How can I tell if slow drainage is caused by a problem in the main sewer line or the venting system?

Question

I recently had the plumbing get backed up and snaked out. Things are working much better now, however, there are certain conditions (notably the use of toilet paper in conjunction with a shower or the washing machine) which result in extremely slow draining for the whole house. This also results in the showers backing up (slightly) and the sinks gurgling.

I'm thinking that the main line may need to be snaked another 20-50 feet but I want to rule out anything within the house. The main is the only common drain between the various rooms impacted so the only other thing I can think of would be the vents.

How can I tell if this is a problem in the main line or in the venting?

Edit: city sewer. At the end of a dead end road if it matters.

Answer 1

I'm afraid it's probably both. You have a venting problem because the sinks gurgle. Watch the water level in a toilet when the washer starts to drain, it'll probably drop a little. The reason you don't have a bigger problem with water getting sucked out of traps is because the main line is obstructed, minimizing the demand for makeup air through the venting system. If the mainline were not obstructed, the backed up water would quickly drain away once the traps were voided of water, providing an alternate path for makeup air.

The fact that toilet paper is a contributing factor also indicates main line problems. The waste water backing up could have caused solids to float up into vent lines and get lodged there. Hopefully snaking through the roof vents will free any vent obstructions. Depending on the nature of the blockage, simple snaking may only temporarily allow drainage. More aggressive clearing may be called for.