If you have an old brick wall in need of restoration, you undoubtedly have old mortar joints in need of repair. Even modern masonry walls are not completely waterproof, and the joint between bricks is the most vulnerable point. Brick manufactured before the 20th century is softer and more porous than modern brick. Restoration will require the use of methods and materials similar to those in use at that time.
Check for differential settling and repair foundations before working on the brick wall itself. Be cautious about causing additional movement -- levelling or straightening walls may cause more damage. Determine whether the wall can be stabilised or if it is leaning so far that it could collapse.
Repair detrimental conditions such as standing water, clogged gutters and improper foundation drainage. The National Park Service guidelines for historic restoration recommend inspecting the masonry structure to determine the causes of deterioration and to document details such as the bond pattern of the brick and the profile of the mortar joints.
Clean the brick using the most gentle method that works. Begin with soft bristle brushes and a garden hose. Never use high-pressure spray, and avoid chemicals if possible. Use mild detergents and rinse well. Do not introduce water into masonry void spaces if there is a chance of freezing weather.
Use small chisels to rake the mortar out by hand. Power tools likely will damage the surrounding brick and make repointing more difficult, with less-than-satisfactory results.
Mix small quantities of high-lime mortar according to package directions. Modern mortar is harder than old brick and much harder than older lime mortar. Mortar that is too hard can destroy the brick. As the building expands and contracts with temperature changes, the mortar will chew up the brick, causing cracking and spalling. According to the Durable Restoration Company, old-style mortars used burnt lime, which makes the mortar softer and highly flexible, able to absorb building movement. You may have to go to a restoration speciality contractor for materials, but using the correct mortar mix is critical to restoring old brick walls.
Replace missing bricks with old salvaged brick to match the appearance, strength and density of the others. Inserting a hard modern brick can cause the same sort of problems as too-hard mortar.
Using a canvas mortar bag, pipe a line of mortar into the cleaned joints. Compress the mortar using a striking tool to match the profile of the existing joints. Brush excess mortar off the brick as you go.
If you want to retain the historic character of your home, the National Park Service guidelines for historic restoration recommend making repairs that look good remain identifiable for later research. For example, do not remove sound mortar so you can repoint an entire wall to avoid an identifiable patch. For major repair work, it is appropriate to include a date in an inconspicuous location.
Carefully evaluate the moisture conditions before applying a clear or penetrating sealer to old brick. The sealer can trap moisture inside the brick and cause spalling. The National Park Service does not recommend the use of sealers or flexible caulking in place of a proper repointing job.