French Drain vs Perimeter Drain: Which Do You Need?

A perimeter drain is the perforated pipe buried at the base of your foundation footing, required by the BC Building Code on almost every house. A French drain is a separate gravel trench out in the yard that intercepts water before it reaches the house. One is foundation infrastructure. The other is site drainage. Confusing them is how people spend money on the wrong fix.

What a perimeter drain is, and why BC calls it that

"Perimeter drain" is the local name for what the rest of North America calls a footing drain or drain tile. That is not a small vocabulary quirk. The US Department of Energy's building science guide for the identical component is titled "Footing Drain Pipe" and never uses the word perimeter. BC's own institutions do the opposite: the Building Code Appeal Board has written about "foundation perimeter drains", and BC Housing's builder guide uses "perimeter drain pipe" throughout.

The practical result is that when you search the term, half the results you get are written for a different regulatory system. The BC Building Code, Sentence 9.14.2.1.(1) in the 2018 edition BC publishes free online, requires that unless it can be shown to be unnecessary, the bottom of every exterior foundation wall be drained, either by drainage tile or pipe around the outside of the foundation, or by a layer of gravel or crushed rock. Pipe must be at least 100 mm in diameter, and covered on top and sides by at least 150 mm of clean crushed stone.

What a French drain is

A French drain is a gravel-filled trench with a perforated pipe near the bottom and filter fabric over the top and sides. BC Housing's Builder Guide to Site and Foundation Drainage (2021) puts it in the site drainage chapter, not the foundation chapter, and describes it as typically oriented perpendicular to the slope so it intercepts surface and near-surface water and diverts it elsewhere. The trench is meant to cut down through the permeable topsoil into the tighter soil below, so perched water sitting on that tight layer can drain into it.

Read that description next to a Greater Victoria lot and the fit is obvious. Water does not soak away through dense marine clay. It sits on top of it and travels sideways, downhill, toward whatever is in the way. A French drain is a tool for exactly that condition. It is not a substitute for foundation drainage.

Side by side

The Code minimum is lower than most people assume

This is the part almost no comparison page states outright, and it changes how you read a quote.

First, the Code does not actually require a pipe. Subsection 9.14.4 lets a builder drain the foundation with a granular layer alone, and BC Housing's guide explicitly labels both of the Code's bare-minimum options as not recommended by best practice, urging a rigid perforated PVC pipe with perforations facing down instead.

Second, the pipe does not have to sit at the footing. Sentence 9.14.3.3.(1) only requires the top of the pipe to be below the bottom of the floor slab. In Appeal Board decision #1345 from 1994, the Board noted the apparent contradiction between that sentence and the requirement to drain the bottom of the wall, and allowed a pipe installed above footing level on a deeper section of wall. The US guide takes a firmer line, telling builders to place the pipe outside of, not on top of, the footings. BC Housing splits the difference: invert below the top of the adjacent footing, at least 50 mm between the top of the pipe and the bottom of the slab, sloped at 1%.

Third, the Code says nothing about filter fabric around the drain rock. BC Housing recommends wrapping the clear gravel in filter cloth, and is specific that the fabric goes around the gravel, not directly on the pipe. On our silty clay soils that detail is the difference between a drain that lasts and one that seals its own perforations.

A perimeter drain can pass inspection and still be a poor system. That is not a scandal, it is how minimum standards work, and it is the single best reason to ask what a contractor is actually installing rather than whether it meets Code.

Nobody has to give your perimeter drain a cleanout

Here is the consequence that follows from the sources and gets stated almost nowhere. The BC Plumbing Code definition of a "drainage system" specifically excludes subsoil drainage pipes, which is why the Appeal Board ruled back in 1990 that foundation perimeter drains cannot form part of a storm drainage system. BC Housing's guide traces that exclusion to its logical end: because perimeter drains are not "drainage systems" under the Plumbing Code, the cleanout spacing rules do not apply to them, so cleanouts are best practice and locally required rather than Code-mandated.

That is why so many older Greater Victoria homes have no way to service the drain short of digging. BC Housing recommends maintaining every element of a drainage system annually at minimum, and clearing pipes by snaking or hydro jetting. You cannot do either through a pipe with no access point.

The same ruling explains a mistake worth checking on your own property. Driveway and area drains cannot be tied into the perforated foundation pipe. They have to run to the storm sewer in solid pipe. A tie-in like that feeds grit straight into the drain that is supposed to protect your foundation.

Where the water is allowed to go

Sentence 9.14.5.1 requires foundation drains to discharge to a sewer, a drainage ditch or a dry well, and where a dry well is used, Article 9.14.5.3 puts it no less than 5 m from the foundation and only where the natural groundwater level sits below it. Municipal rules layer on top. The District of Saanich tells homeowners to make sure roof downspouts, foundation drains and groundwater sump pumps connect to the stormwater system and not the sewer, and separately advises discharging downspouts about 1.2 metres from basement walls.

An outlet is not an afterthought on the quote. It is the part of the job that decides whether the new pipe has anywhere to send water.

So which one do you need?

• Water in the basement or crawl space, and a drain scope showing collapsed or silted pipe. That is a perimeter drain replacement. A French drain will not touch it.
• Water pooling in the yard, sheeting downhill at the house, or building behind a retaining wall. That is what a French drain is designed for, and it is the less expensive of the two because it does not require excavating to footing depth.
• A damp crawl space with no obvious surface water source. Start with crawl space drainage and a look at where groundwater is arriving.
• Both at once. Common on sloped Victoria lots, where upslope water overwhelms a foundation drain that was never sized for it.

Cost follows scope, not the label on the system, which is why we price after a site visit rather than from a table. The factors that move a replacement quote are worth reading before you collect any.

If you do not know which problem you have, that is the normal starting point. Send us the details and we will look at where the water is coming from and tell you which system is the one that needs work.

Sources

• British Columbia Building Code 2018, Division B, Section 9.14 Drainage
• BC Building Code Appeal Board, Decision #1226, Area Drains
• BC Building Code Appeal Board, Decision #1345, Location of Foundation Perimeter Drain Pipe
• BC Housing, Builder Guide to Site and Foundation Drainage: Best Practices for Part 9 Buildings in British Columbia (2021, Horizon Engineering with RDH Building Science)
• US Department of Energy Building America Solution Center, Footing Drain Pipe
• District of Saanich, Sanitary Sewer Guidelines and Flood Information for Homeowners

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