Start with the plumbing fixture that is highest up in the house. For example, begin with the second floor bathroom. Run the water and see if it clogs up. If it does not clog, move on to the next step. If it clogs immediately, it is probable that the clog is in that sink's drain or trap.
Move to the next lowest sink or fixture. Check toilets and shower drains as well. If several drains are clogged but others are not, then the clog is on the branch line that links these fixtures to the main line.
Check the lowest points of the system. If the lowest plumbing fixtures are clogged or all of the fixtures are backed up, it is likely the main stack or sewer line that is clogged.
- You can clear clogged drain traps by pouring ¼ cup baking soda, ½ cup white vinegar and then 1 qt. of hot boiling water down the drain.