Plumb Bob

A plumb bob is one of the oldest tools in construction — a pointed weight (typically brass or steel, 4–32 oz) suspended on a string or cord. Gravity pulls the weight straight down, creating a perfectly vertical reference line. In plumbing, plumb bobs are used to verify that drain stacks, supply risers, and fixture connections are truly vertical before fastening.

Why vertical alignment matters in plumbing: drain-waste-vent (DWV) systems rely on gravity. A stack that leans even slightly can create pockets where waste accumulates, slowing drainage and encouraging clogs. Supply risers that aren't plumb can stress pipe joints over time as framing expands and contracts seasonally.

How to use a plumb bob:

1. Tie the string to a fixed overhead point (joist, top plate)
2. Let the bob hang freely until it stops swinging
3. The string now marks a perfect vertical — measure from it to check pipe alignment
4. For long runs, use two plumb bob measurements to establish a vertical plane

Modern alternatives: laser levels with a plumb-beam mode have largely replaced plumb bobs on larger jobs, but plumb bobs are still preferred in tight spaces (inside wall cavities), in areas where laser beams wash out in bright light, and by plumbers who need a permanent reference mark they can walk away from.

For homeowners: if a drain is draining slowly and you suspect a sagging horizontal drain run, a plumber will use a level (or laser) to check slope — the same principle as a plumb bob but measuring horizontal pitch instead of vertical plumb.