Repairing Stone Chimney Cracks
If caught soon, many cracks are easy enough to repair at home. To do this, clean the area and then simply fill in the crack with caulk or some sort of concrete bonding material. Follow the finishing steps outlined by whichever material you choose, but it's usually a standard procedure of smoothing the bonding materials with a flat edged tool, like a putty knife. If the area is uneven after drying, you can sand it to your liking [source: Repair-home].
If the mortar around the stone has begun to crumble, you'll need to remove it with a small hammer and chisel. Take out portions in a square shape. Try to go at least a half-inch (1.25 cm) deep. Clean and dry the area, so that it is moist, but not wet [source: Repair-home, Warde]. At this point, it's ready for mortar
When you mix the mortar (which needs to be weather-resistant) aim for the consistency of peanut butter. You can use a mortar hawk or a pointing trowel to spread mortar into your damp excavated areas [source: Repair-home, Warde]. The mortar should dry to a stable consistency, but you should still be able to leave a mark with a pressed thumb. Many forms of mortar mix simply need water, so follow the directions and your consistency should be OK. You should start with vertical joints and then move on to horizontal joints, making sure to clear away excess mortar along the way. Repeat this process anywhere you find damage.
To make the most of your repairs, help the mortar set properly by keeping it moist for the first half week it's setting [source: Repair-home].
If your chimney is made of brick rather than of stone, keep reading for more tips.