by Peter Barrett
The major cause of foundation-related problems is water. Hence waterproofing systems are very critical to keeping foundations dry. Like air control above grade, it is important to consider water control below grade as a system solution, and not just as a single component.
A properly installed and performing waterproofing membrane is important, but without an adequate drainage system to allow the liquid to flow away, water can damage any construction project. An improperly conceived waterproofing system can allow the buildup of hydrostatic pressure, turning cracks in the foundation into entry points for water.
When water enters the building interior, there is not only a risk of unsightly water damage, such as rust-colored stains or spots, but also uncontrolled moisture introduces humidity, thereby creating a very desirable home for mold and mildew. The unwelcomed water will also damage building materials.
To properly protect a concrete foundation, the wall must be shielded against the water and the liquid must be directed away from it. Selecting an appropriate below-grade solution for commercial buildings is one of the most important aspects of maintaining structural integrity, indoor air quality (IAQ), and comfortable living or working conditions for all the occupants. To make the right decision, it is wise to first understand site conditions and the science behind movement of moisture from concrete footings up the wall.
Understanding site conditions
In accordance with section 1805.3.2, “Walls,” of the 2018 International Building Code (IBC), concrete or masonry walls retaining earth and enclosing interior spaces and floors below grade shall be waterproofed—designed and constructed to withstand hydrostatic pressures and other lateral loads. When selecting a waterproofing system, it is important to understand the overall water management system goals for site conditions. Soil quality, water table location, and how the system will be installed are all important considerations.
Different soils drain differently. Some hold tonnes of water, and others will let the liquid filter through. A geotechnical report helps outline soils and possible contaminants and chemicals that may need to be addressed before selecting a below-grade system.
The phenomenon of hydrostatic pressure is simply the weight of the accumulated water pushing against the structure. If the soil surrounding a building is saturated with water, intense hydrostatic pressure is applied against the foundation, thereby allowing moisture to push its way through cracks and pores in the concrete walls and up through the floor slab. A metre (4 ft) of water can exert 136 kg (300 lb) of pressure per square foot of wall.
Instead of sealing a foundation against this pressure, it is much more effective and cost efficient to relieve, control, and manage it. The solution to hydrostatic pressure is to remove the water. Drainage systems—combination of grading, granular fill, control of roof bulk water, and a sump—redirect the accumulation of ground and surface water away from the foundation by channeling it to the intended location, but they are generally not enough protection. The installation of a footing drain (also known as perimeter drains) alongside a waterproofing membrane offers the highest degree of protection.