Drainage Systems in Vancouver: Do You Need a Rowboat to Navigate Your Basement?

In the Greater Vancouver area, water damage in homes is a constant threat. If your basement smells moldy or shows signs of moisture or water damage, it may be the result of an improperly functioning drainage system. Since the exterior components of a home are often the most neglected, and the drainage system is largely hidden from sight, this system is especially prone to problems. Professional Home Inspectors know where to look for water damage and mold. The earlier the water damage is identified the better so you can repair the damage before it gets out of hand. Although localized repair is sometimes possible, costs to replace the entire drainage system will be $10,000 – $15,000, making this a costly repair.

There are, however, things you can do yourself to assess the condition of your drainage system and to maintain your Greater Vancouver home.

How the drainage system works

Drainage systems direct roof and ground water away from the foundation of the home. Water is collected in the roof gutters (eaves troughs) and drained via downspouts to draintiles (pipes) laid at the foundation of the house. Surface water also filters down to the draintiles, where it is diverted away from the building, reducing water pressure on the foundation walls and keeping the interior of the home dry and protected.

Draintiles were commonly used in residential construction beginning in the early 1950’s. Prior to that time it is possible that no such drainage system was installed. In older systems, clay pipes were laid around the base of the foundation wall, with each 1′ long pipe section separated by a ¼” space to allow water to enter. The joints were covered with building paper to prevent debris from entering the pipe system. The collected rainwater was diverted into the sanitary sewage system leaving the home.

In modern construction a split system is used. This greatly reduces the amount of water collecting in the draintiles at the foundation level and also keeps rainwater out of the sanitary sewage system. Water collected from the roof flows through the downspouts into a closed pipe located along the exterior walls of the house, just below the soil surface, and is channeled directly to the storm sewer via the sump. Surface water is collected in a separate system of draintiles laid around the base of the foundation wall that also drains to the sump.

The sump is a covered concrete tank usually visible at the soil surface near the house. Drainage water flows to the sump where it is collected, allowing any debris to settle on the bottom. When the water level in the tank rises to the storm sewer outlet level it drains into the municipal storm sewer system.

Common problems and solutions for Greater Vancouver Homes

Lot grading that slopes towards the house directs water to the foundation wall.  As experienced home inspectors in Greater Vancouver we see this a lot and we know that this adds an extra load on the draintiles. Surface levels should slope away from the foundation wall at a rate of 1 inch per foot (1/12) for at least the first 6 feet. Topsoil can usually be added to accomplish this (never sand or gravel). Walkways should have a slight slope to drain water away from the structure.

Soil level higher than the foundation wall can cause water damage to siding and house walls. Soil contact with siding promotes rot and creates an ideal environment for pests. Water damage to house walls can be costly to repair. Ideally, soil surfaces should be kept 6 to 8 inches below siding.

Root growth from trees and shrubs planted close to exterior walls can damage foundations, and clog the draintiles. Tree roots naturally grow towards water and the draintiles provide a steady source. Eventually roots will clog the tile, block drainage, and in severe cases can dislodge foundation walls. Large trees and shrubs should be planted well away from exterior walls.

Leaves and debris on the roof can clog gutters and downspouts. Trim shrubs and trees away from roof and exterior walls. Clean gutters and downspouts annually to maintain proper roof drainage. Do not flush debris into underground drains.

Downspouts draining at a point several feet away from the foundation wall are an indication of existing drainage problems. Redirecting roof water away from the perimeter wall is sometimes effective, but is considered a temporary solution to drainage system problems.

Efflorescence, a white powdery deposit found on the interior of foundation walls is usually caused by excessive moisture penetration through the wall. Efflorescence can usually be removed by scrubbing or chemical cleaning. If the condition is recurring it is an indication that water is penetrating the wall.

Excavation, Damp-proofing and Draintile

When leakage into the basement cannot be controlled by other means, a complete repair may be necessary. The following steps must be taken:

Excavation around the exterior of the structure to expose the foundation wall to the bottom of the floor slab must be done by hand to prevent damage to the foundation wall.

Dampproofing the exterior face of the foundation wall controls the movement of soil moisture into the foundation. A heavy coat of bituminous material, polyethylene or other sheet material is most commonly used. Most older homes were not originally dampproofed.

Draintile made of white PVC piping, with perforations in half of the diameter, is installed holes down, in a bed of drain rock around the base of the foundation wall. More drain rock is added above the piping, and landscape fabric is placed on top of that to prevent soil from filtering down to the drain rock and clogging the system.

Once dampproofing is completed and new draintile has been laid, the excavation is back-filled to bring the soil surface and slope to an appropriate level (see Lot Grading and Soil Level, above).

A properly functioning drainage system will protect the home from moisture and water damage caused by heavy rainfall. If you are concerned about the condition of your drainage system it would be wise to have a professional mold inspection by a qualified drainage company.

Written and published by Vancouver home inspectors at Primus Home Inspections Ltd.