The walls and floors of a home continually shift as the house settles, especially in newer homes. This may cause hairline cracks in the grout between your tiles. Repairing hairline cracks is a simple process. You can use the same grout you used originally, but instead of mixing it with water you will use a latex grout additive that results in more flexible grout that can shift with the house.
Pour the unmixed grout onto a large paper plate. The amount of grout you use will depend on the number of cracks you have to fill. Follow the directions on the package to determine the amount appropriate for your job.
Pour latex grout additive onto the paper plate and mix the two together to form a paste. Follow the package instructions for how much additive to use. Use a paint stirrer to mix the two together.
Scoop up the grout paste with your finger and work it into the hairline crack in the grout. Push it down as far as possible. Fill in the rest of the grout line with the grout so the level is even.
Fill a bucket with water and dip a sponge. Wipe the grout line with the sponge to even out the grout line. Dip the sponge in the water again to rinse it, and wipe the grout line once more. Let the grout dry overnight.
Cracks in grout larger than a hairline may require you to dig out the grout and start over. Consult a contractor if you are unsure if your crack requires more repair.