Keeping tiled floors clean can be a tough task, and grout can be particularly stubborn. The most common cause of dirty grout is using too much water and detergent while you’re washing the floor. Here’s why. Grout is highly porous, which is what makes it effective at protecting your tiles. Unfortunately, every floor clean washes dirty water onto the tiles and into your grout. This creates a black film that can be difficult to remove and in humid areas, be a breeding ground for bacteria and mould.