Washing machine drain keeps overflowing
Hi, at my house, we have had a continuous problem over the years being that the washing machine drain will back-up and overflow from time to time. It occurs about every 6 months or so, and to fix the problem, I pour Assault down the drain, and that takes care of the problem. The washing machine is on the second floor, which has lef to a number of problems over the years with the overflowing. When it overflows, it has created a problem of leaking down the ceiling. Also, after years of using Assault (my mother has owned this house for 29 years), it eventually ate through the cast iron drain pipes. About 4 or 5 years ago, my brother and I tore down the ceiling, and replaced the cast iron drain pipes and then replaced the ceiling (a very big job). For probably about 2 years there was not an overflowing problem with the washer. It eventually came back, and now I am resorting to Assault again, hoping it won't eat through the pipes while my mother still lives here. I know essentially the problem is the cast iron drain pipes are 2 inch pipes, and there is very little angle to the pipes to cause the water and detergent residue to want to run towards the ground... They almost run perpendicular to the ceiling(discovered when we replaced them). And the problem is that with it being the washer, there is always detergent, and with the angle of the pipes, the detergent build up a residue in the drain.
Just some history and stats if they help... The house is about 35 years old, washer is on the second floor, there is a basement, the washer drains into a drain pipe that is about the height of the washer, the drain then runs down about 4 feet or so and then there is the trap, I do know there is a vent right after the trap leading up towards the attic (and I know it is the vent, because there are not any other drains higher than the washer).
Is there a type of detergent we could use that causes less residue in the drain? Is there anything I can do, other then use Assault whenever it overflows? I'm hoping something can be done, because there is already some signs of weakening in the ceiling on the first floor under the washer from the opverflowing (and keep in mind that ceiling is only 4 or 5 years old). Trying to do anything with changing the angle of the cast iron pipes is pretty much out of the question, because that could require extremely extensive remodeling, including the floor joists (I hope I spelled that right), and we live in a condo (more like a townhouse though), and we would need approval to do that, and I don't know if we could get it.
Thanks in advance for your help!