Water is not stopped: it is escorted

The damp basement is the home defect that frightens most and is least understood from quotes. The first surprise is conceptual: against groundwater and infiltration, the modern remedy is not «sealing» — water pushed by hydrostatic pressure always finds the next crack — but escorting: collecting it in a perimeter channel, guiding it to a sump and lifting it out with a pump. The second surprise is economic: the quote presents itself «per metre of perimeter», but around that metre work two items that do not count metres — the dig with its reinstatement and the fixed cost of the machine: sump, pump, hook-up, commissioning. This calculator breaks the quote into its three forces and sums it up in two honest numbers: the effective cost per metre and the index ι.

The formula: metres, dig and the machine

The model is the decomposition of interior drainage. The bare bill is L·c: the metres of earth-retaining perimeter times the unit cost of the system — slotted channel, turned-up membrane, washed gravel, labour. On that bill acts the dig surcharge d: from the 10-15% of a light screed in an empty room to over 60% when the slab is reinforced, the room is tight and the rubble climbs the stairs. Outside the product stays the fixed cost F: the disguised fixed block, which this time is a machine — tenth appearance of the geometry: the sump pit, the pump, the electrical hook-up and the commissioning cost the same over ten metres and over eighty. The index ι = C/(L·c) is dimensionless and holds in any currency.

The perimeter scale: the short wall pays the whole floor's pump

A 200-per-metre system, 30% dig, fixed 1,500; only the perimeter grows:

On the single wall the fixed block is worth more than a third of the price and the effective metre climbs to 410; on the whole floor it dilutes to 6.7% and the metre falls to 279. And the first row hides a connoisseur's detail: its index is exactly 2.05 — the boundary of the top band, touched and shown: the short site structurally lives on the edge of the regime where you no longer buy drainage, but worksite.

A complete numeric example

A typical job: 30 metres of perimeter — the average basement room with three earth-retaining sides — at 200 per metre, dig at 30% (ordinary screed, decent access), fixed block of 1,500 between sump, pump and hook-up. The bare bill is 30 × 200 = 6,000; the dig brings it to 7,800; the machine closes at 9,300. The index is 9,300 / 6,000 = 1.55: every drained metre costs 310 — not the 200 of the list — and the fixed part weighs 16.1%. Whoever compares two quotes «at 180 and at 220 per metre» without reading dig and fixed block is comparing the wrong item.

How to read the quote index

The four bands tell four sites. Below 1.35 the long perimeter dilutes the machine: you buy drainage close to list price, and the real comparison is on the quality of the laying. Between 1.35 and 1.65 the drainage is well quoted: the physiology of the accessible basement. Between 1.65 and 2.05 the dig raises its voice: reinforced slabs, depth, access — survey territory. Beyond 2.05 you buy worksite, not channel: demolition and reinstatement dominate. No band is «wrong»: ι says where to point the lens before signing.

The cost per metre: what it really contains

The variable c sums up the installed system. Inside live the slotted drainage channel facing the wall, the membrane turned up to escort the water without letting it touch the new floor, the washed gravel that does not clog, and the labour that gives the right slope towards the sump. The 80-400 range tells the real market: from the essential system in a utility room to fine drainage under a lived-in floor. The item to inspect with a lens is the slope: a flat channel is a channel that ages badly — and it does not show until it rains for three days.

The dig: the channel is opened once

The variable d is the survey turned into a number. Cutting a light screed in an empty room travels at 10-15%; you climb with reinforced concrete, with the depth of the footing, with access — rubble going up one flight at a time — and with the reinstatement: the floor above the channel must come back as it was. It is the item that produces distant quotes on basements equal on paper, and the operating rule is one: the channel is opened once — every compromise buried today (a scant slope, dirty gravel) is dug up again tomorrow.

International perspective: interior drain, external tanking and the same water

The problem is universal, the schools are not. North America — with lived-in basements as standard — has made the interior french drain with sump pump the market default, to the point that the pump has its own liturgy: redundancy, battery backup, alarm. Europe alternates interior drainage with external remediation — digging to the footing, membrane and drain at the base — more expensive but resolutive when the garden allows it; and in damp climates ventilated crawl spaces and watertight concrete tanks coexist. Everywhere the bill has this model's three items: metres, dig, machine. And the index ι compares quotes across schools on the same scale, because a ratio needs no exchange rate.

Limits of the model and proper use

The model photographs interior perimeter drainage: it does not cover external remediation with excavation, structural watertight tanks, rising damp — a different problem with different remedies — nor the finishing works that follow. The perimeter must be written: which sides, at what depth, with what reinstatement. And water is managed as a chain: the roof drainage calculator says how much rain arrives at the foot of the wall — the first waterproofing is a gutter that works — and the foundation repair cost calculator covers the case where water has already worked too long — because a dry basement is a chain, and the pump is only the last link.

Inside the quote: the dig scale and the machine scale

The six sections that follow put the model on the test bench over the two items that do not count metres. The first is the dig: the same perimeter, opened under ever harder conditions. The second is the fixed block: the machine that lifts the water, from the essential pump to the full kit with redundancy and battery. The two tables use the same job as the example — 30 metres at 200 — and move one knob at a time: the cleanest way to see where the final number really comes from.

