You need an expandable pool liner when your swimming pool is dished out or if your pool has a deep end. Above ground pools can have deeper ends or centers, but it is important to buy the right pool liner as well as install it correctly. If you purchase an expandable swimming pool liner, make sure that it is good quality, 25 gauge, and if possible, try to order a full printed pattern. The full print tends to hide some small imperfections during the installation process. Moreover, make sure that you install expandable liners in 80 degree plus and sunny weather.