Below-grade waterproofing systems protect basement walls and floors from water infiltration. The primary system is typically a membrane applied to the exterior of the concrete wall before backfill. Membrane types include rubberized asphalt, plastic polymer, and spray-applied polyurethane. Rubberized asphalt membranes are self-adhesive or require trowel application. They are flexible and accommodate crack movement. Plastic polymer membranes (PVC or HDPE) are sheets applied with mechanical fasteners or adhesive. Spray-applied membranes are applied with special equipment and form a continuous seal without seams. All membranes must fully cover the below-grade wall surfaces including around penetrations and at construction joints. Detailing at corners and penetrations is critical; improper detailing causes leaks. Perimeter drainage systems collect water that accumulates around the foundation. A perforated drain pipe at the foundation base collects groundwater and directs it to a sump or daylight drainage. The drain pipe must slope continuously toward the outlet to prevent water backup. In sump pit systems, water accumulates and a sump pump removes it. The pump must have capacity adequate for expected groundwater flow. Interior-applied waterproofing products provide secondary protection if water penetrates the exterior membrane. These products are typically crystalline compounds that react with water to seal concrete pores. Proper grading of the site ensures surface water is directed away from the foundation. In high water table areas, interior French drain systems may be installed along the basement perimeter to intercept seepage.