The DuraFlex Top Plate is used to seal the top of masonry chimney and support the flex pipe. Pull the flex length through the Top Plate and attach DuraFlex SS Cap with fast connect band for a secure leak-proof fit. The built-in extension eliminates the use of a storm collar. DuraFlex SS flexible relining is designed for masonry chimneys as well as with gas, wood pellet, oil, or wood appliances. The inner diameter is 3" and the plate measures 16" x 16".