The Dometic SlideTopper is designed to protect the top of an RV's Slideout room from weather, debris and mildew. It is not intended to be 100% waterproof, due to drips and condensation. To determine which size of SlideTopper you require, click on the Product Overview button and follow the instructions. The SlideTopper will not work on rooms with a full extension greater than 44" Note: Need to order hardware separately.