If the cracked grout joints are in expansion joint locations - between tile and a tub, or other non-tile fixture or material - or where a field of tile changes its direction, such as the vertical corners of a tub surround or where baseboard tiles meet the floor tiles, all the old grout should be removed and the joint filled with a resilient material color-matched to the grout. Because these are areas where movement is concentrated, “repairing” them with grout will only cause more cracking.
A tile installation may not fail because of bad grout, but the grout may fail because of a bad tile installation. If the tiles are loose, grouting alone will not solve the problem. If the tiles are parting from the surface of the setting bed, there is a problem with the adhesive or with the structure. These problems must be fixed before permanent repairs can be made to the grout. Grout is not the glue that holds an installation together. Its main functions are to slow down the penetration of water between the tiles, and to protect the tile edges from chipping or flaking.
You need to examine all the grout joints, not just the ones that are visibly failing. Use a sharp utility knife and poke at the joints. If the grout yields easily, or if it is crumbly, flaky, or soft, the entire installation may require re-grouting. Two or three small areas in expected locations, such as near a threshold, a shower floor, or a well-used countertop, are usually not a cause for alarm and may be successfully spot-repaired. If 10% or more of the joints are deteriorating or damaged, though, complete replacement is recommended. Nothing good is accomplished when good grout is applied over bad.
To make a spot repair, use a utility knife or grout removal blade to scrape out all loose material. Remove all the grout bordering one whole tile; by extending the repair area from one joint intersection to another, differences in color are less noticeable. Keep scraping until the blade reaches solid material. When preparing joints in wet-area tile applications where soap film or residue may be coating empty grout joints, the film must be removed prior to the installation of new grout with a phosphoric acid or other suitable cleaner.
On full repairs, or where the grout joints are 1/8-inch or wider, power tools are more efficient at removing unwanted grout than hand tools. Reciprocating tools made specifically for removing grout are available, and are less prone to damage tile edges in the process. The fastest grout removal tool is a rotating dry-cutting diamond blade. Unfortunately, this generates a huge amount of dust and chips. It is also an extremely aggressive tool, making tile damage more likely.
In the process of removing the center section of the joint, much that remains on either side of the joint is also removed. What remains can be sliced away from the tile with a utility knife (illustration 3). With little pressure exerted sideways, edge damage on the tiles is minimized. Most renovation damage is caused when the cutting blade veers off-center. All the grout around the perimeter of any tiles being replaced needs to be removed before the tiles can be chipped out. Removing the grout prevents the shock of hammer-and-chisel demolition from jumping to, and chipping the edges of, the surrounding tiles.
Cutting deeper into the joint than the thickness of the tile may result in damage to an underlying membrane. A deep cut can also substantially weaken an installation by acting like a saw-cut control joint. Cutting away too little is also not recommended. Portland cement grouts get their strength from a thick cross-section. Cuts made as deep as the tile are best. Thin coverings are prone to flaking off. When only a thin layer of grout needs to be applied, 100%-solids epoxy grout is recommended. Very scrupulous preparations are required for thin-layer epoxy grout renovations.
Although epoxy grouts are somewhat self-finishing, Portland cement grouts require hand finishing with a barely-damp sponge and a minimum of wash water on the surface of the tiles. When the grout has set up, striking the surface with a tuck pointer or other striking tool is the last finishing touch (illustration 4).