Basin and bath wastes are less likely to be totally blocked than sink wastes but, when blockages do occur, they can usually be cleared in the same way. They are, however, very subject to partial blockage. The waste water is often found to run from the bath or basin ever more slowly.
This may be due to a build-up of scum, scale and hair on the inside of the waste pipe, and the use of a proprietary drain-clearing chemical will usually clear it. These frequently have a caustic soda base, so they should be kept away from children and handled with care, strictly in accordance with the manufacturer’s instructions. Before spooning them into the bath or basin waste outlet it is wise to smear petroleum jelly over the rim of the outlet to protect the chromium finish, especially with plastic baths or fittings.
Partial blockage of a washbasin waste may often be caused by hair suspended from the grid of the outlet. This may be all but invisible from above, but probing with a piece of wire (the old standby of a straightened-out wire coathanger is useful) can often produce festoons.
If you can’t clear the hair by this means, unscrew the nut that connects the threaded waste outlet to the trap and pull the trap to one side. Now use a pair of pliers to pull the hair away from beneath the grid.