Finally figured out why my toilets were burping and the house smelled like a sewer

I had a similar nightmare with my 1994 colonial last spring. I kept hearing that glug-glug sound every time the washing machine drained, and I swore it was a main line clog. I spent $200 on a rental snake before I realized the issue was actually in the attic. In my case, it wasn't just air pressure; the thermal expansion from the summer heat had caused the PVC to expand and contract so many times that it literally pushed itself right out of the rubber boot on the roof.

When I finally climbed up there with a headlamp, I found the 3-inch stack was leaning at a wierd angle because the original installer never used a riser clamp. I ended up using some heavy-duty Oatey purple primer and medium gray cement to get it back together, then I bolted a metal brace to the rafters to keep it from sliding again. If you haven't already, check if there’s a support bracket holding the vertical weight of that pipe. If it's just hanging by the roof flashing, the weight and the pressure changes will eventually pull it apart again.

I had a similar issue but it wasn't the glue failing or thermal expansion; it was actualy a lack of proper support in the framing. In my 1990s ranch, the plumbers didn't use any riser clamps or J-hooks on the vertical stack in the chase. Every time the heavy 1.6 GPF toilets flushed, the whole column of water whould create enought vibration and momentum that it eventually shimmied the pipe right out of the rubber boot at the roofline.

I found out because I noticed a water stain on the ceiling near the vent. When I got up there with my flashlight, the 3-inch PVC was leaning at a ten-degree angle. Instead of just sliding it back in, I had to install a heavy-duty riser clamp across the attic floor joists to keep it from dropping again. If you're up there fixing yours, make sure the pipe is actually braced to the structure and not just hanging by the roof flashing. If the pipe can move even a fraction of an inch, those vibrations will eventually work it loose again.

While the mechanical failures mentioned are common, one shouldn't overlook the pneumatic positive pressure transients generated when a high-velocity slug of effluent acts as a piston within the drainage system, effectively compressing the air column ahead of it. If the initial installer neglected to properly chamfer the pipe ends per ASTM D2855 standards, the resulting plowing of the solvent cement often creates a structurally deficient joint that remains susceptible to these specific internal pressure spikes. I whould verify that your stack transition utilizes a long-sweep wye rather than a short-radius elbow, as the latter significantly increases the magnitude of those air-pressure surges that ultimately force a weakened slip-joint to fail entirely.

I'm a bit skeptical about internal pressure popping a joint like that. Have you ruled out a 'dry fit' from the original build? It’s pretty common for guys to friction-fit those last few feet in a cramped attic and just forget the glue.

If it was never acutally bonded, it wouldn't take much for it to slip out eventually. Did you see any actual glue residue or primer on the pipe when you pulled it back? If that PVC is clean, it wasn't the pressure—it was just a 20-year-old mistake finally catching up to you.