Leak appearing in basement on bathroom floor beneath vent stack, under main floor bathroom pipes. In the wall between main floor kitchen and main floor bathroom. Erratic leak. Not always after rain but sometimes. Not after shower in main floor bathroom. Main floor bathroom floor near vent stack wall, very slightly springy only along that wall. Suspended ceiling in basement, floor/ceiling beneath kitchen is clean. And so is floor/ ceiling beneath main bathroom. Floor/ceiling around vent stack opening , black rot and damp when I place my hand up and fingers around corner away from vent stack. I've dried and cleaned all the pipes near the stack and am waiting for them to get damp, or see water on floor. Don't want to disassemble kitchen cabinets to get to the wall and pull down drywall, ditto the main bath wall. Any ideas? Under kitchen sink is dry. Vent stack roof to main floor ceiling is one piece, no joints. No water staining. No water staining in attic floor by vent stack. Should I spend the money for a camera view down vent stack; looking at area between main floor ceiling and main floor floor or bite the bullet and start pulling down drywall? Maybe a cup of water visible, visible, twice a month.

Have you checked the boot on the stack? Very common to find a split boot.

Hi , thanks for replying. I have a new roof, 1 yr old, new vents and roof stack at that time. I'm not able to climb up myself right now (had surgery recentl ) but called roofing company to check their work. Not only did he go on roof and state boot was good, but I talked him into going in the attic and checking for water stains around vent stack. Last fall I insulated the stack in the attic space to lessen the chance of ice blocking on top in winter. He said insulation is dry and not stained. We just had a high wind rain this evening so I'm checking periodically to see if any water has made its way down. I half think there might be a leak behind the tub faucets in main floor bathroom, leaking down to basement bathroom. Half think may be a result of overspray. We have a shower curtain in main floor bath/shower. Then I wish I had more halves to help me think this through. If by tomorrow morning no ( rain ) water has made its way down, we'll shower making sure the curtain is secure to the edge of the tub. Then check. And lastly, just pour some water on the floor and see if it weeps through somewhere and that makes the basement bathroom wet. I've thought this might be the reason I do a main floor bathroom reno. So before I start taking drywall down, I guess I should start looking at acrylic vs metal tubs so reassemble is a bit quicker. (25 yr old tub in now is scratched). Will be replacing some subfloor too I think. Not to mention, finding this leak.

Well, I'm still scratching my head. This morning there was some water dripping off the sub floor onto the plumbing stacks. So this was before any showering activity, so I'm ruling out overspray. Could still be a leak behind the main floor bathroom bath tub/shower faucets. Or could still be a crack or bad joint in the vent stack between the walls of main bath and kitchen. I guess a trickle could leave the inside of the stack and migrate around if there's contact beneath where there's a breach. i.e. a brace or framing. I really hope when I finally start the bath project I can find this leak and have a plan to fix it on the spot. I do well on projects when I have a plan and can go start to finish without having to rethink the plan. My past renos have demonstrated how to make a 2 or 3 day project stretch to months as I keep rethinking stuff. ( lose enthusiasm ? ) Any how, after the sub floor stops its slow dripping as it is now, I will pour water on the floor and watch for more dripping, so I can check that off the list. The question is why is it dripping from the subfloor? Then to the visible stacks?