Leaky shower doors are typically caused by a bad door seal. Normal aging and wear causes the strip at the bottom of the door to break, crack or just seal poorly. Replacement seals provide an inexpensive solution to the problem. Although these seals look like plastic, they are more typically made of rubber or vinyl because these materials have a longer life. The type of strip necessary depends on the design of your shower door.
Inspect the bottom of the shower door to determine the plastic seal attachment method. Seals typically slide into a channel at the bottom of the door or they are placed between the base of the door and the metal drip rail.
Remove the adhesive backing from the new seal, if applicable. Not all seals have adhesive.