Deck-O-Seal® or mastic is essentially the caulking between your pool decking and the coping (brick or stone) around the perimeter of your pool. The sealant acts as the expansion joint that expands and contracts during warm and cold weather.
Pool mastic should be replaced when you notice it peeling, cracking, and/or badly receding (pulling away from the coping or deck). Other things that indicate that you may need to replace your mastic would be pool tiles falling off, cracks in your deck, or heaving (rising) of the pool deck. Aquanomics Pools can replace this seal to maintain the integrity of your pool.
The life expectancy of mastic is very high. Once you apply mastic silicone, you can expect it to stay in place for up to five years before it requires replacing.