All plumbing systems have vent stacks that allow air to escape and water to flow through and out to your sewer or septic system. These vents are in the walls and travel up and out above the roof level of your house and can be seen by looking at your roof. They will be pipes sticking up out of the roof. These vents can become clogged with debris from falling leaves, etc and an occasional small animal may take up residence and leave a nest behind. If these vents are clogged then your drains can act as you describe because the air trapped in front of the escaping water has no way to escape thus slowing the drains and bubbling or backing up. These vents usually can be cleaned easily by someone climbing on the roof and using a garden hose at high pressure to blow them out, forcing the clog on down your drains. This does not diminish the possibilty that you have a root clog in your drain lines that will need to be augered or "snaked" out. The transient nature of this type of clog can be affected by how saturated the soil is from rains, or a septic or sewer system problem.
Hopefully it was a vent problem and has cleared itself up.

I concur with Faucetman.

ok thanks! I am up on my roof all the time cleaning up leaves, and never thought about vents! I'll try to clear them out, since the toilet is still bubbling, though there is no drainage problem. Can I do any damage doing this with a hose? If there is a blockage, am I basically blowing it back into the pipes and then out with the sewage? We are on city plumbing; no septic tank.

Use your drain auger on the vents first and pull out any debris you can. Then you can flush some water down. I wouldn't recommend relying on pure water pressure to clear a vent, as it can do exactly as you state and blow debris back upline and create a potentially more difficult blockage to deal with.

Thank you!

I have my little project for tomorrow! :)

A word of caution. If you run water down the vent, have someone inside (eg. lowest drain or toilet) looking for anything backing up (so they can yell for you to "STOP!!!" before you flood something, in case there's still an obstruction somewhere).

I have a customer that yearly augers their sewer line in the front yard (roots due to a mammoth tree, and repair estimates are prohibitive for them now) - they know when it needs augering again when they see water coming up the basement floor drain....and any other sewage coming down the drain adds to the mess.

i agree, have a helper before running water down the vent. 2 years ago the drain line for our kitchen sink backed up, acting like the vent was clogged. well i go up on teh roof and run teh hose down it and proceed to run it full blast(had a pressure adapter on it to really get a good jet). water is coming up out the vent so i think it is still eating thru the clog. next thing i know my wife is running outside telling me to stop. turns out the vent WAS clogged where it came off the main drain line, but i just forced the clog thru a little where it hung up again. i had been running the hose for several minutes, unaware that water was shooting from teh drain in teh kitchen and all over everything! since this line served only teh kitchen, it was a 2" line and must have had enough grease in it to catch the debris from teh vent.

Thanks for the added comments! I'm going to do this little project when my fiance and I are both home, and I would NOT have thought of flooding inside! Yikes!

Faucetman,
I read your description and can't figure out one thing. You say that there are air bubbles because there is no place for the air to go. There should be no pressure in the sewer line. Can't the air just be pushed out in front of the outgoing water? Of course assuming there is no clogged pipe.
Thanks, Mark

that is why there are bubbles. the line should be filled only part way with fluid, then rest is air above it. when you send anythign down a drain, it has to compress the air which results in a minute backpressure. the air then bubbles up thru the traps.

Apropos of these vents, mine seem to tie from 2" or so into one 4" one which seems to be the norm here in British Columbia, and I notice my vent, and all the neighbours, all have no cage or cover over the vent. This area does get snow but very little precipitation overall, is the idea of no cover to allow rainwater to keep traps charged? Seems odd to me, and when I lived in Australia, all vents had cages or covers on them - admittedly being a warm climate, there is probably more potential for critter entry there.

Assuming I can even find a cover - just a slotted one to keep critters and leaves out - is it advisable or justified? I wouldn't cement it on, just leave it loose for future access.