When the water draining from one appliance wells up through the waste of another, the trouble probably lies in the main drain or its branches, which channel waste into the sewer. Since waste flows downwards, any blockage in the main drain or soil stack will stop up all the appliances above that point. In addition, anything that blocks the vent, which keeps air flowing through the system, will cause waste to drain away sluggishly, and there may be an unpleasant odour in the house. Before starting work, plot the course of the pipes in your waste system to help you pinpoint the blockage. You may find it helpful to sketch the layout on paper. Once you have decided where the obstruction might be, try a rubber plunger of the appliance closest to the trouble spot this can be effective even if the blockage is in the main soil and waste stack. If that fails, try cleaning out the stack with a cleaning rod or drain auger. Work from the appliance closest to the blockage.
To make the job easier, hire an electric auger up to 30 metres long from a tool-hire firm. Even an electric auger, however, will not work effectively if it has to go round too many bends. Always take the straightest possible route to the problem area. In the main stack that usually means working through the nearest inspection chamber or, more commonly, through a cleaning eye. A blockage below ground level, in the drain to the sewer, will have to be reached through a gully or inspection chamber.
In most plumbing repairs speed is essential, but unclogging the main drains calls instead for patience. Avoid precipitate action. The column of water trapped by the blockage may extend above the point where your cleaning eye is located, and it will gush out as soon as you open the eye. Wait for at least two or three hours after you have spotted the trouble before starting work, to allow as much as possible of the waste to seep past the blockage. Even then, you will need mops, buckets, rags and old newspapers to soak up any overflow. When you have finished clearing up, dispose of the rags and newspapers in sealed plastic bags and disinfect the site.
Locating the blockage
The first step in finding blockages is to check which fittings are draining normally; clogs in a soil and waste stack will always be below the level of the lowest blocked fitting and above the highest working fitting. In the plumbing system above, a blockage near the cleaning eye would block both the basin and the W.C., but not the bath. Should all the first-floor fittings be blocked, the problem must lie below the point where the bath waste feeds into the stack. In either case, the blockage should be cleared through the cleaning eye. If the sink on the ground floor is blocked, the fault must lie in the gully or the pipes leading to or from it. Try pouring water through the gully. If it passes freely through, the blockage is in the sink waste, which should be cleared through the sink trap; if not, clear the gully with a hose or auger. When all the appliances are blocked, the blockage must be in the drain to the public sewer, and should be tackled through the inspection chamber. Often the first sign of such an obstruction will be flooding from the gully, the lowest drain outlet in the system.