How much more does exterior cost?

Roughly double per linear foot, and more once the yard is involved. Interior is published at $70 to $180 a foot across five Ontario companies. Exterior is $100 to $350 from the waterproofing firms and $250 to $475 from excavation contractors pricing by basement depth. On a 110-foot perimeter that is about $8,000 to $20,000 interior against $11,000 to $39,000 exterior before the two costs that only exist outside: hand-digging a narrow side yard at $50 to $100 a foot, and putting the garden back at $40 to $95 a foot.

Does exterior really last twice as long?

One Ontario source states it plainly. Rock Basements publishes that if done properly, interior waterproofing should be good for at least 30 years while exterior work can keep a basement dry for 50. Gamcon's repair table separately gives exterior waterproofing a lifespan of 25 years and up. Divide either cost by either lifespan and the annual figures sit much closer together than the up-front ones, which is the arithmetic the calculator above does for you and labels as arithmetic.

When is interior the right answer rather than the cheap answer?

When the wall itself is sound and the problem is water reaching the floor edge, and when digging is impractical. A four-foot side yard, a mature tree against the foundation, a deck, an addition, or a neighbour's wall a metre away all make exterior work expensive or impossible. Interior work happens entirely inside the building envelope, so none of that applies, and the price does not change because of what is in your garden.

When is exterior the only real answer?

When the water is a symptom of the foundation itself, rather than of drainage. If the original membrane and weeping tile have failed, an interior system is managing an ongoing leak instead of ending it. If the wall is deteriorating from freeze-thaw, keeping the water off the outside face is the point of the work and an interior drain does nothing about it. And if you are digging anyway for a walkout, an underpinning or a new drain, the marginal cost of doing the membrane while the hole is open is far lower than doing it later.

Can I do part of one and part of the other?

Yes, and on a Toronto semi it is often the sensible answer. Most leaks are on one wall, and OCM Excavation prices one 30-foot exterior wall at $8,500 to $13,500 against $25,000 to $55,000 to wrap all four. Doing the wall that leaks from outside and leaving the rest, or running an interior system and excavating only the one problem elevation, are both real quotes rather than compromises.

Interior or exterior waterproofing: which is worth it on your house?

Last updated: August 26, 2026By Below-Grade Records Desk

Skip to the comparison

The honest version of this decision is not which one works. Both keep a basement dry. It is that exterior costs about twice as much per foot and is published as lasting about twice as long, and that everything which makes exterior expensive is in your garden rather than in your basement. Put your wall length in and see both, side by side, with the per-year arithmetic done.

Compare them on your own wall

How many linear feet of wall?

A standard Toronto perimeter is 100 to 120 feet. One wall on a semi is about 30.

linear feet

How deep is the basement? (this only changes the exterior side)

Not sure how deep the basement isAbout 7 feet, an older Toronto basementAbout 8 feet, 1960s or newer

Excavation contractors price exterior work by depth and land above the flat band the waterproofing companies publish. Picking a depth swaps which of those two published views you are comparing against.

Two things that only cost money on the outside

The side yard is too narrow for a machineInterior work does not care about your gate. Exterior work cares enormously.There is a garden, deck or interlock to put backThe other cost that exists on one side of the wall and not the other.

Interior

A trench inside the footing, membrane up the wall, tied to a sump. Nothing outside is touched.

$8,701–$22,374

with 13% HST. Before tax $7,700–$19,800.

110 linear ft of wallProject Landscaping; DryShield; Nusite Group; Rock Basements; AquaMaster Plumbing: $7,700–$19,800

Per linear foot: $70–$180Project Landscaping; DryShield; Nusite Group; Rock Basements; AquaMaster Plumbing

How long it is published to last

30 years (Rock Basements), which works out at $290–$746 a year.

Published whole-project figure for a typical GTA home: $8,000–$18,000 (Nusite Group).

Exterior

Excavate to the footing, parge, membrane, dimpled board, new weeping tile, backfill.

$17,402–$55,313

with 13% HST. Before tax $15,400–$48,950.

110 linear ft of wallNusite Group; DryShield; Rock Basements; AquaMaster Plumbing; Gamcon: $11,000–$38,500
Putting the yard backOCM Excavation & Construction: $4,400–$10,450

Per linear foot: $100–$350Nusite Group; DryShield; Rock Basements; AquaMaster Plumbing; Gamcon

How long it is published to last

50 years (Rock Basements), which works out at $348–$1,106 a year.
25 years (Gamcon), which works out at $696–$2,213 a year.

Published whole-project figure for a typical GTA home: $15,000–$35,000 (Nusite Group).

Reading the two together

At the top of each range, exterior costs about 2.5 times what interior does on this wall. Rock Basements publishes interior as lasting at least 30 years and exterior 50, so the per-year figures on each card are much closer together than the up-front ones. Those per-year numbers are our arithmetic on their two published figures, not a price anyone quoted.

