How to Construct a Chimney Crown
A chimney crown or mortar cap might seem to be a simple and sometimes insignificant part of a chimney. In many situations, it is not even recognized as a part of the chimney. But, in fact, this part of the chimney provides a vital function in keeping your house dry and your chimney working efficiently to remove smoke from your fireplace or stove. Without a crown that is constructed correctly, the chimney in time will deteriorate, allowing water to run down through your roof and into your ceiling. If you plan to install a chimney, you will need to know how to properly install its crown. The following information will help you with that installation
Step 1: Remove Old Mortar
If you plan to replace an older, existing crown, you will need to first remove the old mortar. Use a brick chisel and hammer and chip away all the existing mortar. When finished, use a wire brush to clean off the brick surfaces. If left, this mortar debris and dust will likely prevent the new cap from properly bonding to the older brick surface.
Step 2: Construct a Frame
Make a wood frame out of 2x4 boards. Rip the board with a 15-degree bevel. Nail this beveled board to a 1x6 board. The bottom of the 2x4 (the square edge) should be flush with the bottom of the frame. Cut this piece into 4 sections, allowing the 2"x4" to fit snugly around the top layer of chimney bricks and creating a cap that is 2-inch thick and has a 1 ½-inch overhang. Using your drill, drive wood screws into the wood pieces at each of the four corners
Step 3: Set the Frame on the Chimney Brick
When the form is made, set it on top of the chimney brick, and use a load binder or band clamp to reinforce it. Inspect the form for any spaces between it and the chimney top. If you find spaces, use duct tape to seal them before pouring mortar, forming an expansion joint. This will prevent the concrete from cracking as the flue heats up and expands.
Step 4: Create an Expansion Joint
Place corrugated cardboard around each flue and keep it in place with duct tape to create an expansion joint. This will keep the concrete from cracking when the flue heats and expands. To prevent the form from sticking when stripped, oil its inside surface. Avoid getting oil on the brick surface. Otherwise, the concrete will not bond to the brick surfaces.
Step 5: Add Concrete Mix
Mix the concrete you will pour into the form, using small aggregate. Dampen the chimney brick with water, and pour 1 inch of concrete into the form. Insert No. 3 rebar, leaving it 2 inches in from the edges of the form. Then fill the form with concrete. Smooth and taper the edges with a trowel.
Step 6: Remove Forms and Caulk
On the following day, remove the forms and seal flue joints with a high-temperature caulk, and seal the crown with waterproof coating.