So my main drain backs up, at least when I run the clothes washer. Whenever the washer drains it backs up into the bathtub. Ok easy enough right, so I go through the process of trying to snake the drain for the tub, nothing comes of it. I go into the crawlspace and notice a P-trap on the main line right beofre it enters the stack (so the P-trap is up stream of the stack) I assume, and you know what that spells, that the P-trap must be clogged...so easy enough right, i cut out the P-trap and put in a new one, but before I do that I run my snake back to the stack, no clogs found, and the P-trap is not clogged either. However, after replacing the trap the drain doesn't back up as bad. The toilet never backs up either and I never found a clog up stream of the stack. So i'm confused. I half wonder if the vent may be clogged and when I cut the p-trap out that extra moment of releasing the pressure is what has made the drain not back up as bad. I checked the vent pipe at the top of the roof and can't see anything blocking it, but I suppose its possible there is a clog in it lower down. I have not snaked the main line from the stack back to the sewer, but if that was clogged wouldn't my toilet be backing up too? I just don't know, I'm a sparky, plumbing is not my area of expertise but I am plenty mechanically inclined so I'd like to fix the problem without having to call a plumber. Thanks all!