How to Build a Floating Chimney

The Form

• Measure the dimensions of the chimney and of the flue, then cut a ¾-inch plywood sheet that’s the same dimensions as the chimney. Cut out a square in the center of the sheet that measures the same dimensions as the flue. Grab an electric screwdriver and screw 3½-inch flat plywood panels to all four sides of the perimeter of the big square to create short walls. Do the same for the interior of the smaller flue square, this time using 4¾-inch-thick panels. You now have the base form: a large square with a smaller square in the center, each having walls that create a trench between them. The size differential will create a slope, once you add concrete, that aids in moving rainwater away from the chimney opening.

Fill It In

• Prepare a mix of “stiff” quick-drying cement by intentionally using less water than stated on the package’s directions – about half the suggested amount should do the trick – then fill in the space in between the walls of the squares with the stiff cement. Use a masonry float to plane the cement and smooth the contour between the walls of the squares.

Take It Off

• Wait a full 48 hours for the cement to cure completely, then disassemble the outer wall of the frame by simply unscrewing all the screws. It's likely you’ll need to chisel the plywood from the sides of the cement, as they can be a bit stubborn. When done, chisel the walls of the smaller interior square and you’re ready to set the float on the chimney.

Float Installation

• Hoist the cement chimney float onto the roof – chances are you’ll need assistance with this step – then place the float over the chimney, threading the flue through the center opening. There will likely be a small gap between the flue and cement; seal it with fire-rope (nylon cording that's heat resistant) and caulk the edging to finish the job.