In Plain English
Quick Answer
The main drain pipe inside a building that collects all the wastewater and directs it to the sewer line outside.
Definition
Definition
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.
Context
Why It Matters in Bidding
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.
Example
Example
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.
See Also
Related Terms
FAQ
Questions Contractors Ask
What slope is required for a building drain in a bid?
Most codes require 1/4 inch per foot for pipes 3 inches and smaller and allow 1/8 inch per foot for larger diameters. Estimators must confirm the spec'd slope, since flatter runs need deeper trenching at the far end, raising excavation and backfill costs the further the run extends.
How does the building drain differ from the building sewer in a takeoff?
The building drain is measured inside the structure to a point a few feet past the foundation; beyond that line it becomes the building sewer. Splitting the two matters because they often carry different materials, bury depths, trade responsibilities, and inspection scopes within the bid.
What material is typically priced for a building drain?
Cast iron and PVC are the most common choices. Cast iron carries higher material and labor cost but is often specified for sound attenuation or fire-rated assemblies, while PVC reduces cost. Always verify the spec, since substituting materials without an approved addendum can affect bid responsiveness.
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