Keeping your basement dry


Home drainage systems

There are two distinct drainage systems in the typical home. There is the wastewater (or sewer) system that collects water from toilets, sinks and tubs/showers, etc., which is taken to a treatment plant for processing before it is released back into the environment. The other system is the stormwater system. We will be discussing this system here. Many older homes do not separate the two systems and all water/waste is collected in the sewer system and sent to the treatment plant for processing.

Since 1987 in Vancouver, all new storm drainage systems (including one and two family dwellings) have been required to convey stormwater from roof and paved areas to a sump. A sump is essentially a barrel buried in the ground where rainwater from the roof and grounds is piped into it and an outlet pipe connects to the city storm sewer.  See diagram 1

 In diagram 2 we see the ideal perimeter drainage system for homes in Vancouver and the surrounding areas. The roof and foundation drainage systems are further separated into their own system and sent to the sump independently. This system helps keep the majority of the rainwater away from the buildings perimeter drainage system by collecting the rainwater from the roof and sending it independently to the sump.


                                 Diagram 1

                                 Diagram 1

 The rainwater that soaks into the ground around the building is collected in the perimeter drainage system by the perforated piping found at the footing below the foundation walls where it is redirected to the sump. In modern systems, the perimeter drainage is generally made up of perforated plastic PVC pipes. Systems more than 20 years old are usually ABS plastic or “Big 0”. Older installations are generally cement or clay tile. Older systems on the West Coast are usually a single system where roof downspouts are connected to the perimeter drain and the water is then sent to connect with the city storm sewer.

                                                                                    Diagram 2

                                                                                    Diagram 2

Beyond Perimeter and Roof Drainage

Rainwater management alone is not enough to maintain a dry basement in the wet winter months. We also need to keep the moist ground from soaking the foundation walls thereby bringing moisture into the basement. Traditional methods include coating the outside of the foundation walls with a dampproofing product and installing a drainage composite mat to the walls to seal out moisture. In the past, problems have arisen when basements were renovated and converted into bedrooms and suites, etc. In many instances, improper building practices have sealed moisture inside the wall assembly and mold and rot has resulted. Best practices include insulating the foundation on the outside only and keeping the foundation wall dry and heated from the inside. The ideal system for preventing moisture related problems in a basement is an extensive interconnection of various systems briefly summarized in the following diagram.

  1. Wall Assembly from outside to in:
  • Plastic drainage composite mat
  • 50mm (min) XPS insulation (styrofoam)
  • Dampproofing coating
  • Concrete foundation wall
  • 10mm air space
  • Stud wall
  • Gypsum drywall

2)  Floor Assembly from outside to in:

  • Undisturbed soil
  • Course clean granular fill (gravel)
  • 50mm (min) XPS insulation
  • Vapour barrier
  • Concrete slab
  • Interior finishings

3) Perimeter drainage pipe


For a fully underground basement, a separate roof drainage piping system would also be required.

During a typical home inspection

The majority of a drainage systems is not readily visible during a typical home inspection. Looking around the home, a competent inspector can find indications of how the drainage system is preforming. Such as:

  • Clean outs and drain covers can be opened and looked into to see of they are clear, or blocked, or full of water, 
  • An inspector can see how the downspouts are connected to the stand pipes of the drainage system,
  • Sump covers can be removed and the sump area can often show indications of performance,
  • The ground closest to the foundation may be sloped toward the house, or sunken in areas.

Other evidence of perimeter drainage performance can be gathered from inside the home.

  • Relative humidity inside compared to outside can be a good indication, 
  • Smell can also indicate problems,
  • Moisture meter readings around the outside walls is also informative,
  • Areas of exposed concrete can show signs of previous or present water infiltration. 

Often this is enough to make an educated guess as to the performance of the perimeter drainage system and the foundations ability to keep moisture out of the building. Often it is not. During long dry periods, for example, all evidence of water ingress can disappear and not reappear until after extended periods of heavy rain. For this reason it is a good idea to have the perimeter drainage system inspected every five years. 

Perimeter drainage repair or replacement

An inspection may discover perimeter drainage problems. Repairs or replacement of the system may be required. This can be costly as the foundation wall would have to be exposed for perimeter drain replacement. A common and less expensive upgrade would be the installation of a new shallow roof water drainage system, thus separating roof water from perimeter ground water. Oftentimes this is enough of a relief on the perimeter drainage system to prevent moisture related problems from effecting the basement.

Upgrading to a two pipe system involves exposing the foundation walls down to the footings and the removal of the existing drainage system and the installation of a new PVC perforated pipe ground water system at the bottom of the footings along with a separate, shallower roof water drainage system and dampproofing the foundation walls. 

Since the foundation walls would have to be exposed for the perimeter drain replacement, it may be a good time to also consider upgrading the insulation strategy for the basement by adding an insulation layer around the outside of the foundation walls. 

Preventative maintenance

There are a few things a home owner can easily do to help prolong the life of the perimeter drainage system. 

  • Keep soil and debris out of the standpipes where they join the downspouts,
  • Keeping your gutters clean will reduce the amount of organic matter that can flow through the downspouts and clog the system,
  • Keep trees and large growing shrubs away from the house. Their roots can penetrate some drain tiles and clog them,
  • Keep the bottom of the sump pit clean and check it once or twice a year. If it is suddenly more dirty than usual, it may indicate a problem,
  • Run a hose into the cleanouts and check in the sump area to see if water is flowing freely through the system,
  • Be aware of new musty odours in the basement. They are a sure sign of a problem somewhere in the system.

Sometimes, hiring a competent home inspector is the best idea to have your perimeter drainage system checked for defects. He has experience in such matters and knowledge of how the systems work and the proper tools to look for evidence of a problem situation. Correcting a defect before it becomes a serious problem can go a long way toward saving you a lot of trouble and expensive repairs to your home in the future.