Blocked shower drains are quite common. The usual culprit is a build-up of hair. When mixed with soap scum, it gradually begins to congeal and stick in the pipes, causing a blockage. The first indication is when water drains away slower, until finally it stops draining all together and instead sits on the floor.
These blockages can be plunged away, and you can always try clearing the blockage yourself. Items such as ‘Draino’ can be bought to get rid of the problem. But sometimes blocked shower drains take a bit more effort and professional help to clear (especially if the grate doesn’t come away easily from the floor). In some instances a plumber may need to use acid to help clear the problem. After plunging, the plumber will pour drain acid down the drain and let it sit for a while. The acid eats away at the remaining hair and soap scum caught in the pipes, clearing away the last of the blockage. Once it’s been left to sit for a while, the acid is washed away, to clear the last of the mess for you.
Remember to always advise the plumber if you have applied drain cleaning products for his personal safety.