Most truck bed toppers are water resistant but none are 100 percent waterproof. Additional features and seals can be purchased to reduce the chance of water leakage.
Rain may get into the truck bed through unsealed gaps, such as around the sides and bottom of the tailgate and around the front bulkhead. If the truck has a plastic bed liner, often the outside edge of the front bed rail is left uncovered, allowing water to seep in. If a truck owner takes steps to seal all of these areas, the truck bed is better protected from the rain and other elements.