How basements start leaking

A rainstorm or melting snow temporarily raises the groundwater level. When water accumulates around the foundation, hydrostatic pressure builds up and causes the basement to leak. Clay-rich soils do not drain well and hold rainwater right against the foundation walls. Water pushes its way inside through cracks or joints and the pores in concrete.

As houses settle, concrete develops stress cracks that leak water. Exterior waterproofing cracks and disintegrates or separates due to the "alkali attack." Water also corrodes imbedded steel, which expands and cracks the concrete.

Water penetrates into the pores in concrete, dissolves alkalis, and enlarges the pores. As concrete ages, it becomes more and more porous. Initially, the seeping water evaporates, leaving on the surface lime that reacts with carbon dioxide in the air and forms limestone. Efflorescence is a telltale sign of capillary water seepage.

Slabs get even less protection than basement walls. The plastic "vapor barrier" soon disintegrates due to the lime in concrete and over time, the layer of gravel ("drainage pad") silts up. The concrete slab pulls in groundwater against gravity by capillary suction.