To repair a chimney cap, remove the cap from the chimney, clean the cap, rejoin any dislocated joints, clean the chimney's crown and repair any cracks on it, and then remove any loose mortar from the chimney and replace it with powdered mortar. Screwing the cap back into position completes this process.
Start by wearing loose-fitting clothing and rubber-soled shoes. Using a ladder, climb up the roof to access the chimney cap. To remove the cap from the chimney, use a screwdriver to loosen and remove any screws that secure the cap in place, and then remove the cap. Climb down from the roof with the cap.
To clean the cap, use a chisel and a hammer to clear the cap of creosote, taking care to protect your hand with gloves during this step. Once the cap is free of creosote, examine the joints for dislocation, and rejoin them with self-tapping screws. Climb up the roof again with the repaired cap, and place it in a safe place. Clear the crown of algae by scrubbing it with a brush, and rinse it with water. Examine the crown for cracks, and fill them with a crown sealant as needed.
Next, chisel away any loose mortar from the chimney, and use a trowel to replace the mortar with a mixture of powdered mortar and water. Allow the mortar to dry. Finally, replace the cap, and secure it in place with screws.