OCM Excavation puts the difference in kind rather than degree: “Interior systems manage water that’s already entered the wall, fine for finished basements where the wall is sound. Exterior excavation actually stops water from entering, protects the foundation from freeze-thaw damage, and is the only fix when the original membrane and weeping tile have failed.”

Every per-foot and per-year figure traces to a named Ontario company, widened where companies disagree and never averaged. The cost-per-year lines are division, shown with both inputs named, and are the only derived numbers on this page.

Method

How this comparison is built

Seven Ontario companies publish a per-foot rate for one side or both, and WaterproofingIndex read all seven in August 2026. Each side of the comparison is the widened range across the companies that price that side, multiplied by your wall length, with 13 per cent HST added. Hand-digging and yard restoration are added to the exterior side only, because that is where they exist.

One number on this page is derived rather than published: the cost per year. It is the with-tax total divided by a published expected life, and it is shown with both inputs named beside it every time. That division is the whole point of the page, because the up-front prices and the lifespans are always published separately and the comparison nobody makes is the one that matters.

The exterior lifespan is shown against BOTH published figures rather than one. Rock Basements says 50 years and Gamcon says 25 and up. Averaging those into 37 would produce a figure neither company printed, which is the thing this site does not do, so both are shown and the reader can decide which contractor they believe.

Ranking

What actually decides it, once the money is equal

Is the wall sound, or is the wall the problem? Interior work manages water. It does not protect the outside face of a deteriorating foundation from another thirty freeze-thaw cycles.
Can anyone actually dig there? A four-foot side yard, a mature tree, a deck, an addition or a neighbour's wall can move exterior from expensive to impossible.
How much finished basement is in the way? Interior work means the bottom of the drywall, the flooring and anything against the wall come out. On a newly finished basement that is its own bill.
How long do you intend to own it? The per-year figures only pay off if somebody is there to collect them. Warranty transferability matters more than usual on this decision.
Is the hole being dug anyway? A walkout, an underpinning or a weeping tile replacement all open the ground. The membrane is far cheaper while it is open than as its own project later.

Frequently asked questions

Is exterior waterproofing better than interior?: OCM Excavation answers it directly: yes, for permanent dryness. Its wording is that interior systems manage water that has already entered the wall, which is fine for finished basements where the wall is sound, while exterior excavation actually stops water from entering, protects the foundation from freeze-thaw damage, and is the only fix when the original membrane and weeping tile have failed. Both keep a basement dry to somebody standing in it. They differ in what happens to the wall over thirty winters.
How much more does exterior cost?: Roughly double per linear foot, and more once the yard is involved. Interior is published at $70 to $180 a foot across five Ontario companies. Exterior is $100 to $350 from the waterproofing firms and $250 to $475 from excavation contractors pricing by basement depth. On a 110-foot perimeter that is about $8,000 to $20,000 interior against $11,000 to $39,000 exterior before the two costs that only exist outside: hand-digging a narrow side yard at $50 to $100 a foot, and putting the garden back at $40 to $95 a foot.
Does exterior really last twice as long?: One Ontario source states it plainly. Rock Basements publishes that if done properly, interior waterproofing should be good for at least 30 years while exterior work can keep a basement dry for 50. Gamcon's repair table separately gives exterior waterproofing a lifespan of 25 years and up. Divide either cost by either lifespan and the annual figures sit much closer together than the up-front ones, which is the arithmetic the calculator above does for you and labels as arithmetic.
When is interior the right answer rather than the cheap answer?: When the wall itself is sound and the problem is water reaching the floor edge, and when digging is impractical. A four-foot side yard, a mature tree against the foundation, a deck, an addition, or a neighbour's wall a metre away all make exterior work expensive or impossible. Interior work happens entirely inside the building envelope, so none of that applies, and the price does not change because of what is in your garden.
When is exterior the only real answer?: When the water is a symptom of the foundation itself, rather than of drainage. If the original membrane and weeping tile have failed, an interior system is managing an ongoing leak instead of ending it. If the wall is deteriorating from freeze-thaw, keeping the water off the outside face is the point of the work and an interior drain does nothing about it. And if you are digging anyway for a walkout, an underpinning or a new drain, the marginal cost of doing the membrane while the hole is open is far lower than doing it later.
Can I do part of one and part of the other?: Yes, and on a Toronto semi it is often the sensible answer. Most leaks are on one wall, and OCM Excavation prices one 30-foot exterior wall at $8,500 to $13,500 against $25,000 to $55,000 to wrap all four. Doing the wall that leaks from outside and leaving the rest, or running an interior system and excavating only the one problem elevation, are both real quotes rather than compromises.

Sources

Where these numbers came from

Seven Ontario waterproofing and excavation price lists, each read on its own page in August 2026 and stored with the sentence carrying the figure, plus the provincial tax rate. Canadian dollars.

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