When I do a load of wash, it backs up in the shower. HELP! Six plumbers in 3 yrs. have not been able to solve this problem. House is built on slab and there’s a septic thats been pumped out four times in 3 yrs, field bed replaced last year. Nothing helps.
If this is a consistent problem, even just after the septic system was pumped, you may have a blocked or clogged drain vent. A common scenario is that the washing machine and shower both share the same drain. Each fixture has a trap, holding a small body of water, to prevent the sewer gasses from entering the home. If not enough air is available to allow the water to drain properly, it can create a negative pressure in the vent pipe, and siphon the water to the easiest escape route . . . into your shower drain rather than down the drain into the septic tank.
The blocked vent could be anywhere in the vertical vent system, but it is often at the termination point on the roof. Common blockages are due to leaves or debris, or bird or rodent nests. A common plumbing snake can be used to clear the drain from the roof.
Sometimes it is useful to attack a complicated situation with the simplest solutions. That which is most likely causing the back up is the blockage in the vent system. So That means you would need to make aggressive use of the plumber’s snake. And with a vertical vent you have both the drain to address and the vent stack where debris such as a bird or squirrel’s nest to contend with. So that as I have illustrated should be your point of your attack. And if you start on the roof with the snake and work your way down to the drain, you can address both issues.