Adding a Metal Smoke Pipe to a Leaky Masonry Chimney
You can also stop smoke leaks from the masonry joints of an unlined chimney by adding a stainless-steel liner. The flue must be straight from the smoke chamber to the chimney tip, and large enough for a liner matched to the fireplace opening.
To begin the job, take preliminary measurements on your fireplace. Lower a steel-tape ruled down through the flue until the end of the tape is level with the juncture of smoke chamber and flue – have a helper peer up through the damper opening with a flashlight to monitor this action.
To this measurement add 8 inches for the liner extension above the chimney, plus at least 3 additional inches for every 3 feet of length to allow for liner joints. The final figure is the total length of steel pipe you will need.
Next, measure the size of the chimney opening. If the opening is a rectangle, the diameter of the flue pipe should be 1 inch less than the shorter dimension – unless a smaller flue would increase the efficiency of the fireplace.
From a sheet-metal fabricator, order 24-gauge stainless-steel pipe in 3-foot lengths, crimped for 3 inches at one end of each piece so that the parts can be easily joined. Buy enough self-tapping screws to put four at each planned joint. And get a sheet of 24-gauge stainless steel at least 4 inches wider and longer than the flue opening – it will be used to make the fireplaces’ collar on which the bottom of the finished liner will rest.
Once the metal liner is installed, you must fill the space between the pipe and the chimney walls with a special insulating mixture of portland cement, sand and perlite treated with silicone; all three are available at masonry dealers. To 2 cubic feet of portland cement – enough for a 12-by-12-inch flue on a two-story house – add 6 cubic feet of sand; then to one part of this mixture, add five parts of silicone-treated perlite. Place this mixture in a mortar-mixing box and use a hoe to combine the ingredients, then mixture throughout without making it runny. Use a bucket and a sturdy rope to haul the insulating mixture up to the roof.