Redoing the French drain is a daunting and costly task: you need to be informed. The purpose of the following is not to make the house owner an expert in the subject matter. The aim is to know the basic vocabulary, to have an understanding of the usefulness of the materials and to be able to visualize how these materials manage the groundwater.
French drain replacement steps
1) The ground is excavated to expose the foundation walls and footing. The excavated earth is stored on the sides of the excavation trench during the work.
2) The foundation is cleaned with high-pressure water jets.
3) Foundation cracks, when present, are repaired by polyurethane injection.
4) A liquid rubber sealing membrane is applied and glued to the foundation walls and to the foundation wall-footing junction. The function of the sealing membrane is to protect against water infiltration and to prevent the transfer of humidity from the surrounding soil to the basement.
5) Rigid insulation with a thickness of 25mm and insulation factor R-5 is installed over the rubber membrane. The function of the insulation is threefold: to protect the membrane during the backfill step, to protect the membrane against the friction of the ground movements during the freeze-thaw cycles and finally to reduce the losses of heat of the basement.
6) The French drain is installed at the base of the footing around the entire perimeter of the building. The function of the French drain is to capture groundwater at the base of the foundation and direct it to the municipal rainwater network.
7) Drainage stone (3/4 net) is placed over and next to the French drain. The function of the drainage stone is to create a porous space around the drain in order to direct the water to the drain.
8) A geotextile fabric membrane is placed over and around the drainage stone. The function of the geotextile membrane is to separate the stone from the surrounding earth. The geotextile membrane allows the water to pass to the stone without letting the soil pass through.
9) The excavated earth is replaced against the foundation. Because of the addition of stone on the drain, it is normal to have to transport a certain amount of soil off-site. It is also normal to return the land to a level higher than pre-work to compensate for the collapse of the embankment during the first year following the replacement of the French drain.
How to detect a French drain problem?
ANSWER: water, humidity and their consequences.
| SIGN | DESCRIPTION | CAUSE |
| Efflorescence | Traces of whitish powder on the surface of the concrete. | Humidity transfer from the outside of the foundation to the inside of the basement. The whitish powder is composed of mineral salts present inside the foundation concrete that have been transported by humidity transfer. On arrival at the interior surface of the basement, the humidity evaporates while the salts remain on the wall. |
| Excessive humidity | In winter, air humidity should be 30% or less. In the summer, a rate of 55% is acceptable. | Ineffective sealing of the foundation. The sealing membrane is either of low quality or worn out by ground movements or non-existent. |
| Mold | Smell and visible signs of mold: green, red or black agglomerated stains. | An inefficient French drain coupled with a permeable sealing membrane allows an increase in the humidity content beyond the limit values. A high humidity content promotes the growth of molds already present in the basement, but until then dormant. |
| Water infiltration | Infiltration of water in the basement. Most often at the junction between the concrete slab of the basement floor and the foundation wall. | The French drain is inefficient: the water around the foundations accumulates instead of being evacuated. The accumulation of water creates pressure: this pressure pushes the outside water inward. |
What are the most frequent causes of an inefficient French drain?
| SIGN | DESCRIPTION |
| Landscaping | Unsuitable installations such as gutters connected directly to the French drain or curb drains without grids allow the entry of leaves and other debris that obstruct the French drain. |
| Connection | Inadequate connection of the French drain to the municipal rainwater system: inverted slope, connection too high, use of perforated pipe instead of rigid pipe. |
| Original installation | Failure to comply with up to code original installation. Examples: absence of geotextile membrane, insufficient amount of drainage stone, counter-slope. |
| Roots | The roots of some trees such as maples or oaks as well as some shrubs such as cedars or lilac can bud roots in the French drain. |
| Old age | Over time, the French drain becomes obstructed by soil, sand or mud that has gradually infiltrated through openings designed to allow water to drain into it. |
IMMEDIATE ASSISTANCE
CLICK 514-699-0006
MONTREAL AND SOUTH SHORE