Repointing a chimney involves raking out part of the old mortar from all the joints between the bricks and "repointing" or refilling the joints with new mortar. Ideally, the new mortar should be a 3-1-1 type — three parts building sand, one part sharp sand and one part cement. Repointing is necessary when gaps begin to appear in the mortar of the chimney.
Repointing should be done in sections 3 to 5 feet wide. If the mortar is removed in the entire structure before beginning to repoint, its integrity could be compromised, and the structure might collapse. To make sure the bricks are not damaged during the removal of the mortar, a handheld grout saw or cold chisel should be used. An angle grinder is also very effective at removing mortar, although care should be taken not to cut into the brick. For best results, the mortar should be removed about 1/2 inch deep. Repointing in sections also prevents the mortar mixture from drying too much before it is applied.
During the task of repointing a chimney, it is a good idea to renew the mortar at the haunch or top of the chimney. It is also a good opportunity to check the lead work between the chimney at the roof tiles and repair or replace it if it shows signs of wear, particularly if it is more than a few decades old.