Cast iron drain failures, root intrusion, slab drain leaks. Cat 2/3 water requires multi-stage IICRC S500 sanitization plus standard drying and reconstruction.
Drain line leaks behave differently from supply line leaks. Drain water is gravity-fed (low pressure), but it carries soap residue, hair, food debris, fecal matter, and other organic contaminants. IICRC S500 classifies drain water as Category 2 (grey) or Category 3 (black) depending on source. Houston's older cast iron drain lines (common in Heights, Montrose, Eastwood, and Inner Loop pre-1970s construction) develop pinhole corrosion and section failures that can release sustained drain water into wall cavities and crawl spaces undetected for months.
Heights, Montrose, Eastwood, and older Inner Loop neighborhoods commonly have original cast iron drain pipes. After 60-80 years, sustained sewer gas corrodes the pipe from the inside, producing pinhole leaks and full sections fall out.
Houston's mature live oaks, pecans, and water oaks send roots into drain joints. Once inside the pipe, roots expand and crack the pipe. Sustained low-pressure drainage seeps through the crack.
Houston's clay soils shift seasonally. Drain pipes installed at proper slope (1/4″ per foot) can settle into negative slope, causing standing water in pipes and accelerated corrosion at low points.
Under-sink P-trap connections work loose from vibration and freeze cycles. Slow drips into cabinets eventually saturate cabinet base and floor below.
Inadequate venting causes drain siphoning, which empties P-traps and allows sewer gas back into the home. Often combined with drain line leak symptoms.
Slab-on-grade homes with drain lines through the slab can leak below the slab undetected for years. Symptoms: damp slab perimeter, sewer odor, mold along baseboards.