A specialized caulking, called Mastic, is installed in the expansion joints between the coping and decking of the pool and in any gaps found around masonry. When Pools, Patios and Walkways are built they must have expansion joints. These gaps are necessary to allow for settling and insure your pool deck has room to move without causing damage to the pool coping. One of the most important expansion joints exists between the swimming pool coping and the decking around the pool.