There is no one cause as to why water flows slower than normal out of a drain. The culprit might be directly in front of you, right in the drain opening, or the cause might be a problem farther down the drain pipe or in a different pipe. The farther the cause is away from the drain opening, the more difficult it becomes for do-it-yourselfers to fix.
Dirty Pop-Up Stopper
On bathroom sinks, showers and tub drains, hair and soap scum that do not go down the drainpipe will collect around the drain opening. With drains that have a pop-up stopper, a buildup of tangled strands of hair and caked-up soap scum, along with other foreign matter, can impede the flow of water down the drain. Disengage and remove the stopper from the drain and clean it.
The trap is the curved section of pipe located underneath the drain on fixtures such as sinks, tubs and showers. It's easy for objects that fall down a drain -- such as rings -- to get caught inside the trap. However, other items, such as grease and debris also build up inside the trap over time to create either a partial or full clog. If you have a slow sink drain and want to eliminate a clogged trap as the cause, place a bucket underneath the trap and unscrew the slip nuts on each end of the trap to remove it. Be careful since water will run out when you disconnect the trap. Clean out the trap and reinstall it.
Drain Pipe Clogs
A partial clog can be located farther down a drain pipe than the trap. To reach these deeper clogs, you generally need to use an auger, or what is commonly called a snake. To reach whatever is causing the blockage in a sink's drain pipe, remove the trap underneath it, and insert the auger's flexible cable directly into the horizontal or vertical pipe that leads through the wall or floor. For a bathtub, remove the overflow plate that is located on the far end, inside the tub, and insert the cable down through the open overflow tube. Insert the cable down through the drain inside a toilet bowl to snake a toilet drain clog.
Problems with Multiple Drains
If you have eliminated the possibility of a clogged trap, and can't find any clogs inside a fixture's drain pipe, the cause is likely within the house drain or the soil pipe. This is definitely true if more than one fixture has a slow drain, or backs up when other fixtures are used. The house drain is accessed through clean-outs, which are short sections of pipe that have a removable plug, allowing you access to clean out the line. The soil pipe is accessed through the vent stack, which protrudes from the roof. Use a long auger on either one of these areas to clean out the pipes.