How to Cement a Chimney Top
Being able to build your own chimney top is a great advantage when you consider the expense of hiring a mason. All it takes is a little common carpentry techniques and instructions, and you will be able to do a beautiful job for yourself.
Things You'll Need:
- 2 to 4 two-by-four and 2 to 4 one-by-six pieces of lumber, depending on the size of your chimney
- Portland cement
- #3 rebar
- Large clamps
- Membrane flashing
- Duplex nails
- Concrete waterproofing
- High temperature sealant
- Remove the existing mortar with a chisel and wire brush, then measure the dimensions of the outside of the chimney at the top bricks.
- Apply the membrane flashing so that it extends past the face of the brick and up the flue.
- Use an expansion joint made of newspaper, plastic bags or cardboard one-half inch thick to prevent cement from contacting with flue. Plastic will provide easier removal once the cement has set up. This padding should overlap and extend up about eight inches.
- Construct a frame from two-by-fours making a 15-degree angle cut along the length of one short side of the boards. This will help to direct rain from the bricks. Nail or screw the boards in place with the highest part of the angle to the inside. Then screw in place the one-by-six boards in an upright position to where they are flush with the outside bottom of the perimeter or the frame.
- Use the duplex nails and run a series of them through the one-by-six boards just past the highest point of the two-by-fours. These should be 18 inches apart and extend about an inch past the edge. The frame is then placed over the chimney with the screws as support along with the clamps. Cracks between the chimney and frame should be filled with scrap wood and duct tape. Coat with oil or drape this with plastic bags for easy removal of the frame.
- Cut the rebar approximately four inches shorter than the frame width.
- Place Portland cement into a bucket and prepare it for use.
- Use water to moisten the chimney base and begin pouring the cement into the frame, and force it into the angle you have prepared. Once the cement is an inch above the rim, place the rebar on the frame width, centered between the flues and the frame ends.
- Pour the remainder of the cement in with more at the flue, trowel it in with an angle that goes downward to the edge of the frame and allow it to dry.
- Remove the flue pad and frame, apply high temperature caulk to fill in the gaps between the flue and the cap and waterproof the cement and you are done.