Interior drainage vs exterior waterproofing: an honest cost comparison
Two solutions to what looks like one problem, with a threefold price difference. Neither is a scam. Neither is always right. The symptom decides more than the budget does.
You have water in the basement, you have collected quotes, and they fall into two camps that differ by a factor of three. This is the decision, laid out without a product to sell.
What each one actually does
Interior perimeter drainage
The slab is cut around the inside of the perimeter, a perforated pipe is laid in gravel next to the footing, and it runs to a sump where a pump lifts the water out.
Read that again, because the important part is easy to miss: it does not stop water entering the wall. It collects water that has already arrived and removes it before it reaches the floor. The wall stays wet. For a utility basement that is perfectly acceptable. For a basement you intend to finish and put timber and plasterboard against, it is a different proposition.
Exterior excavation and membrane
The ground is dug out to the footing, the outside face of the wall is cleaned and sealed, a drainage board or dimple membrane goes on, and a footing drain is laid in gravel. Water is kept out of the wall rather than managed after it is in.
Most of what you are paying for is not waterproofing. It is excavation: eight feet down around a house, keeping the trench safe, and putting the yard back. The membrane is cheap.
The comparison
| Interior drainage | Exterior waterproofing | |
|---|---|---|
| Modelled range | $55–$115 per linear foot, plus $1,200–$2,800 for the pump | $95–$270 per linear foot |
| Time on site | 2–4 days | 1–2 weeks |
| Disruption | Inside: concrete dust everywhere, contents moved | Outside: landscaping, patios, decks, mature planting |
| Wall stays wet | Yes | No |
| Depends on electricity forever | Yes | Only if the drain cannot reach daylight |
| Handles water under the slab | Yes — this is what it is for | No |
| Expected life | Pipe: decades. Pump: 7–12 years | 20–30+ years if backfilled properly |
| Can be impossible | Rarely | Often — access, additions, boundary walls, trees |
Run both for your dimensions and state. Ranges are modelled from published editorial figures, not measured transactions.
What should decide it
| Your situation | Points to |
|---|---|
| Water at the wall-floor joint, or coming up through the floor | Interior. Exterior work does not reach water that is under the slab. |
| Water high on the wall, or from a window well | Neither yet. That is surface water; fix grade and downspouts first. |
| One leaking crack in a poured wall, not moving | Neither. Inject it. |
| Poorly drained soil, shallow water table | Interior plus serious pump redundancy. Exterior helps but does not remove the pump. |
| Well drained soil, deep water table, wet wall anyway | Find the construction cause before buying either. Here the ground is usually not the culprit. |
| Finishing the basement as living space | Exterior becomes much more attractive. A dry wall and a managed wall are different things behind plasterboard. |
| Historic stone or rubble foundation | Specialist. Interior drainage that undermines a shallow rubble footing is a real failure mode. |
| No exterior access on the wet side | Interior, by elimination. Say so out loud so nobody pretends it was a preference. |
Check what your county's soil is classified as before the visit.
The question that reveals which one you are being sold
“What happens to the water after your system collects it, and what happens during a four-hour power cut?”
An interior system has to answer honestly: it stops, and the pit fills. That is not a reason to reject it. It is the reason to put the backup pump in the same contract rather than discovering the gap during the storm. A battery backup runs $800–$2,000; a water-powered backup runs $600–$1,600 and never needs charging, but needs municipal water pressure.
The middle path most quotes skip
Before either: extend the downspouts well past the foundation, bury the discharge, restore positive grade for the first three metres, put a drain and a cover in every window well. Total spend a few hundred to a few thousand dollars, and it resolves a meaningful share of wet basements outright.
The exception, and it is important: if water is arriving up through the floor, none of the surface work can help, because the water is not coming from the surface. See how many gallons your roof puts next to the wall in an hour — the number is usually larger than people expect, and it is the cheapest thing on this page to fix.
Questions people actually ask
Which one do the big franchises sell?
Overwhelmingly interior drainage, because it is repeatable, weather-independent, needs no heavy plant, can be sold and installed in a week and carries a warranty that is easy to write. None of that makes it wrong — for water arriving under the slab it is the correct product. It does mean the recommendation is not soil-driven by default.
Can I do interior now and exterior later?
Yes, and it is a common sequence. Nothing about an interior system prevents later excavation. The reverse is also true. What you should not do is pay for interior drainage, discover it does not achieve what you wanted because you intend to finish the basement, and then pay for exterior too.
What about the coatings sold as waterproofing?
Interior coatings handle vapour and very light seepage. Against hydrostatic pressure they fail: the water finds the weakest point or pushes the coating off the substrate. They are sold vastly more often than they work, and a bid at half the price of a drainage system is usually a coating being described as one.
Does exterior work eliminate the need for a pump?
Only if the water table stays below the slab. A footing drain has to discharge somewhere; on a sloping lot it can go to daylight by gravity, which is the ideal. On a flat lot it often still ends at a sump, and then you are back to depending on electricity.
Related
Condensation or a leak? The four tests that tell you which
Damp basement walls are either humid air condensing on cold masonry or water coming through it. Four free tests separate them, and the answer changes …
What your soil's drainage class says about your basement
The USDA has classified the drainage of nearly every acre in America. Those seven classes predict basement behaviour better than rainfall maps, and al…
Radon and wet basements: the connection nobody mentions
Radon and water use the same route into a house. Why drainage work changes your radon level, what a sealed sump lid is worth, and the rough-in that co…
Get quotes from waterproofing contractors
We pass what you enter to the contractor networks listed below. You choose whether to hire anyone.
Page data updated