The main drain pipe inside a building that collects all the wastewater and directs it to the sewer line outside.
The lowest horizontal drain pipe within a building that receives discharge from all soil, waste, and drain lines and carries it to the building sewer outside the structure. The building drain runs from the base of the main soil stack to the point 2 to 3 feet outside the building foundation. It must be sloped to maintain a self-cleaning velocity.
Building drain runs are a core line item in plumbing takeoffs, where pipe diameter, slope, material, and depth of bury drive both labor and material costs. Underestimating excavation depth or slope requirements is a common source of bid errors and change orders on tight-grade sites.
A plumbing estimator measuring a restaurant's underslab piping quantifies 60 linear feet of 4-inch cast iron building drain at 1/4-inch-per-foot slope, then prices the trenching and bedding separately as a sitework allowance.
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