Slab leak (water leak under foundation) — San Francisco, California

A slab leak is a water or sewer line break beneath your home's concrete foundation. Because the pipe is buried under several inches of concrete and soil, the leak is invisible until secondary symptoms appear: unexplained spikes in the water bill, warm spots on the floor (hot-water line leak), the sound of running water when all fixtures are off, damp carpet or warped flooring over the slab, cracks in walls or baseboards from foundation movement, or mold and mildew smell with no visible source. Left unchecked, a slab leak erodes the soil supporting the foundation, causing differential settlement that cracks walls, jams doors, and can make the home structurally unsafe. Detection requires specialized equipment — electronic amplification, tracer gas, or infrared thermography — which is why DIY detection is rarely successful. Repair options range from spot repair ($500–$2,000, breaking through the slab to fix one section) to rerouting the line through the attic or walls ($2,000–$6,000, bypassing the slab entirely) to full epoxy pipe lining ($4,000–$15,000, coating the interior of existing pipes without excavation). A licensed plumber experienced in slab leak detection and repair is essential — general handymen should not attempt this work.