How to Construct a Chimney Crown

A chimney crown (mortar cap) often seems 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

Things you will need:

  • 2x4-inch board
  • 1x6-inch board
  • Nails
  • Hammer
  • Miter box
  • Protective goggles
  • Gloves
  • Brick chisel
  • Hammer
  • Wire brush
  • Saw
  • Wood Screws
  • Drill
  • Screwdriver drill blade
  • Loadbinder or bandclamp
  • Bench saw

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 loadbinder 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.