A chimney cap or crown acts as a roof for the chimney. The cap's slope allows water to flow away from the chimney. When water seeps into the chimney structure, it can cause the masonry to crack. Installing a chimney cap on your chimney will help to prevent damaged caused by water. These caps undergo a lot of stress because of the changing temperature of the chimney and flue. They should be inspected annually for cracks and damage.
Measure the top of the chimney and the length, width and height of the flue.
Build the bottom of the mould out of 1.8 cm (3/4 inch) plywood. Take the measurements of the top of the chimney and add 5 cm (2 inches) so the cap will overhang the chimney by 5 cm (2 inches) on all sides. Cut the plywood to that measurement by using a table saw or circular saw.
Create the cutout for the flue. In the centre of the plywood draw the outline of the cutout for the flue by using the flue's measurements. Add 1.8 cm (3/4 inch) to each side of the outline to make the gap between the flue and cap. Cut on this line to make the flue cutout.
Cut the pieces to form the flue cutout first. Cut four pieces of ½1.2 cm (1/2 inch) plywood to 11.8 cm (4 3/4 inches) tall by the width of the flue. Attach these pieces to the bottom of the mould to form the sides of the cutout. Stand each piece against the inside of the cutout so they stand 11.8 cm (4 3/4 inches) tall. Fasten the bottom of the sides to the bottom of the mould with screws. Fasten the pieces to the ends of the piece beside it to create a small bottomless and topless box in the centre of the mould.
Cut 1.2 cm (1/2 inch) plywood to make the sides of the mould. Cut four side pieces to 8.7 cm (3 1/2 inches) high by the width of the chimney. Attach each piece to the sides of the bottom of the mould. Stand the side pieces up so they stand 8.7 cm (3 1/2 inches) tall and screw the bottoms into the bottom of the mould. Screw the ends of the sides to the ends of the ones beside it. This will create a topless box with a smaller topless and bottomless box in the centre.
Create a drip edge around the cap by gluing 9 mm (3/8 inch) wooden dowels to the bottom of the box 2.5 cm (1 inch) from the sides of the mould.
Coat the interior of the mould with vegetable oil to prevent sticking. Mix Portland cement following the manufacturer's instructions. Pour the cement into the mould. Smooth the cement with a wooden float. Let the cement set up to the manufacturer's recommendations. Cure the cement by spiriting it with water daily for a week.
Unscrew the sides of the mould with a screwdriver to release the chimney cap.
You can substitute a stiff mortar mix for the Portland cement.
Cement can cause skin, throat and eye irritation. Wear gloves and protective goggles, and cover your mouth when working with it.