There are several signs when your hot tub cover may need to be replaced. The most obvious one is weight. If you are finding your cover is difficult to lift, it has most-likely become water logged and it is time to replace it. Water-logging means that moisture has become trapped inside the cover. This can also cause an odour, which can be an indication of mold or bacteria growth inside the cover. If you notice any odour, the cover should be replaced right away.
Also watch for any type of material damage or tears. If the material is beginning to deteriorate or show signs of wear, this is how water will get inside the cover and it should be replaced.