Site Grading and Drainage to Achieve High-Performance Basements

Proper site grading and foundation drainage strategies are required in order to prevent water damage to basements and their contents. This Update reviews current construction practices used for basements, discusses some of the key issues and deficiencies that lead to problems, and provides practical suggestions for improving drainage and construction.

The causes of basement moisture problems are mainly external, but they can be addressed by providing diversion of water away from the building, drainage along the basement perimeter, protection of the foundation walls against moisture, as well as by effective and durable grading near the basement and over the entire lot. (see Figure 1).

Well-considered design of foundations and their interface with surrounding soils, combined with effective site drainage, can eliminate most of the moisture problems found in basements.

Figure 1. Key elements involved in providing moisture protection for basements

Moisture in Basements

Basement moisture problems can be caused by water penetration (leakage), dampness in the form of water and water vapour migrating from the soil through the basement enclosure, and sump pump failures and sewer backups; they can also be caused by internal sources (humidity and plumbing leaks).

Leakage occurs when there is an accumulation of water outside a basement wall or floor that is adjacent to a crack or defect. Surface water that is not directed away from the building can enter the basement through unintentional openings (e.g., joint cracks) at or below grade. By providing adequate drainage around the perimeter of the basement, this situation can be avoided. Alternatively, the basement can be waterproofed to resist hydrostatic pressures and thus control leakage. A fluctuating water table that rises above the basement floor level may cause leakage unless the basement is waterproofed or the groundwater level around the basement is lowered (to below floor level) by the operation of a sump pump.

Waterproofing: Treatment of the surface or structure to prevent the passage of water through the basement envelope under hydrostatic pressures. Dampproofing: Treatment of a surface or installation of a technology to resist the passage of moisture caused by differences in moisture content, vapour pressure and temperature across basement envelope components. Note: Most foundations in new Canadian houses feature footing drainage systems with complementary dampproofing elements in the wall. This approach normally precludes the need for waterproofing. The basement systems discussed in this Update use drained approaches, without waterproofing.

Achieving complete basement protection requires that appropriate control measures be provided for each form of moisture. There are five major considerations:

1. Site grading and drainage provide the first line of defence against exterior water entering the basement.

2. 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, provide the second line of defence.

3. Incorporating special design details and building in redundancy or factors of safety are ways of addressing weak links in these two lines of defence. Preventing leaks at window wells and sump pump failures are two examples of this approach.

4. Selecting the proper materials is an integral part of achieving complete basement protection (see Construction Technology Update No. 70 Options in the Selection of Materials for Basement Construction for additional information).

5. Eliminating and/or controlling indoor sources of moisture is the final step in providing adequate moisture protection in basements.

The first three of these considerations are discussed in the following sections.

Site Grading and Drainage

Improper grading of the lot can result in poor surface drainage, ponding or flooding around the basement wall, foundation settlement or other damage, and basement dampness and other undesirable effects.

It is one of the main generators of customer complaints, call-backs, warranty repairs and legal proceedings following the completion and occupancy of a dwelling.

The two most common types of lot grading are:

Back to front. With this type of grading, the rear lot line is the high point. First of all an elevated apron must be created around the house. This allows the surface drainage to flow towards the back and then forward to the street at the edges of the property. Back-to-front grading is typical of properties on a hillside or mountainside.

Split. With this type, the house is the high point and the lot is graded so that surface drainage flows forward to the street and backward toward the rear lot line, which is then generally drained by a swale and catch basin system (see Figure 2). This is the more common type and is typical of housing developments on relatively flat land.

Figure 2. Spilt site-grading and drainage plan

Most surface drainage problems occur along the side lot lines between two houses, especially where the houses are close together. In these areas, a well-defined swale between the houses should be formed, which allows surface water to be diverted from the house and directed to the swale and then forward or backward, depending on the type of lot-grading approach used.

Downspouts from the roof should discharge at grade onto a splash pad at least

1 m (3 ft.) away from the building; they should not be directed toward adjacent property but rather toward swales established between the properties and within the property boundary.

In addition, the following guidelines should be followed:

• Minimum height of top of foundation wall above grade: 200 mm (8 in.)
• Minimum slope away from house for at least 1.5 m (5 ft.): 5%, i.e., approximately 19 mm per 300 mm (3/4 in. per ft.).
• Minimum slope on rest of lot: 1.5%, i.e., approximately 6 mm per 300 mm (1/4 in. per ft.)
• Minimum elevation of lot, at house, above street level: 450 mm (1.5 ft.)
• Minimum depth of swales: 150 mm (6 in.)
• Surface drainage should be directed away from window wells, exterior stairwells and decks
• Planned elevations at the peri­meter of the lot should not be altered as a consequence of gardening, landscaping or fencing projects.

Patios next to the house basement . This is a common landscape feature and one that invites problems. In time, patios almost always end up sloping toward the house. The 5% recommended slope could pose problems for interlock patios and related walkways next to the house. These require additional planning consideration. A lesser slope may have to be used as a compromise, for reasons of practicality and safety. Gradual and consistent sloping away from the house (with a slope of 1%), using laser technology, in combination with thorough tamping, should allow acceptable outward water flow to be achieved during heavy rain events. Such patios can provide a more consistent surface for water runoff than other landscaping, hence the exception to the 5% rule.

Site grading and drainage require careful planning, as this work will not be executed until the building is nearly completed. However, moisture-protection measures must be considered and addressed during the construction process.

Source : http://irc.nrc-cnrc.gc.ca