Thoughts on rebuilding an entire tub drain assembly vs just repairing the leak point?

I'm in the camp of preserving clearance every single time, especially between floors. If the existing assembly is heavy 17-gauge tubular brass, slapping in a fresh beveled poly washer and a smear of RectorSeal will easily buy you another fifteen years without creating clearance nightmares. The issue is that alot of guys reflexively cut everything out beacuse code in many areas forbids concealed slip joints behind drywall without an access panel. The second a sub opens that ceiling, they feel obligated to glue up a solid schedule 40 P-trap so they don't get married to a mechanical joint they didn't install.

That said, dropping the trap so far that it's pressed tight against a 1/2-inch copper supply line is just trading a small leak for a future pinhole from pipe vibration. When I run into tight 2x8 joist bays, I'll fight to keep the original geometry intact or use a low-profile solvent weld assembly rather than wedge ABS against hot and cold lines.

I get the desire to keep that extra inch of headroom in the joist bay, but as soon as you close that drywall back up, leaving mechanical slip joints behind is a massive liability. I used to nurse old brass setups along with new poly washers and True-Blu, but after having to pay out of pocket to repaint a first-floor dining room ceiling beacuse tub vibration worked a slip nut loose three years later, I completly stopped doing that. If the ceiling is allready open, those mechanical joints are getting cut out every single time. Glued schedule 40 ABS doesn't care about tub flex, temperture cycling, or someone dumping caustic drain cleaner down the overflow. Your sub definitly shoud have made sure the trap wasn't physically binding against that copper line—I always slip a scrap of felt pipe wrap or foam insulation between them to kill any creaking from thermal expansion—but converting the whole drop to solvent-weld was the right long-term move.

What gets overlooked in these retrofits is the vertical dimension between the strainer flange and the trap weir, beacuse dropping the P-trap lower to accomodate standard Schedule 40 hub dimensions alters the developed length of that vertical drop, wich—aside from creating physical interference with the adjacent supply lines—can induce unnecessary turbulence and self-siphonage if the trap arm slope wasn't properly recalibrated at the sanitary tee. If the objective was code-compliant solvent-weld permanence without stealing space in the joist bay, using a dedicated low-profile solvent-weld waste kit (somthing like a Watco 901) whould have preserved the original hydraulic profile far better than cobbled-togather standard ABS fittings.

Kids these days see brass and panic beacuse they don't know how to run a torch on it anymore. everthing has to be glued up like plastic tinker toys. Back when we ran heavy brass, if code didn't allow slip nuts buried in a ceiling, you’d just sweat the whole waste and overflow solid with 50/50. Bulletproof, slim profile, and you never had to worry about wedging bulky Schedule 40 hubs against a water line. Now a sub opens a ceiling, hacks out good metal, and forces fat plastic fittings where they don't belong because it's the only trick in his bag.

Quick update — went back with a tape measure after reading the point about the trap weir and drop length. Dropping it to fit those bulky ABS hubs put the vertical drop from the strainer at nearly 18 inches, and the bottom of the trap is literally touching the drywall plane. For those who brought up developed length: does extending the drop that far risk self-siphoning, or is it strictly a clearance headache?