Our house was built in 1997. We bought it in 2001 in the winter. There is a problem with sewage odor coming from the washing machine drain in the laudry room when the weather is warm. A couple of years ago we had the septic tank checked and waste water lines extended (I'm not sure if that's what you really call those lines --pipes in the ground with holes in them). We had hoped that the septic line work would solve the problem but it has not. Does anyone have a clue as to what is going on here and how it can be
Permanently fixed? I can control the odor somewhat by putting baking soda down the washing machine drain after I do laundry and have been doing that for the last few years; I would really like a permanent fix. Thanks to anyone who can help.

I found a similar problem with a house on a septic system. Turns out the vertical drain went directly into a cast iron main drain in the floor, there was no trap. Without a trap thefumes from the septic tank came back into the house. To fix the problem we made a new vertical drain with PVC and with a trap. We cut off the old drain a few inches above the floor after opening the drywall. The PVC was attached to the old drain with a hub.

You definitely need a trap on this drain. You might have a clogged or missing vent stack, too. I say this because it seems possible that when it is warm, and the septic tank is really percolating, gasses might be able to push past the water in the trap (if it does exist) if they have nowhere else to vent to.