When regrouting a tile wall or floor in the bathroom, you may wonder if you can place new grout directly on top of the old grout. Removing the grout between floor tiles is an arduous task, and removing grout from wall tiles can cause the tiles to fall from the wall and complicate a bathroom remodel. If your grout is not in too bad of shape, you likely can put your new grout directly on top of your old grout and save yourself considerable hassle, at least temporarily. Realize, however, that this repair will likely not last as long as a complete regrouting.
Sealed Versus Unsealed
Sealed tile grout has a shine to it, almost like the tile itself. Unsealed tile grout appears rough, with a cement-like texture. You can safely put new tile grout on top of old unsealed tile grout, but attempting to do the same with sealed grout will not work. Sealed grout is not sufficiently porous to allow the new grout to bond with it, and you will wind up with tile grout falling out from between tiles within a matter of months. Remove the sealed grout, and replace with your new grout.
Depending on the condition of the grout, apart from cosmetic appearance you may or may not want to even attempt to grout on top of the old grout. When you put new grout on the old stuff, you are relying on the grip the old grout has on the wall to keep the whole thing together. If your old grout is already crumbling and falling away, putting new stuff on top of it will not solve the problem. If the old grout is still secured to the wall but is cosmetically an issue, give the old grout a once-over with a grout saw to remove any mold or mildew, and apply the new stuff directly on top of the old.
Depending on the depth of the grooves between the tiles, you may or may not want to attempt to put new grout on top of old. You do not want what should look like grout-valleys between your tiles to begin to look like grout-mountains as this will give water a place to get trapped and give mold and mildew a home. If the old grout between your tiles is filling in the grooves between your tiles, use a grout saw to remove the old stuff before adding any more.
Many tiled walls and floors have been caulked over due to leaks or as a precautionary measure. Ensure that you are not looking at a caulked seam before attempting to apply new grout on top of the old grout. If there is any caulk present whatsoever, the new grout will not get a firm grip on the old stuff, and the whole thing will give way and allow water to become trapped behind it. Place a few drops of water on the old grout. If it absorbs the water, it is unsealed grout; but if the water beads up and rolls off, it is sealed grout or grout with caulk. Remove and replace the grout.