Regrouting tiles can be risky as it can cause damage, therefore, you should only regrout tiles if absolutely necessary. Regrouting should only be required if the grout begins to crumble and the tiles become loose. The old grout should be removed from the tile joint with either a grout rake or multi tool to a depth of 2-3mm and then cleaned to remove contaminants and dust. This will leave space for the new flexible grout to be successfully installed into the joint.