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Basements: Back to Basics

A major study carried out by the National Research Council Institute for Research in Construction (NRC-IRC) reinforces conventional wisdom that links long-term basement performance to proper site grading and foundation drainage. This article summarizes some key basic principles from that study.

Moisture in Basements
Basement moisture problems originating from the exterior can be caused by leakage, water migration through the foundation walls and floor, sump pump failure and sewer backup.
Leakage is prevented by directing water away from a foundation by means of site grading and providing drainage around the foundation perimeter, or by waterproofing the foundation. Most foundations in new Canadian houses feature footing drainage systems with complementary wall and floor dampproofing. This approach normally makes waterproofing unnecessary.
Basement protection against exterior water requires:
Site grading and drainage;
Foundation drainage (weeping tile and/or a granular drainage layer), combined with effective drainage around the basement walls and appropriate moisture protection of the below-grade basement envelope surfaces;
The use of special design details and building in factors of safety; and

Site Grading and Drainage
Improper lot grading can direct water toward the foundation, causing water leakage or settlement - one of the main reasons for customer dissatisfaction and call-backs for builders.
Most surface drainage problems occur between houses, especially where houses are close together. A well-defined swale should be constructed between the houses to allow surface water to be diverted away from the houses, and downspouts should discharge at least 1 m (3 ft) away from foundations and be directed away from a house and adjacent, neighbouring houses.
Specific guidelines should be followed with respect to the minimum height of the foundation wall above grade; the minimum slope away from the house; the minimum slope on the rest of the lot; the minimum elevation of the lot above street level, at the house; the minimum depth of swales; the direction of surface drainage relative to certain building or landscaping features; the impact of landscaping and fencing projects on planned elevations at the site perimeter.
The site grading and drainage plan should be put in place prior to construction but cannot be executed until construction is complete.

Foundation Drainage and Moisture Protection
Designing to keep water out of the foundation drainage system is crucial. This involves taking measures such as providing adequate slope away from the building, as well as an impermeable cap. Any water that can't be directed away from the building through site grading and drainage must be dealt with by the foundation drainage system.
Proper foundation drainage requires a path to convey water to the bottom of the foundation wall and a drainage system to take the water away. Both free-draining backfill and drainage membranes provide a vertical path that allows the water along the soil/wall interface to drain (see figure), with the dampproofing membrane or coating providing additional protection.
Foundation drainage is usually composed of drainpipe (also known as weeping tile) installed around the perimeter of the foundation wall footing and covered with granular material before backfilling. When there are long runs of drainpipe, a second connection to the storm sewer or sump pump can maximize flow through the pipe. Placing the drainpipe on a layer of free-draining material can offer better flow because any silt deposition will occur in the granular material rather than in the pipe.

Special Design and Construction Measures
Window wells pose design and construction challenges because they are usually below grade and accumulate snow and surface water, and the window units they surround are not designed to resist hydrostatic pressure. Water in the window well needs to be conducted to the horizontal weeping tile by means of a vertical drainpipe and granular material, and the ground surface inside the window well should be well below the bottom of the window opening.
Preventing sump pump failure involves carefully considering various factors, such as the area and configuration of the basement (to determine how many sump pumps will be needed and where they should be located), the permeability of the soil, and the provision of adequate back-up, especially when the basement is used as a living space.

More information on basement grading and drainage can be found in Construction Technology Update No. 69, available at: http://irc.nrc-cnrc.gc.ca/pubs/ctus/index_e.html. The full report from the NRC-IRC basement study is available at: http://irc.nrc-cnrc.gc.ca/pubs/rr/rr199/index_e.html.

Mike Swinton is a principal research officer in the Building Envelope and Structure program at NRC-IRC. He will address basement construction at the NRC-IRC cross-Canada seminar series for 2008/09, Single and Multi-Family Houses: Improving Performance Through a Systems Approach. www.bsi.gc.ca, e-mail: mike.swinton@nrc-cnrc.gc.ca.

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