Perhaps because it sits in an exposed, hard-to-access spot on the roof, the sight of a crumbling brick chimney is not uncommon, even when the rest of the walls of a brick home are in solid condition. Like any brick structure, a chimney could be falling apart because the mortar has degraded, because the bricks are cracked, or both. To fix a crumbling chimney, a homeowner must be prepared to both repoint the mortar and to replace damaged bricks.
Measure the length of any damaged, irregular bricks in the chimney. Shortened, custom-cut bricks are often seen in chimneys. Replacing these shorter bricks requires cutting your normal replacement bricks to fit the existing space.
Mark a replacement brick with a paint pen or another heavy marker, drawing a straight line all around the brick at a point determined by your measurements.
Cut a groove along this line by chipping away with a hammer and a cold chisel. Set the chisel on the line at a 60-degree angle and tap it with the hammer, shifting from left to right as you progress to cut a more even groove. Put a brick set into the groove and strike it hard with the hammer, splitting the brick. Do this for every shortened replacement brick required.
Chip out the mortar around a damaged brick with the hammer and chisel. When the brick is loose enough, pull it out of the wall. Brush out any remaining bits of mortar from the now mostly empty space with a wire brush. Repeat this process for a few more damaged bricks, but do not remove too many bricks from the chimney at once.
Make enough mortar for roughly 15 to 20 minutes worth of work, adding water as directed by the manufacturer.
Lay mortar into the bottom of the empty space with the trowel, matching the thickness of the mortar in the adjacent joints. Butter the replacement brick by spreading a layer of mortar onto the ends and the top. Put the brick into the space.
Scrape off excess mortar with the trowel, then use the trowel to shape the wet mortar to match the appearance of the appearance of the old mortar. Move on to replacing the next brick.
Remove all cracked or crumbling mortar from the chimney's joints with the hammer and cold chisel. Some mortar may be soft enough to remove with your fingers or with a wire brush.
Make enough mortar for roughly 30 minutes worth of work, following the manufacturer's instructions. Pour the mortar into a mortar bag.
Place the nozzle of the mortar bag into the hollow joints. Squeeze the bag to push mortar through the nozzle and into the joint until the joint is full.
Scrape off excess mortar and shape the wet mortar to match the adjacent joints.
Brick masonry dating to before the mid-20th century used lime and sand mortar, which is softer than modern mortar. The bricks were softer as well, so combining modern, cement-hard mortar with old bricks might crack the brick. Examine the mortar of an old chimney carefully to determine what sort of mortar it uses, and therefore what sort of mortar should be used to fix it.
Take care not to remove too many bricks from a chimney at once. Chimneys are usually small, free standing structures, so removing too many bricks at once can easily cause the chimney to collapse. Plan your brick replacement work so that it proceeds in stages, with only a handful of bricks removed at a given time.