Question: i WOULD LIKE TO PUT A SIX FOOT CHIMNEY EXTENSION ON MY EXISTING. MORTAR CHIMNEY. I AM A WELDER AND WOULD MAKE IT OUT OF STAINLESS STEEL, USING A DOUBLE WALL SYSTEM WITH AN APPROVED ROCK WOOL BLANKET IN BETWEEN. THE REASON I NEED THE EXTENSION IS THAT I'M GETTING SMOKE IN MY SECOND STORY WINDOWS (A NEW ADDITION) IT ALSO HAS AFFECTED MY DRAFT. Answer: There is no "approved" way of doing this, but the method you mention would probably be very effective. The main thing is to support the chimney extention as it extends upwards. Other than that, should work fine.