A GOOD tile surface should last for many years without any problems. Eventually, however, the grout in the joints between the tiles may deteriorate or crack. While it is possible that crumbling grout joints can signal a possible problem with the underlying surface, usually a few cracks here and there are indicative of grout that was improperly mixed and applied.
It is not a good idea to apply new grout over the old because grout is an inflexible material and is not strong or durable in thin layers. With the years the added grout will flake off and expose the deteriorated material below. To correct the problem, you must first remove the old grout.
You can use a utility knife or a grout saw to scrape away all traces of the crumbling grout. Clean the gaps down to the substrate, but be careful not to damage the substrate surface. Use a vacuum cleaner to suck out all the dust and debris from the gaps; then clean the surface of the tiles.
Many tile stores sell products made specifically to clean grout residue (often called 'grout haze'). Or you can make a good substitute by mixing a cup of vinegar in a gallon of clean water. Allow the area to dry completely before applying the grout.
Applying the grout properly involves more than simply pushing it into the gaps between the tiles. A good grouting job can often mask imperfections in a tile installation, while a poor job can mar an otherwise outstanding tile installation. To look good, the grout must be smoothed so it forms an even border around each tile.
The job can be divided into four stages: mixing the grout, applying it, removing excess, then cleaning the joints. There are pre-mixed grouts on the market; these are useful for filling in small gaps. It's better however, to buy the powdered grout and mix it with liquid just before installation. This grout will be stronger and more durable than the pre-mixed.
Depending upon the product, the grout can be mixed with either water (it should be clean and free of impurities) or a special liquid additive. Carefully read the instructions on the package to find out the exact amount of liquid to mix with the powder. It's best, however, to start with only two-thirds of that amount.
Pour the liquid into a clean, dry bucket. Slowly add the powder until the mixture becomes plastic and spreadable. If it becomes too thick, add more liquid. You can use a hand trowel for mixing the grout or a power mixing paddle chucked in an electric drill. If you choose the electric drill method, keep the motor speed below 300 r.p.m. Otherwise, the paddle will force air bubbles into the mix.
It is important to get the right mix. Too much liquid will weaken the grout, while to little will make it stiff and hard to work. Properly mixed grout should be firm enough to hold peaks, yet plastic enough to be spreadable without running. After the grout is mixed, allow it stand for about 10 minutes. This waiting period is called ''slaking.'' After the slaking, the mix may be slightly harder. If so, add a little more liquid, then wait another 10 minutes.
When the grout is properly mixed, it can be applied to the joints. Ideally, it should be applied when the ambient temperature is between 65 and 75 degrees Fahrenheit. Use a grout trowel to spread the grout into the joints. (It has a rubber face so it cannot scratch the tiles; different versions are available for wall or floors at tile stores and home centers.) Hold the trowel at a 30-degree angle, and push the grout into the gaps.
It is important to pack the joints to eliminate all voids. Next, hold the trowel almost perpendicular to the tile surface, and work across the tiles to scrape away the excess grout but watch that the corners of the tool do not gouge any grout from the joints. When most of the surface grout has been scraped off, remove the remaining material with a wet wipe.
Use a grout sponge (a thick sponge with round corners) to wipe the residue from the face of the tiles. The sponge should be damp but not wet, so it will not add moisture to the grout in the joints. It's best to go over the entire surface, three times lightly, to remove the residue without disturbing the joints. Use the round corners of the sponge to smooth the joints and remove any lumps in the surface. Allow time for the grout to dry; then wipe the surface with clean, dry cheesecloth.
As it may sound it is not difficult to try this yourself. Remember it is your investment and you need to take care of it for years to come.