Sewer Line Installation in Floor Joists

Common Joist Types

• Bracing and boring practices for sewer line installation vary according to joist type. Traditional joists consist of solid lumber, often 2-by-8 or larger. Engineered joists are a common alternative to solid joists. Engineered joists, also called I-joists, consist of a vertically oriented chip-board panel glued between a set of laminated boards. Drilling is permissible through both solid lumber joists and engineered joists. However, many engineered joists have perforated "knock-out" holes for plumbing and electrical lines. If the prefabricated holes are too small to accommodate drainpipes, plumbers enlarge them with a standard hole saw.

Drain and Waste Piping Materials

• Metal and plastic pipes are the most common materials for drain and waste piping systems. While allowable materials inevitably vary according to building design and local building codes, plumbers frequently use either cast-iron or ABS pipes to construct in-floor drain piping. Cast-iron pipes often connect via mechanical, clamp-tightened fittings, and plastic ABS pipes join with a glue-like process called solvent welding.

Boring Through Joists

• Although boring details for solid lumber joists vary according to site-specific conditions, joist span and wood species, building codes offer general guidelines. Holes should not sit closer than 2 inches from a joist's edges. Additionally, hole diameter should not exceed one-third of the joist's depth where depth refers to the size of the joist's face, such as the 6 in a 2-by-6. Note that the guidelines apply to 2-by-6 and larger joists; 2-by-4 joists are too weak to withstand boring for sewer lines. For engineered joists, consult your joist manufacturer's guidelines to determine hole size and placement.

Bracing and Support

• Plumbers support pipes with bracing or strapping where pipes must run parallel to joists or below joists. Lumber blocks are a common bracing technique. Plumbers cut block to fit perpendicularly between joists and fasten the blocks with screws or nails. The blocks might support pipes from beneath or sit above pipes to provide a nailing surface for strapping materials. Pipe strapping materials include "plumber's tape." Not to be confused with pipe-thread seal tape, the term plumber's tape refers to a roll of perforated, metal or plastic banding. Plumbers attach the banding to joists or blocks with nails or screws.