This cap attaches to the mortar crown around the flue tile with mounting flanges and provides a very custom look. When sizing for this style cap, the base size is the inner opening; add 3" to each side to make sure you allow for the mounting flanges. For instance, if the OUTER DIMENSION of the flue tile measures 12" x 12", you would choose the SS13x13 cap and verify that the mortar crown can accommodate the mounting flanges which would measure 16" x 16 (base size + 3" in each direction for the mounting flanges.
