The foundation drain, often called a French drain, goes unnoticed as long as it works. Laid at the base of the foundation walls, it collects groundwater and carries it away before it reaches the basement. Quebec's Construction Code requires one at the base of exterior foundation walls, unless it can be shown to be unnecessary (article 9.14.2.1). In some soils, that drain can be clogged by iron ochre (ocre ferreuse).

What is iron ochre?

Ochre deposits depend on soil characteristics and groundwater conditions. The Régie du bâtiment du Québec and the Garantie de construction résidentielle (GCR) describe the mechanism this way:

Microorganisms present in generally water-saturated soils extract oxygen from elements such as iron, which they reduce to ferrous ions. Once reduced and dissolved, this iron migrates through the soil to the foundation drains, where it can clog them.

These iron bacteria turn the iron in the soil into a substance that settles back out inside the drain. It looks orange and gelatinous, a lot like rust. Over time the deposit blocks the perforations and then the pipe itself. Water is no longer carried away, it builds up against the foundation, and the risk of basement moisture or leaks goes up.

Ochre can't be eliminated

GCR's technical sheet stresses one point. A drain built to the BNQ 3661-500 standard does not stop the phenomenon or make the ochre go away. It prevents clogging because it can be inspected and cleaned as often as needed. The design goal is a drain that can be maintained.

What the BNQ 3661-500 standard provides

Where there is a risk of ochre, the recommended system differs from an ordinary drain in several ways:

  • a rigid 100 mm PVC pipe with a smooth, easy-to-clean inner wall, instead of the usual flexible corrugated drain;
  • perforations on the top of the pipe only, never underneath, which makes it less prone to clogging;
  • at least two pairs of access and cleanout chimneys at opposite corners of the building, so the entire drain can be inspected and cleaned;
  • installation at footing level, in clean-stone backfill with a separation membrane;
  • a connection to the sump pit through a submerged elbow, which keeps ambient air, and therefore oxygen, from reaching the drain and feeding the iron bacteria.

The drain performs better, is laid with a slope and, above all, can be inspected and cleaned for the life of the building.

Assessing the risk before building

Before construction, the standard calls for a visual assessment of the site to look for signs associated with ochre. If there are any, soil and water laboratory testing is strongly recommended. The results help estimate the clogging potential and choose the right approach. Keep in mind that a BNQ 3661-500 drain does not solve a water level that is too high relative to the footings. That situation needs a professional's analysis.

GCR generally recommends installing a system that complies with BNQ 3661-500. It's a simple, inexpensive preventive measure that can avoid costly work if the clogging potential develops or gets worse later.

If the house is already built

Some signs should get your attention: orange or rust-coloured water in the basement or sump, a gelatinous deposit, a distinctive smell, a damp basement, or a sump pump that runs very often. The BNQ 3661-500 standard also includes a diagnostic component for existing buildings. Even with a good system, owner-occupants need to plan inspections of the drain to determine how often it should be cleaned.

We design foundations and drains that meet GCR requirements and can look at your situation, whether you're building on an at-risk site or suspect an ochre problem in an existing home. Get in touch.

Sources: Garantie de construction résidentielle (GCR), technical sheet FT-9.14.-02, "Ochre Deposits in Building Drainage Systems"; BNQ 3661-500 standard; Quebec Construction Code, art. 9.14.2.1. This article is provided for information purposes only.