Below-Grade Waterproofing Systems
Waterproofing of below-grade structures is essential for protecting buildings from water infiltration that can cause structural damage, mold growth, and deterioration of interior finishes. The selection of a waterproofing system depends on the depth of the below-grade space, the groundwater conditions, and the acceptable level of risk for water intrusion. Sheet membrane systems using modified bitumen or polymeric sheets are bonded to the prepared concrete surface to create a continuous waterproof barrier. The sheets are applied with overlapping seams that are sealed by torching or with self-adhesive backing. The membrane thickness typically ranges from 40 to 80 mils for below-grade applications, with thicker membranes used for deeper installations where hydrostatic pressure is higher. The membrane must be protected from damage during backfilling by a protection board that is placed over the waterproofing before the backfill is placed.
Liquid-applied membranes are spray or trowel applied to the concrete surface, curing to form a seamless elastomeric coating. The absence of seams eliminates the potential for seam failure that exists with sheet membranes. Liquid-applied polyurethane and polymer-modified asphalt systems are the most common types, with application rates of 60 to 80 square feet per gallon depending on the specified dry film thickness. The concrete surface must be clean, sound, and properly cured before application. Surface defects such as form tie holes, honeycomb areas, and cold joints must be patched and sealed before the membrane is applied. The liquid membrane bonds directly to the concrete surface, providing excellent adhesion that resists water migration beneath the membrane.
Bentonite clay panels provide a unique self-sealing waterproofing system that uses the swelling properties of sodium bentonite clay. The clay is sandwiched between two layers of geotextile fabric or corrugated cardboard to form panels that are attached to the foundation wall before backfilling. When the clay contacts water, it expands to many times its dry volume, filling cracks and voids in the concrete and creating an impermeable barrier. The swelling pressure of the hydrated clay forces it into any openings in the concrete surface, providing a self-healing capability that is not available with other waterproofing systems. Bentonite systems are particularly effective for applications where conventional membrane systems are difficult to install or repair.
Interior Water Management
Interior water management systems address water that has already penetrated the building envelope. These systems are often used as remedial solutions in existing buildings where exterior waterproofing is impractical or too costly. The interior drain tile system consists of a perforated pipe installed around the perimeter of the basement floor, either in a trench cut into the existing slab or in a new slab construction. The pipe is embedded in washed gravel and wrapped in geotextile fabric to prevent clogging. bentonite clay waterproofing systems for basement walls. sump pump battery backup for basement flood protection. french drain installation around building foundations. Water that enters the wall-floor joint or seeps through wall cracks is collected by the drain tile and directed to a sump pit for removal.
Sump pumps are the active component of interior water management systems, removing water that collects in the sump pit and discharging it to the exterior at a location where it will not re-enter the building. The pump capacity must be sufficient to handle the expected water inflow rate during peak storm events. Typical residential sump pumps have capacities of 2,000 to 4,000 gallons per hour at a 10 foot discharge head. Battery backup systems are essential for sump pumps because power outages frequently accompany the heavy storms that cause basement flooding. A backup pump powered by a deep-cycle marine battery can operate for 8 to 24 hours depending on the pumping demand. Some systems use a water-powered backup that operates using municipal water pressure, providing indefinite pumping capacity without electricity.
Subsurface drainage systems around the foundation perimeter intercept groundwater before it reaches the building. French drains consisting of a trench filled with washed gravel and a perforated pipe are installed at the footing level around the building perimeter. The drain pipe slopes to a suitable outlet such as a storm sewer, dry well, or daylight discharge point. The depth of the French drain must be below the lowest level to be protected, with the invert of the pipe at or below the footing elevation. The spacing of the drain pipe from the foundation wall depends on the soil permeability, with more permeable soils allowing wider spacing. Geotextile fabric wrapped around the gravel prevents soil migration into the drainage system that would eventually clog the pipe and reduce its effectiveness.
Exterior Drainage and Grading
Proper grading around the building is the first line of defense against water infiltration. The finished grade should slope away from the foundation at a minimum slope of 5 percent for the first 10 feet. This slope of 1/2 inch per foot directs surface water away from the building rather than allowing it to pond against the foundation walls. Inadequate grading is one of the most common causes of basement water problems and can often be corrected by regrading or by installing swales that direct water around the building. The grade should be maintained at least 6 inches below the finished floor elevation at all points around the building to prevent water from splashing against the siding and entering the wall assembly.
Gutter and downspout systems must be properly sized and maintained to handle the roof runoff from the building. A clogged gutter system can discharge thousands of gallons of water against the foundation during a single storm event. The gutters should be cleaned at least twice per year, in the spring after pollen and seed drop and in the fall after leaf drop. Downspouts should extend at least 5 feet from the foundation wall and discharge onto a splash block or into an underground drain pipe that carries the water to an appropriate outlet. Underground downspout connections must be watertight and sloped to drain completely between storms to prevent standing water that can attract mosquitoes.
Repair of Existing Water Problems
When existing basements or below-grade spaces develop water problems, the source of the water entry must be identified before repairs can be specified. Water entry through wall cracks can be stopped by injecting low-viscosity polyurethane or epoxy into the crack from the interior side. The injection material flows through the crack and seals it from the exterior side, blocking water entry. Active leaks under pressure require injection of a fast-setting polyurethane that reacts with water to form a flexible foam seal. The foam expands as it cures, filling the crack and stopping the flow of water within minutes.
Water entry at the wall-floor joint, called the cove joint, is one of the most common basement water entry points. The joint between the foundation wall and the floor slab is a natural location for water to enter because it is a cold joint between two concrete placements. Repair involves cutting a groove along the joint, cleaning it thoroughly, and filling it with a flexible sealant or hydraulic cement. For persistent cove joint leaks, a more comprehensive solution involves installing an interior drain tile system that collects water at the joint and directs it to a sump pump. The combination of crack injection, joint sealing, and interior drainage provides comprehensive protection against water entry for most below-grade spaces.