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If old mortar has cracked or deteriorated, get advice from a structural engineer about whether it is a sign of serious structural problems or simply due to age. If age-related deterioration is the cause, mortar can be repointed or "stitched" (a technique involving special "stitching rods" and an epoxy resin). If the problem is more serious, follow your engineer's advice before attempting to tackle it.
Use a joint raker to remove any loose, crumbly bits of old mortar until you reach sound material (Image 1).
Use a two-pound hammer and chisel to "peck" out the more obstinate, solid chunks of mortar from between the bricks (Image 2).
Dust out the old mortar joints to remove all debris (Image 3).
Use a spray to wet the bricks and mortar joints. This stops bricks from soaking up the new mortar and ensures that it will adhere.
Excerpted from Do It Yourself Home Improvement
© Dorling Kindersley Limited 2009