How to Pour a Chimney Crown

Build a Frame

• 1

  Remove any loose mortar from the top of the chimney with a cold chisel and hammer. Scrub the top of the bricks with a wire brush and sweep the top clean. There should be several inches of the flue protruding past the block or masonry chimney.

• 2

  Measure each side of the chimney. Add 6 inches to each measurement. Cut four 2-by-4-inch boards and four 1-by-6-inch boards to the measurement. You should have one board of each size for each side. Rip the length of one side of the 2-by-4-inch boards with a 15-degree bevel, using a circular saw.

• 3

  Place a 2-by-4-inch board against a 1-by-6-inch board with one of the flat sides flush with the other. The bevel should be pointing against the 1-by-6-inch board. Nail the boards together with finishing nails. Cut the ends of the boards at a 45-degree angle to form a miter corner, ensuring you cut the ends to fit each side of the chimney

• 4

  Join the boards to form a box frame and drive 2-inch lag screws through the corners to secure the frame together with a drill and socket adapter. Drive 16d nails through the 1-by-6-inch board, with the shank of the nail extending over the 2-by-4-inch board. The nail shanks act as hangers.

• 5

  Position the box frame over the top of the chimney. The flat sides of the boards should face down, and the frame will hang from the nail shanks on the chimney. Tie a rope around the chimney under the frame.

• 6

  Wrap the outside of the flue with cardboard, using duct tape to secure the cardboard tightly. This provides an expansion joint for the concrete around the flue. Rub oil on the inside of the frame and the outside of the cardboard with a rag. This helps when it is time to remove the frame and cardboard.

• 7

  Cut pieces of rebar 3 inches shorter than each side with a grinder or chop saw. You should have four pieces of rebar.

Pour the Chimney Crown

• 8

  Mix a concrete mix of small aggregate according to the directions on the bag. This is best done from the ground. Ask a helper to fill a 5-gallon bucket with the concrete mix. Hoist the bucket up to the roof with a rope and pour it over the chimney top. Continue adding concrete until it is at the top of the form.

• 9

  Insert the rebar into the concrete 2 inches from the flue. Push the rebar halfway into the concrete with a trowel. Smooth the concrete with a trowel and cover it with plastic. Allow the concrete to set as recommended on the manufacturer's instructions -- usually 24 to 48 hours.

• 10

  Remove the lag screws from the corners of the frame with the drill and socket adapter. Tap the inside of each frame side with a hammer to separate it from the concrete. Pull the cardboard away from the flue and concrete.

• 11

  Seal the joint between the concrete and flue, using high-temperature caulk and a caulk gun. Apply a concrete sealer to the top of the crown according to the directions on the can.