I have a similar problem as Intrepid, however, my drains are all PVC, the main line out of my house is SCH 40 4" PVC, and it has a good slope, it just started backing up last night while my wife was doing her third load of laundry. Could a 4" drain need snaked? I just have a hard time believing it. This morning it drained fine when the washer drained the first time, but after the spin cycle I was down there monitoring it, it backed up. Please help a carpenter out! I thought maybe the vent on the roof was blocked up with snow, but there isn't any over the top of it but should I climb up there and look down it? It's only on a first story roof no biggie. I got a 3/8" snake, but if I need to snake a 4" drain, that won't do I'll have to get something bigger. Any help would be greatly appreciated!