Exterior perimeter drains keep water away from the foundation and cost more because they require excavation. Interior drains collect water that has already arrived and cost less because they do not. Exterior work is the more complete repair and the right default where access allows it. Interior work earns its place where excavation is blocked, and on a pre-1990 home it brings a floor-cutting question with it.
Here is how the two approaches actually differ, what each one does and does not solve, and how to tell which conversation your house belongs in.
What is the actual difference between the two?
Where the water gets intercepted, and therefore what problem gets solved.
An exterior perimeter drain is the traditional system: a pipe in gravel at the base of the footing, on the outside of the wall, below the basement floor level. Water moving down through the soil reaches it before it reaches the wall, and it carries that water away to daylight, to a storm connection or to a sump. The wall stays comparatively dry because the water never builds up against it.
An interior perimeter drain sits inside. A channel is cut around the inside edge of the basement floor, a pipe is laid in gravel below the slab, and it runs to a sump pit where a pump lifts the water out. Water still arrives at the foundation, still passes through or under the wall, and is then collected at the floor edge before it can spread across the basement.
That difference sounds technical and is actually the whole decision. One system prevents the wall from getting wet. The other manages the water after the wall is already wet.
Which problems does each one actually solve?
Match the system to the symptom rather than to the price.
| Symptom | Exterior replacement | Interior system |
|---|---|---|
| Water pooling on the basement floor at the wall edge | Solves it | Solves it |
| Damp or wet foundation wall, efflorescence, staining | Solves it | Does not address the wall |
| Musty smell from chronic wall dampness | Usually solves it | Partial at best |
| Collapsed or root-blocked exterior drain pipe | Replaces it directly | Leaves the failed pipe in place |
| Failing foundation damp-proofing | Can be re-treated while open | No access to the wall |
| Poor grading or backfill against the house | Corrected during backfill | Not addressed |
| Excavation blocked by a slab, addition or property line | Not possible on that side | This is its strongest case |
| Hydrostatic pressure under the slab | Partially reduced | Directly relieved |
Two rows carry most of the decision. If the complaint is a wet wall, an interior system is not aimed at your problem. If the obstacle is access, an interior system may be the only thing physically available.
What does each cost?
Exterior is the bigger number, and most of the difference is the excavation and everything that comes with it. Costs below are indicative Fraser Valley market estimates as of August 2026, not quotes.
| Item | Indicative cost |
|---|---|
| Camera inspection to diagnose before choosing | $300 to $600 |
| Full exterior perimeter drain replacement | $15,000 to $30,000 |
| Sectional exterior repair, one failed side | A few thousand dollars |
| Flushing an intact but silted line | $600 to $1,500 |
| Adding a sump pit and pump | $2,500 to $6,000 |
| Foundation waterproofing membrane, while the wall is open | Adds $3,000 to $8,000 |
| Asbestos sampling, per material, where the floor or wall is disturbed | $100 to $150 |
Notice what the exterior number buys beyond the pipe. With the wall exposed, the damp-proofing can be inspected and upgraded, the backfill can be corrected, and the grading can be fixed. Those are included opportunities rather than extra trips. Our full cost breakdown for perimeter drain replacement covers what moves the number within that range.
There is also a cheaper branch that both options skip past. If the line is intact and merely silted, flushing at $600 to $1,500 may be the entire fix, which is why the camera inspection comes first. See cheap fixes before replacing perimeter drains and drain flushing versus replacement.
What does an interior system mean in a pre-1990 basement?
It means cutting the floor, and in an older house that is a question before it is a task.
An interior drain requires a channel around the entire inside perimeter of the slab. In a basement that has been finished at some point across the last sixty years, that channel runs straight through whatever flooring is down there, and floor tile and the black adhesive under it are both on published lists of suspect asbestos-containing materials.
The regulation treats this as demolition. Section 20.1 of WorkSafeBC’s OHS Regulation Part 20 defines demolition to include the “breakup, razing, or removal of the whole or part of a building or structure,” and a slab is part of the structure. Section 20.112 then requires a qualified person to identify hazardous materials before that work begins, placing the duty on the owner as well as on the employers doing the work.
