excavation to footing depth around the outside — landscaping, paths, steps and services all in the way, and shoring where it is deep. | Breaking out the slab perimeter inside, or a single pit. Dusty and contained rather than site-wide. |
| Maintenance | Rodding points and inspection chambers if they were built in; very little otherwise, which is also why failure is invisible. | A machine with a service life. Test it, clear the pit, and expect to replace the pump — which is why a spare or a backup unit is standard on anything that matters. |
| Behaviour in the event that matters | Passive. It works in a power cut, in a storm, and while nobody is home. | Needs power at exactly the moment a storm is most likely to remove it. Battery or water-powered backup is the standard answer and is part of the real cost. |
| Effect on the wall itself | Usually paired with re-tanking or a drainage membrane while the excavation is open, because the wall is exposed and will not be again. | Leaves the external face untouched, so a wall that is failing as a barrier keeps failing as one — the water is simply collected afterwards. |
| Shape of the cost | Front-loaded and largely excavation: it scales with depth and with what is in the way, and it is close to maintenance-free afterwards. | Lower to install, then a continuing cost — electricity, servicing, replacement pumps and a backup — for as long as the building stands. |
Which one, and when
Choose french drain, gravity outfall when…
- There is a genuine fall to a legal discharge point, and the ground is open enough to trench.
- The wall is being excavated anyway for tanking, insulation or repair, so the trench is already paid for.
- The building must stay dry without power — a basement holding anything that a pump failure would ruin.
- The water table is seasonal and shallow, and intercepting it upslope removes most of the load.
Choose sump pit and pump when…
- There is nowhere downhill to go, which is the common case on a flat or low-lying plot.
- Excavating outside is impossible or absurd: a terrace mid-row, a boundary wall, a mature garden, services everywhere.
- The water arrives episodically in volume and the requirement is to clear it fast rather than to stop it.
- The work has to happen inside, in winter, without opening up the outside of the building.
Now run your own numbers
This page holds no prices on purpose — a national average is wrong for almost every real project. Quantify both options with your dimensions and your local quotes.
Frequently asked questions
- Which should I do first?
- Neither. Walk the outside in heavy rain first and watch where the water actually goes. Downpipes discharging at the wall, a silted gully, a path falling toward the house, a flowerbed built up above the damp-proof course — each of those delivers more water to a foundation than most groundwater does, and each costs a fraction of either system on this page. The order of work is surface water, then ground shaping, then subsurface drainage, and skipping to the third is how people end up with an expensive drain that still does not keep the basement dry. It is also the order that tells you how much load is left for the drainage to handle, which is the input both calculators need.
- Does a French drain need a geotextile, and does the type matter?
- It needs one and the type matters a great deal, because the failure mode of a French drain is not collapse — it is BLINDING. Fine soil particles migrate into the stone, fill the voids, and the drain stops conducting water while looking exactly as it did on the day it was buried. The fabric's job is to let water through and hold the fines back, which means its pore size has to be matched to the surrounding soil's grading rather than chosen by brand. Getting it wrong in one direction lets fines through; in the other, the fabric itself clogs. This is why the stone is graded rather than whatever was cheapest, and why a drain wrapped in the wrong fabric can fail faster than one with none at all.
- How often do sump pumps fail, and what do I do about it?
- Often enough that a single unguarded pump is not a design, it is a hope. The three common failures are a float switch that sticks or is obstructed by the pit's own debris, a power cut during the storm that created the demand, and a discharge line that freezes or blocks so the pump runs against a closed pipe. Each has a standard answer: keep the pit clean and the float clear, fit a battery or water-powered backup for the power case, and run the discharge with a fall and an air gap so it drains rather than holding water to freeze. A high-water alarm is cheap and tells you the day something is wrong rather than the day the floor is wet. If a pump failure would be expensive, two pumps on separate circuits is normal practice.
- Where is the pumped water supposed to go?
- Far enough away and downhill enough that it does not come straight back, which is the detail most often got wrong. A discharge that ends a metre from the wall is a recirculating system: the pump lifts water out, the ground returns it, and the pump runs continuously wearing itself out. It also must not go into a foul sewer, which is prohibited in most jurisdictions and overloads treatment during storms. A surface discharge with a proper outfall, a soakaway sized for the flow and set back from the building, or an approved storm connection are the routes — and which of those is permitted is a question for the drainage authority rather than a matter of preference.
- Can I have both?
- Yes, and on difficult sites it is the normal arrangement rather than belt-and-braces. The external drain intercepts what it can by gravity and removes most of the load; the internal system and pump catch what gets past and deal with the episodic peaks that exceed the gravity drain's capacity. The two are complementary because they fail differently — one silts up slowly and invisibly, the other stops suddenly and audibly — so a site that cannot tolerate water uses both and monitors the pump for signs the drain has stopped working. The tell is a pump that has started running far more often than it used to with no change in weather.
- Will either of these fix damp, or only water?
- They deal with liquid water under pressure, which is one cause of a damp basement and not the only one. Rising damp through a wall with no functioning damp-proof course, condensation on cold surfaces in an unventilated space, and vapour moving through a slab with no membrane beneath it all produce a basement that feels and smells wet without any water arriving as water. Those need a different intervention — a membrane, ventilation, insulation to raise surface temperatures — and a drainage system installed against them will not help. The diagnostic worth doing first is simple: tape a square of plastic sheet tightly to the floor or wall and leave it. Moisture on the underside is coming through the structure; moisture on the room side is condensation.