The Perimeter Trench: How Deep, How Wide, and What BC Rules Govern It

A perimeter trench is the excavation cut around the outside of a foundation so the drain sitting at the footing can be exposed, replaced and rebedded. In British Columbia it is almost always a regulated trench by definition: over 1.2 m deep and under 3.7 m wide at the bottom. That definition, more than the pipe itself, sets how the work has to be done.

What counts as a perimeter trench

The word has a legal meaning here, and it is narrower than the everyday one. WorkSafeBC defines a trench as an excavation less than 3.7 m (12 ft) wide at the bottom, over 1.2 m (4 ft) deep, and of any length (OHS Regulation Part 20, section 20.1). A shallow scrape along a garden bed is not a trench. A cut that follows the footing around a house with a basement almost always is.

That matters because the whole of Part 20 turns on it. Once the excavation crosses 1.2 m, a set of requirements attaches to the hole itself, separately from anything the Building Code says about the pipe going into it. Homeowners reading a drainage quote usually price the pipe. The trench is the larger part of the job, and it is the part governed by rules you can look up.

How deep does a perimeter trench have to be?

Deep enough that the top of the drain pipe sits below the bottom of the floor slab. That is the requirement of Sentence 9.14.3.3.(1) of the BC Building Code, quoted directly in a Building Code Appeal Board decision on exactly this question (BCAB #1345, Location of Foundation Perimeter Drain Pipe).

The decision is worth reading because it settles something contractors and building officials disagree about. A homeowner appealed after a building official refused to accept drain pipe laid above the bottom of the footing along a 20 foot section of wall that extended deeper than the rest of the foundation. The Board found that the pipe does not have to sit at the bottom of the wall or at footing level, provided it stays below the level of the floor slab.

So the slab, not the yard and not the footing, is what fixes the depth. On a typical Greater Victoria house with a full basement, that puts the bottom of the trench well past the 1.2 m line, which is why this is machine work rather than shovel work.

How wide is it, at the bottom and at the top?

Two different widths, and the gap between them is what surprises people. At the bottom the trench only has to clear the footing, which projects out past the face of the wall, plus room to bed the pipe in not less than 150 mm of crushed stone (Section 9.14. Drainage). At the top it is wider, and how much wider is not a choice.

Before a worker enters any excavation over 1.2 m (4 ft) deep, the sides have to be sloped, benched, or supported by shoring or a trench box (OHS Regulation section 20.81). Where sloping is used instead of shoring, Figure 20-1 caps the excavated face at 3 horizontal to 4 vertical in hard and solid soil, and caps any vertical portion at 1.2 metres (4 feet). Slope a 1.8 m face at that maximum and roughly 1.35 m of ground opens up on the outboard side, before the spoil pile is anywhere.

What the trench has to satisfy, and the rule that sets it

The rule nobody quotes: your house is the adjacent structure

Here is the part every competing page skips. Excavation work has to follow the written instructions of a qualified registered professional when an improvement or structure is adjacent to the excavation, and Part 20 defines adjacent as within a distance less than or equal to the overall depth of the excavation, measured from a vertical line through the toe of the excavation face (OHS Regulation sections 20.78 and 20.1).

Read those two together against a perimeter trench. The toe of the face is at the footing. The structure sits directly above it, at a distance of nothing at all. On the face of the regulation, a perimeter trench at a house is the textbook case of the condition, not an edge case, and where support structures are used the instructions have to come from a professional engineer.

That is not a reason to be alarmed about a quote. It is a reason to ask a specific question: how the trench will be held open along your walls, and who is specifying it. A contractor who has thought about this has an answer. Sloping, benching, and trench boxes are all permitted routes, and which one fits depends on soil, depth, and how much room exists between the wall and the property line.

What that means for the ground above the trench

The strip of yard that gets disturbed is wider than the pipe and wider than the footing, because the top of a sloped face is set by the depth below it. Beds, shrubs, patio edges and lawn inside that strip are part of the job, not collateral damage. Restoration belongs in the scope of work and in the quote.

The excavated material has to go somewhere too, and it cannot simply be stacked at the lip. Once a spoil pile beside a supported excavation averages more than 60 cm (2 ft) deep, the added lateral pressure has to be accounted for in the shoring selected (OHS Regulation section 20.86). Trees are the other constraint the Regulation names: those adjacent to the area being excavated must be removed or secured if they could endanger workers.

A mature tree close to the wall is therefore a scoping question before it is a landscaping one, and municipal tree bylaws in Greater Victoria may govern what happens to it. That is a conversation to have before the machine arrives, which is one of several reasons this kind of work needs an on-site inspection rather than a phone estimate.

What has to happen before anyone digs

The location of all underground utility services must be accurately determined before excavating with powered equipment (OHS Regulation section 20.79). In British Columbia that starts with a free locate request through BC 1 Call, whose members respond to locate requests within 3 business days of notification, or 5 business days for large projects (BC 1 Call). Water and sewer laterals, gas, electrical and communications lines commonly cross the same ground a perimeter trench follows.

Two more things get settled at the same stage. Water must not be allowed to accumulate in the excavation where it could affect stability, which is a real scheduling constraint in a wet Greater Victoria winter. And where the new drain connects to the municipal stormwater system, a permit applies: the City of Victoria requires a plumbing permit for that connection (Plumbing Permits). Getting the outlet settled first is what stops a finished trench having nowhere to send water.

So which of these applies to your house?

• A scope report calling the pipe collapsed, crushed or silted. That is a full perimeter drain replacement, and the trench follows the whole loop for the reason above: the failure lives in the path, not at the symptom.
• Water pooling in the yard or running downhill toward the house. That is surface water, which a French drain intercepts in a shallower trench along the path the water takes, not around the footing.
• A damp crawl space with no obvious source above ground. Start with crawl space drainage, where an interior solution sometimes avoids an exterior trench altogether.
• Not sure which one you are looking at. Read where the drain actually sits first, then send us what you are seeing and we will arrange a site assessment. We replace perimeter drains across Greater Victoria, and any judgement about your foundation requires an on-site inspection.

Sources

• WorkSafeBC, Occupational Health and Safety Regulation Part 20, Construction, Excavation and Demolition (sections 20.1, 20.78 to 20.95 and Figure 20-1)
• Province of British Columbia, Building Code Appeal Board, BCAB #1345, Location of Foundation Perimeter Drain Pipe, Sentences 9.14.2.1.(1) and 9.14.3.3.(1)
• British Columbia Building Code 2018, Division B, Section 9.14. Drainage
• BC 1 Call, free underground utility locate requests
• City of Victoria, Plumbing Permits

Estimates vary by property. Book a site assessment for a scope and price specific to your home.