A chimney cap can refer to a decorative copper or a metal cage-like device placed on top of a chimney to prevent rain and small animals like birds and squirrels from entering the chimney and to reduce the risk of sparks exiting the chimney. The term chimney cap may also refer to the cement topper covering a brick or stone chimney, exclusive of the flue. A cement chimney cap is also known as a chimney crown, wash or splay.
Chisel off any existing mortar, and then use a wire brush on the chimney surface to ensure all remnants of mortar are gone. Measure the outside dimensions of the chimney at the level of the top row of bricks.
Add membrane flashing that extends off the brick face and goes up the sides of the flue liner.
Make an expansion joint to prevent the cement from touching the flue. This can be done by wrapping a disposal "pad" around the flue. A pad can be made from at least a 1.2 cm (1/2 inch) thickness of newspaper or corrugated cardboard (like from a packing box) inside bin bags or plastic shopping bags. The plastic will make it easier to remove after the cement dries. When wrapping the expansion pad around the flue, overlap the ends rather than butting them together. The pad should touch the chimney bed and extend up at least 20 cm (8 inches). Secure the pad in place with duct tape. The newspaper or cardboard will compress some with the weight of the cement pressing against them.
Build a frame from 5 x 10 cm (2 bx 4 inch) boards with one short side of each board cut along its length at a 15-degree angle. The angle will help drain rain away from the bricks of the chimney. With the angled edges upward and the high part of the angles inward, nail or screw the four boards together according to the dimensions measured in Step 1, to create the base of the frame. Screw the 2.5 x 15 cm (1 x 6 inch) boards upright and flush to the bottom of the outside perimeter of the 5 x 10 cm (2 x 4 inch) board frame to form the walls of the frame.
Run a series of duplex nails through the 2.5 x 15 cm (1 x 6 inch) boards and just over the highest part of the 5 x 10 cm (2 bx 4 inch) -- just above the point of the bevelled edge. This is where the cement cap will be slightly angled down from the brick on its bottom outer edge) The screws should extend out at least 2.5 cm (1 inch) and be placed about 45 cm (18 inches) apart. Place the frame on the chimney using the screws to support it on the top row of bricks. Add clamps across the width of the frame for support. Fill cracks between the frame and the chimney with small pieces of scrap wood or even duct tape. Draping polyurethane or cut-up bin bags, or carefully brushed-on cooking oil, on just the wooden frame can make it easier to remove the frame after the cement dries.
Cut rebar for installation later. The #3 rebar should be about 10 cm (4 inches) shorter than the width of the frame. For a single flue stack, two rebars are used on either side of the flue. Increase the number of rebars for multiple flues so each has a rebar midway between it and the edge of the frame or the next flue.
Prepare the Portland cement mix, which will need to be pulled up to the roof in a bucket.
Moisten the base of the chimney pad and pour in the cement, forcing it into the angle cut. When the cement is about one inch over the brick rim, lay the rebar the width of the frame, centring each bar between flues and the ends of the frame.
Pour in the last of the cement, adding more near the flue(s). Trowel the cement from the flue on a slight downward angle to the edges of the frame to create a draining surface for rain. Allow the cement to dry and partially cure overnight.
Remove the frame and flue pad. Apply high temperature caulk around the flue to fill the gap and apply waterproofing to the cement.
If it's windy or very hot, cover the newly cemented crown with polyurethane to avoid rapid drying or evaporation.