A sewer scope found a dent in a new home's 4-inch pipe. The builder says 7.5% deflection is allowed

The owner described the findings in a post on Reddit's r/Plumbing. According to the post, the builder's contractor said the dent is within tolerance and that cutting in a coupling would not leave the line any better. The owner is deciding whether to accept that answer or have the line scoped again before the warranty runs out.

The 7.5% figure comes from ASTM D3034, the standard for PVC gravity sewer pipe. The Uni-Bell PVC Pipe Association says the limit carries a safety factor of 4, since PVC pipe tolerates more than 30% deflection before it risks failing. Uni-Bell defines deflection as the drop in the pipe's vertical diameter divided by its original diameter: the round pipe squeezed into an oval by soil load. On a 4-inch pipe, 7.5% is about 0.3 inch, so a 3/16 to 1/4 inch reduction, roughly 4.5% to 6%, falls inside the limit if it is that kind of deflection. Uni-Bell says a pipe deflected 7.5% loses less than 1% of its flow capacity.

A dent in one spot is a different question from an evenly squeezed pipe. The International Residential Code targets the conditions that put point loads on buried pipe. Section P2604 requires solid, continuous support under a building sewer, bars resting pipe on rocks or blocks, and calls for backfill free of rocks, broken concrete and debris until 12 inches of tamped earth cover the pipe, placed evenly on both sides.

The clock also matters. The Federal Trade Commission says builder warranties generally cover plumbing for two years and most workmanship for one. The FTC advises filing claims in writing, by certified mail, and keeping records of every exchange. Owners in this spot can ask the builder for a written measurement of the dent and a second scope near the end of coverage to see whether it has changed.