The dig scale: the same channel, different floors

30 metres at 200, fixed 1,500; only the surcharge d grows:

From the empty room to the by-hand site the total grows by two thirds — from 8,100 to 13,500 — without one extra metre of channel. And the first row is another exact boundary made visible: ι = 1.35 precisely, the frontier between the regime where the machine dilutes and the one where the quote is well balanced. This calculator's two tables thus show both frontiers of the model — 2.05 on the short wall, 1.35 on the easy screed: the boundaries are not theory, they are sites that exist.

Reading the scale: the floor decides more than the water table

The index column tells an uncomfortable truth: between drainage at 270 per metre and drainage at 450 the water does not change — the floor above the channel does. That is why the serious survey starts on its knees: what screed it is, how reinforced, where the rubble exits. And it is also the most concrete negotiating lever: emptying the room before the site, accepting a technical reinstatement instead of a fine one where the room allows it, opening the best access — these are the client's choices, worth whole points of surcharge, that is hundreds on the total.

The machine scale: from the bare pump to the full kit

Same 30 metres at 200, dig held at 30%; only the block F grows:

The last row travels with the flag lit: a third of the quote inside the sump. But here the flag must be read intelligently: on the full kit — second pump, battery backup, level alarm — that high share is often the best-spent item of the whole site. The right question is not «why is the fixed block high?» but «what is inside it?»: the share turns on the lens, the specification decides the judgement.

The pump: the only item that works forever

Channel and membrane, once buried, ask for nothing more; the pump works every time it rains — and it is the only component of the system that can fail. The items worth their price: the quality float switch (the first cause of failure), the second pump or the battery where the basement holds value, the level alarm that warns before the damage, and the yearly test — a bucket of water in the sump, thirty seconds. Perfect drainage with a stopped pump is a flooded floor with an excellent channel underneath.

Before signing: the three-item checklist

A readable quote always breaks out the model's three forces. Demand in writing: the metres with the sides marked on the plan and the channel depth; the dig-and-reinstatement item with the screed type declared and where the rubble goes; the fixed block itemised — sump, pump with brand and flow rate, hook-up, commissioning. Then two questions worth more than any discount: where the pump discharges (never into the foul sewer without a valve, never against the freshly drained wall), and what happens to the price if a surprise appears under the screed. Whoever answers with numbers is selling a dry basement; whoever answers with adjectives is selling a bucket.

Frequently asked questions

How much does basement waterproofing cost?

With the decomposition C = L·c·(1+d/100) + F: the metres of perimeter times the drainage system cost, increased by the dig, plus the fixed block of sump, pump and hook-up. Thirty metres at 200 with a 30% dig and a 1,500 fixed block close at 9,300 — an effective 310 per metre.

What is the drainage quote index?

It is the ratio ι = C/(L·c) between the total and the bare channel bill. Below 1.35 the long perimeter dilutes the machine; between 1.65 and 2.05 the dig raises its voice; beyond 2.05 you are mostly buying demolition and reinstatement.

Why does the short site cost more per metre?

Because sump, pump and hook-up cost the same on 10 and on 80 metres: on the single wall the fixed block is worth 36.6% and the effective metre climbs to 410; on the whole floor it dilutes to 6.7% and falls to 279. It is the disguised fixed block that this time is a machine — the pump does not count metres.

Interior drainage or external remediation?

Two schools: the interior collects the water that arrives and lifts it out — less invasive, always feasible; the external digs to the footing and stops water before the wall — more expensive, resolutive when garden and depth allow it. The model photographs the first; a serious comparison puts both on the table.

What does the cost per metre include?

The slotted drainage channel, the turned-up membrane, the washed gravel and the labour with the slope towards the sump. The 80-400 range tells the market: from the essential utility-room system to fine drainage under a lived-in floor.

How much does the dig weigh on the price?

It is the surcharge d of the model: 10-15% for a light screed in an accessible room, over 60% with reinforced concrete, depth and rubble climbing the stairs. The channel is opened once: proper reinstatement is part of the price, not a luxury.

What does the fixed block include?

The collection sump, the pump with its float, the dedicated electrical hook-up and the commissioning. They are the items that do not grow with the metres — and the ones that dominate the short site's quote.

Does the pump need redundancy?

In basements that hold value, yes: a second pump or a battery-backup pump, plus a level alarm. The storm that floods often arrives with a blackout: the night the pump truly matters is the night the power is out. It is the fixed-block item not worth haggling on.

Does the calculator work for the American interior french drain?

Yes: it is exactly that system — interior perimeter channel, sump pit and sump pump. The names and the quoting unit change, not the structure: metres (or feet), dig, machine. The index ι stays comparable because it is a ratio.

What about rising damp?

No: rising damp is a different problem — water climbing inside the wall by capillarity — with different remedies (chemical barriers, macroporous renders, ventilation). Perimeter drainage manages free groundwater and infiltration: if the symptoms sit mid-wall and climb, another diagnosis is needed.

Does interior drainage also dry the walls?

It removes the hydrostatic pressure and the free water at the foot: the walls stop weeping and the floor stays dry. But the earth-retaining wall remains a managed damp wall, not an external one: delicate finishes still ask for ventilated linings or suitable renders.

What does the computed cost NOT include?

External remediation with excavation, watertight concrete tanks, rising-damp treatments, the finishing works that follow (linings, floors) and the surprises inside the slab. It is the photograph of interior perimeter drainage: the rest is added item by item, with the perimeter in writing.

Updated July 2026