The perimeter drain around a house is a perforated pipe buried against the outside of the foundation footing, running the full loop of the building. It sits below the level of the basement floor slab, slopes toward one collection point, and carries groundwater away from the walls. British Columbia's Building Code sets its size, its bedding and where it is allowed to discharge.
What the perimeter drain around a house actually is
It is an assembly rather than a single part. A perforated pipe sits in a bed of washed stone, the stone is wrapped in filter fabric, and the whole thing follows the outside of the footing all the way around the building.
The US Department of Energy describes the same component for builders: the pipe surrounds the full perimeter of the house and is laid at a slight slope, so water entering anywhere along the loop drains to a single collection point. From there it leaves in solid unperforated pipe and discharges above grade at least 10 feet from the foundation, or into a drywell, an approved storm sewer, or a sump pump (Footing Drain Pipe, Building America Solution Center).
In BC the same component carries a different name and its own Code article. Sentence 9.14.2.1.(1) requires that, unless it can be shown to be unnecessary, the bottom of every exterior foundation wall be drained by drainage tile or pipe laid around the exterior of the foundation, or by a layer of gravel or crushed rock (BC Building Code 2018, Section 9.14. Drainage).
Where it sits against the house, and how deep
Lower than most people picture. Sentence 9.14.3.3.(1) requires drain tile or pipe to be laid on undisturbed or well compacted soil so that the top of the pipe is below the bottom of the floor slab, or below the ground cover of the crawl space (Section 9.14. Drainage). The US guide is blunter about the horizontal position: put the pipe outside of, and not on top of, the footings.
Two things follow from that for anyone reading a quote. Reaching the pipe means excavating to below slab depth right around the building, which is why replacement is a machine job and not a shovel job. And the trench has to clear the footing, which projects out past the face of the wall, so the dig is wider at the bottom than the wall line suggests.
What is buried around the pipe
The pipe is the part people picture and the part that matters least on its own. What surrounds it decides whether groundwater can reach it, and whether silt can too. The BC Building Code and the US Department of Energy guide describe the assembly differently, and the gaps between them are the useful part.
| Part of the assembly | BC Building Code 2018 | US Department of Energy guide |
|---|---|---|
| The pipe | Drain tile or pipe not less than 100 mm in diameter, to ASTM, BNQ or CSA material standards | Perforated plastic drain tile, laid with the perforations facing down |
| Under the pipe | No bedding depth stated for a pipe system | At least 2 inches of washed gravel |
| Over and beside it | Not less than 150 mm of crushed stone, no more than 10% of it fine enough to pass a 4 mm sieve | At least 6 inches of half inch to three quarter inch washed gravel |
| Joints | Butt joints laid 6 mm to 10 mm open, top half covered with sheathing paper, 0.10 mm polyethylene or No. 15 felt | Continuous pipe, so no open joints to cover |
| Filter fabric | Not mentioned | Landscape fabric under, around and over the washed stone |
| Slope | Not specified | Enough fall to carry water to one collection point |
| Where it ends | A sewer, a drainage ditch or a dry well | Daylight well clear of the foundation, a drywell, an approved storm sewer, or a sump pump |
Read down the middle column and you can see how much the Code leaves open. It fixes the pipe at not less than 100 mm in diameter and the cover at not less than 150 mm of crushed stone containing no more than 10% of material that will pass a 4 mm sieve (Section 9.14. Drainage). It says nothing about filter fabric and nothing about slope. Those two silences are where a drain in silty clay soil fails early, because the fabric is what keeps fines out of the stone and the fall is what stops water standing in the pipe.
Why you cannot see it, and what you can check instead
Nothing about the loop is visible from the yard except its two ends. That is the honest answer to the question homeowners are really asking, which is how to tell what is down there without paying to dig.
Three things above ground are worth checking. The first is where the water leaves: an outlet at the property line, a connection into the municipal storm system, or a sump. A permit is required for a connection to the stormwater utility in the City of Victoria (Plumbing Permits), so a connection made properly leaves a record you can ask about. The second is whether roof downspouts tie into the same pipe, which loads a foundation drain with roof water it was never sized for. The District of Saanich advises keeping downspout discharge about 1.2 metres, or 4 feet, away from basement walls (Flood Information for Homeowners).
The third check is the one that surprises people. The house may have no pipe at all. Subsection 9.14.4 accepts a granular drainage layer as the alternative, laid to a depth of not less than 125 mm beneath the footing and extending not less than 300 mm beyond its outside edge (Section 9.14. Drainage). A house built to that option has legal drainage around the foundation and nothing anyone can put a camera into.
None of these checks tell you whether the pipe between those two ends is still open. A camera scope does. Any judgement about a specific foundation, and about where the water reaching it is coming from, requires an on-site inspection.
The loop is only as good as its lowest point
Here is the consequence that follows from the way the system is built, and it gets stated almost nowhere. Because the pipe is laid at a slope toward a single collection point, a blockage holds back everything upstream of it. A length crushed by a root at the back corner can put water against a foundation wall two sides away, at the point where the pipe runs highest and has the least fall left in it.
So a wet spot in one corner is weak evidence about where the fault is, and replacing the drain along one wall sometimes changes nothing. The failure lives in the path, not at the symptom.
The Code's own tolerances explain why silt is usually the culprit. Butt-jointed tile is laid with 6 mm to 10 mm open joints, and the stone around it may contain up to 10% of material fine enough to pass a 4 mm sieve (Section 9.14. Drainage). The gaps that let groundwater in let fines in as well, and across forty or fifty years those fines settle into the low spots. The symptoms that show up first are usually the last stage of that process, not the start of it.
Where the loop is allowed to end
Sentence 9.14.5.1 requires foundation drains to discharge to a sewer, a drainage ditch or a dry well. Where gravity drainage is not practical, Article 9.14.5.2 sets the sump pit at not less than 750 mm deep, not less than 0.25 m2 in area, and covered, with an automatic pump to move the water out. A dry well has to sit not less than 5 m from the foundation (Section 9.14. Drainage), and only where the natural groundwater level is below the bottom of the well.
Municipal rules sit on top of the Code. The District of Saanich tells homeowners to make sure roof downspouts, foundation drains and groundwater sump pumps connect to the stormwater drainage system and not the sewer system (Sanitary Sewer Guidelines). Older properties are where that matters most, because a cross connection made decades ago is still a cross connection today.
The outlet decides whether a new pipe has anywhere to send water, which is why it is one of the harder parts of a job to scope from the curb.
So what do you do with this?
- Water inside, or a scope report calling the pipe collapsed or silted. That is a perimeter drain replacement, and it is a whole-loop job for the reason above.
- Water pooling in the yard or sheeting downhill toward the house. That is surface and near-surface water, which is what a French drain intercepts. The two systems solve different problems and are easy to confuse.
- A damp crawl space with no obvious source above ground. Start with crawl space drainage and with finding out where groundwater is arriving.
- No idea which of these you have. That is the normal starting point. Send us what you are seeing and we will arrange a site assessment.
Sources
- British Columbia Building Code 2018, Division B, Section 9.14. Drainage (Province of British Columbia)
- US Department of Energy Building America Solution Center, Footing Drain Pipe
- City of Victoria, Plumbing Permits
- District of Saanich, Flood Information for Homeowners and Sanitary Sewer Guidelines
Estimates vary by property. Book a site assessment for a scope and price specific to your home.