How to Make Concrete Chimney Crowns

Measure across the top of the chimney to determine the width of the stack, then measure the width of the opening of the flue (the smaller ceramic stack jutting up from the chimney’s interior). Cut the ¾-inch plywood square to the measurements from the chimney, but add 2 inches in width to create a small lip that will make holding and installing the cement cap easier in a later step.

Draw a square in the center of the larger square according to the measurement of the flue. Cut out the square to accommodate room for the flue. Cut each 3 ½-inch thick plywood panel to the same measurement of the chimney square, then cut each 4 ¾-inch plywood panel to match the measurements of the smaller center flue square.

Screw each 3 ½-inch thick panel to each side of the perimeter of the larger square. Do this by laying each panel parallel to the lip of the board and screwing completely through, going from the outside of the panel all the way through to the plywood square. Once done, this creates four “walls” around the square’s perimeter. Repeat for the center square, this time using the 4 ¾-inch thick panels to make walls. This creates a trench that runs through the interior of the square that you’ll need to fill with cement.

Mix the quick drying cement as directed by the manufacturer but use roughly half the advised amount of water. This makes the cement slightly stiffer, a texture you’ll need when forming the slope on the crown. Fill the trench space between the walls of the square with the stiff concrete and smooth the contoured slope between the walls using a float. This gradual incline is needed to allow rain to run off the peak of the crown and not into the flue.

Wait a full 48 hours to allow the cement to dry completely. Unscrew all the wood panels; chances are some will stick and will need to be chiseled away. Hold the lip on the crown, carry it up the ladder to the roof and place the cap over the chimney, threading the flue through the smaller square in the center of the crown. Seal the small gap between the flue and the crown with heat-resistant fire-rope and caulk.