As your chimney ages, the mortar in between the bricks can become brittle and develop cracks. It isn't safe to leave the chimney in this condition. It could literally come crashing down around you! Even if the worst case scenario doesn't happen, cracked mortar can allow moisture into your house, causing serious damage to the fireplace system and the surrounding walls.
But that doesn't mean you have to give the whole chimney up for lost. If you catch the cracks in time, they probably aren't that deep. If you want to put in the time and effort, you can remove the old mortar to the depth of the cracks and re-grout the chimney yourself.
There are four special tools you'll need in order to attempt this process. Those are a grout rake, a brick chisel, a masonry hammer, and a wire brush. You're probably familiar with the last three and they can be found in any home improvement store. The grout rake may be a little less familiar. This is a specialized tool that removes loose mortar. You hold it by a handle and roll it along the brick on its two small wheels. A hardened masonry nail digs into old mortar, enabling you to remove it. On the flip side from the nail, this tool can be used to spread the joints slightly.
Here are the basic steps to repairing you chimney. This is a messy, dusty process. Chips of mortar can fly. So remember to wear safety goggles:
1. Use the grout rake to start digging out old, loose, cracked mortar from your chimney. The masonry hammer and brick chisel can be used for stubborn spots, but be careful not to over do it.
2. Use the other side of the grout rake to enlarge the joints just a little.
3. Vacuum up the mess with a shop vac.
4. Use a wire brush to scrub the surface and remove even more dust. Repeat the vacuuming procedure with the shop vac.
5. Mix up some mortar and fortifier according to the instructions that came with it. The mix should be thick but still able to be worked into the grooves. Set the mix aside to give it time to soak up all the water.
6. While that's going on, moisten the joints with a water spray bottle. If the old mortar is damp, the new mortar will adhere to it more effectively.
7. Use a tuck-pointing tool or other small device to pack the mortar into the joints as tightly as you can. Any mortar that gets on the bricks will be difficult to chip off after it's dried.
8. After half an hour, use a trowel to scrape off the excess so the joints are flush with the brick.
9. Spray the mortar with water again. You don't want it to dry too quickly.
10. After a few days, brush the chimney with a stiff broom to get rid of any residual dust.