In practice this is small and fast if it happens at the right time: a single material test runs about $100 to $150 with results in one to three business days, which fits comfortably inside the window where you are still comparing quotes. It is only expensive when it is discovered after the saw has already been through the floor. The asbestos-side guide to a pre-renovation survey covers what that scope looks like, and it is a separate trade from drainage.
Exterior work has its own version of this question, since the black damp-proofing on the outside of the wall is also a suspect material once a crew starts stripping it back for a new membrane. Neither route avoids the topic entirely on a pre-1990 house. They just put it in different places.
How do you decide?
Work through these in order. The answers usually converge quickly.
- What is the symptom? Water on the floor, or a wet wall? A wet wall points outside.
- What does the camera show? Intact and silted, or collapsed? Intact may mean neither system is needed yet.
- Can you actually dig? Walk the full perimeter. A garage slab, an addition, a deck on footings, a neighbour’s fence at the property line, or a mature tree can each rule out excavation on one side.
- Is there a gravity outlet? If water can fall to daylight or a storm connection, an exterior system may need no pump at all. If not, a sump is in the plan either way.
- What else would you do while the wall is open? If the damp-proofing is failing or the grading is wrong, exterior work bundles those in for far less than doing them separately later.
- Is the house pre-1990, and what is on the basement floor? This shapes the interior option’s real cost and timeline.
A hybrid is a legitimate answer. Exterior replacement on three accessible sides and an interior run along the side blocked by the garage slab is a common and sensible configuration.
The short version
Exterior drains stop water reaching the wall. Interior drains collect it after it has. The exterior route costs more, solves more, and is the right default wherever a machine can get to the foundation. The interior route is the answer when access is genuinely blocked, and on a pre-1990 home it comes with a floor-cutting question worth settling before the saw arrives, not after.
Start with the camera inspection either way. At $300 to $600 it is the only step that tells you whether you need either system at all.
Trying to work out which route your home needs? Call (604) 800-7217 or see what perimeter drain replacement and drain camera inspection involve. We work across Abbotsford, Chilliwack, Mission and Langley.
Frequently Asked Questions
Is an interior drain system cheaper than digging outside?
Usually on the invoice, and not always in total. An interior system avoids excavation, landscaping restoration and the weather delays that come with an open trench, which is where much of the exterior cost sits. But it manages water that has already reached the foundation rather than keeping it away, and in a pre-1990 basement it means cutting the floor, which brings its own testing and disposal questions. Cheaper up front is not automatically cheaper.
Does an interior system fix a wet foundation wall?
No, and this is the most important limitation to understand. An interior drain collects water at the base of the wall and moves it to a sump. The wall itself still gets wet, because the water is still arriving from outside. If your problem is a damp wall or efflorescence rather than water pooling on the floor, an interior system may not address what is actually bothering you.
Why would anyone dig outside if inside is less disruptive?
Because exterior work treats the cause rather than the symptom. Digging out gives access to replace the drain, inspect and re-treat the foundation wall, add a proper waterproofing membrane, and correct the backfill and grading all at once. That is a genuinely more complete repair, and on a home where the wall itself is failing it is the only one that addresses it.
Does cutting the basement floor need asbestos testing first?
In a pre-1990 home, the flooring and adhesive in the cut path should be sampled before the saw starts. WorkSafeBC's OHS Regulation defines demolition to include the breakup of part of a building or structure, and section 20.112 requires a qualified person to identify hazardous materials before that work begins. A single material test runs about $100 to $150 with results in one to three business days.
Can you do both?
Yes, and on some homes that is the right answer. A common combination is exterior replacement on the sides where access is straightforward and the wall needs attention, with an interior system on a side where a garage slab, an addition or a property line makes excavation impractical. A camera inspection and a walk around the house is what determines whether a hybrid makes sense.
Published August 20, 2026 · Last updated August 20, 2026 · Fraser Valley Drainage