I am not sure if our problem has to do with the washing machine or the plumbing. I tried reading other answers to similar questions on this forum, but not sure if the answers apply to our problem. We have been using the same washer in our basement for 7 years. It had an overflow problem at the drain hose/stand pipe juncture once before, when both the washer drain pipe and the kitchen sink backed up. We fixed it by pouring drain declogger down the kitchen sink. Everything worked fine for 3 years after that. However, recently we have had problems again. This time our kitchen sink is draining fine, but the overflow problem at the standpipe is worse than before. We tried putting drain declogger down both the sink and the standpipe but it didn't help. We even opened up the standpipe and it was clear of clogs. At the main metal pipe which drains both the sink and washer out of the house, the washer drain line is connected first, then the kitchen sink drain line enters a few inches down the metal pipe, which then leads out of the house. This would suggest that there would be a clog somewhere prior to the kitchen drain connection. However, we poured a gallon of water down the stand pipe, it seemed to drain fine. Is there anyway it could be the pump in the washer? It is old. Is there anyway back pressure would build up somewhere because of the washer pump? Thanks.

There is an immense amount of water flowing out of a washer when it drains. Pouring a gallon of water down the standpipe is not a reasonable comparison. I think that you have a partial clog. You can confirm this by simulating the washing machine by using your garden hose. Depending on your water pressure, this more closely approximates the amount of water coming out of a washing machine.

Drain cleaner will not remove stubborn partial clogs. The pipe you describe leads out of the house. Consider using a power auger - even with a root cutting head on it. You've not mentioned the age of your home, or the proximity the drain lines are to tree roots, etc. Also, note since it shares the line with the kitchen sink, there could be other "fun stuff" in the lines, grease, food debris, etc.

You may need to enlist the services of a professional plumber or rent appropriate drain cleaning equipment to clear the line.

Stew