Since our house was built in 1997, we have had at least six times a backup in one plumbing line that drains the washing machine and laundry tub in the laundry room and the toilet, tub, shower, and two sinks of the bathroom that shares a common wall. We have had the rotorooter people out several times, and while he removed some blockage, it was just the stuff you'd expect to be going through the line from the toilet (low flow, compressed air type) == nothing that should be causing a problem. The last time we had a problem, for example, the water from the washing machine came up through the tub and the shower stall all over the floor, bringing with it a bit of fecal material. Our builder's plumber has no idea what is wrong. Pipe has been replaced between the toilet and the other bathroom appliances, but it didn't help. The soil stack/vent goes horizontally out through soffits in the overhang of our flat roof. The distance between the common drain pipe for all of this in the basement and the vertical of the vent is under 12 feet, but then the vent runs about four feet horizontally at ceiling/roof level before it opens at the under side of the overhang. What is our problem and how do we cure it? We have other bathrooms venting into the overhang without problem. Advice would be very, very much appreciated!!
Now that I have looked at some of your other messages, I believe I need to provide some more information. The ceilings on the floor where the problem occurs are 12 feet high. Thus I guess we have 12 feet vertical and maybe 15 feet horizontal on this. Would a fan placed in the vent stack somewhere as it goes up inside the wall help? Or a vent opening in the run between the washer to the laundry sink, which is on the outside of the wall?