Plate C — Belly (vertical sag).

§ I · The PACP coding

PACP v8 codes sags by depth in inches per foot of host pipe:

• SAS — sag, small. Up to ¼"/ft. Tolerable with periodic flushing.
• SAM — sag, medium. ¼–½"/ft. The customer is noticing slow drains.
• SAL — sag, large. Over ½"/ft. Backups during heavy use.

By way of comparison, residential sanitary lateral design grade per the NYS Uniform Plumbing Code is ¼" per foot for a 4-in. line, ⅛" per foot for a 6-in. line.¹ A 6-in. lateral with a ½"/ft sag is locally negative grade — the pipe is sloping back uphill — and the only thing carrying flow through the dip is upstream pressure from the next flush.

§ II · Why CIPP is the wrong answer

This is the most common misdiagnosis in our service area. A customer comes to us with a CIPP recommendation from another contractor that includes "lining over the sag." The lining will harden in the sagged geometry. The dip remains. The standing water remains. The customer paid $7,000 to confirm that the geometry of their pipe is wrong.

CIPP cannot raise the host pipe. It conforms to whatever shape the host pipe is in, including a sag. A liner installed across a half-inch-per-foot sag will trap solids in the dip exactly as the host pipe did, and will require periodic cabling exactly as the host pipe did. The only difference is that the customer now believes the pipe is "new" and is surprised when the symptoms return.

§ III · What is the right answer

To raise the host pipe, you must excavate to the host pipe. The methods for a true sag fix:

• Pipe bursting with bedding correction at the pit. The bedding inside the pit can be replaced and the pipe can be re-laid at the correct grade for the section between pits. The mid-run grade between pits is determined by the original bedding profile and is not corrected by the burst alone.
• Open-cut excavation along the full sag length, with a re-laid bed at design grade. This is what we recommend at SAL severity. The cost reflects the dig — typically $11,000–$18,000 for a residential lateral with a documented mid-run sag.
• Sectional spot repair + re-grade if the sag is short and shallow (under 4 ft of length, ≤ ½" total depth). One pit, one bedding correction, one re-laid section. $3,400–$4,800.

§ IV · How sags happen

Most Buffalo sags trace to one of three causes: (a) original bedding compromised by groundwater migration during the first decade after install (we see this in Allentown, where the water table sits high and the 1900s bedding was unwashed sand); (b) post-install excavation alongside the lateral that disturbed bedding (a 1962 oil-tank install, a 1991 gas-service replacement, a 2014 sidewalk slab pour); or (c) seasonal frost-heave under marginal-depth installs. Cause (b) is the most common in our dossiers — the lateral is not the original cause of its own sag.