Tips and Tricks for Extending Your Schluter-KERDI-SHOWER Tray with Ease. Learn how to make the most of your Schluter-KERDI-SHOWER trays with our handy tips. These trays come with built-in waterproofing and in a variety of sizes and configurations. These trays can be easily cut to fit your shower space and easily extended with dry pack mortar. In this video, we’ll show you how to extend your KERDI-SHOWER tray using dry pack mortar. Follow along for simple and practical advice to improve your shower setup