Water management and perimeter drainage

Jan 20, 2017 | News

The purpose of a perimeter drain system is to disperse the water collected from the roof surface away from the foundation wall of a house. The roof drainage system and the perimeter drain system need to work together to manage the water around a building. These are important systems given our location on Vancouver Island in a temperate rainforest zone.
Mike Baker

Serving Courtenay, Comox, Cumberland, Royston, Merville, Black Creek, Union Bay, Fanny Bay, Campbell River, Parksville, Qualicum Beach, Errington, and Coombs.

Areas such as the Comox Valley see about 1100 mm of rain annually, closer to 1500 mm in Campbell River, and over 3000 mm in Tofino. Generally, we don’t mind the rain here on the Island, just put a rain jacket on and get out there regardless of the weather. But we do need to pay attention to our houses and how the water is managed on a piece of property.

Roof Drainage System

Ideally each house should have a roof drainage system. Gutters often fill with leaves and debris and should be cleaned periodically to keep the water flowing into the downspouts. How often should we clean our gutters? Well that largely depends on how many trees are dropping vegetation on the roof surface. Most homes without a large amount of trees around it do well with annual cleaning, preferably after fall when the leaves of the deciduous have dropped, and before the winter rain. Some houses need cleaning multiple times a year due to a larger amount of trees. Most home owners get a feel for how often gutter cleaning should take place. Preferably before the water overflows the gutters, but that is a sure sign.

Perimeter Drainage System

The water flows from the gutters to the downspouts, then ideally the downspouts tie into a perimeter drain system. Not all homes are equipped with perimeter drainage. Some older houses don’t have any perimeter drain system at all. In these cases sometimes the best solution is to extend the downspouts 6 feet away from the building, this will reduce the amount of water pushing against the foundation walls, also know as hydraulic pressure. Downspout material is relatively inexpensive. There are some fancier products that roll out when it rains. Installation of a perimeter drain system is costly, a ditch needs to be dug around the house, back-filled with drain rock and drain pipe then topped and re-landscaped.

Slope and Grade

Another consideration for houses without perimeter drains is the slope and grade of the lot and location of the house. Perimeter drainage systems deal with ground water as well. The foundation wall on the uphill side of a house without perimeter drainage can see excessive hydraulic pressure. This can damage a foundation wall and compromise the structural integrity of a house. All the more reason to hire a qualified home inspector when purchasing a home. Baker Street Home Inspection provides an analysis of the water management for every house we inspect.


Houses equipped with perimeter drainage systems have the advantage of being able to move water away from the foundation walls. But these systems do need maintenance as well. The same leaves that collect in the gutters can find their way into the perimeter drains, build up then create blockages. Roots from trees and shrubs can grow into the drainage system and block the flow of water. Drains at the bottom of driveways or sunken stairwells need to be cleared of leaves and debris. There are contractors who provide perimeter drain scoping. They have equipment that removes the blockages and inspects the perimeter drain system. Annual perimeter drain scoping is recommended.