I’ve got a partially sunken tub and shower that’s got a mixture of different tiles. The tub is cast-in-place concrete and was hot-mopped with asphalt to prevent water leaks. The grout between the mosaic tiles is eroding slowly, but the wall tile grout is in very good condition. Is mold causing the tile grout to erode? Will the eroding grout cause leaks? Should I just demolish the tub and shower and start over, or is it possible to repair the grout? How do I stop the mold from growing in this gorgeous tub and shower area? — Marie J., Santa Barbara, Calif.

Oh, my goodness! Thanks for the photos you sent in. Your tub and shower is a work of art! By all means do everything possible to restore it. I cringe at the thought of someone hitting that masterpiece with a sledgehammer to take it out.

Sunken tubs like yours are a dying craft. There are still master tile setters who can form one and then cover it with expert tile work, but each year there are fewer of these talented individuals. There’s a very good chance that your tub and shower could be a major reason someone buys your home in the future.

Here’s a general note: Before you consider removing a specialty fixture from your home, always check with a knowledgeable real estate agent. In certain areas of the nation, there are communities with period homes that are highly sought after. A top-performing, experienced agent can tell you what buyers are looking for.

Now, regarding your sunken bath, if you’re not experiencing any leaks at this time, then the job is just going to be tedious. The good news is you can re-grout the mosaic tile and make it look brand new. Fortunately, there are modern tools you can use that will allow you to get the job done much faster than what it took in the past.

Let’s talk about the grout deterioration first. Mold and mildew are not the cause of the erosion of the grout. The black organic growth is more a nuisance and a cosmetic blemish than anything else. It’s hard to say what’s causing the loss of grout, as I don’t know when it started and how fast the deterioration progressed.

The sanded grout that’s between your mosaic tub tiles is just a mixture of extremely hard, fine silica sand and Portland cement. When mixed and finished properly, this grout can last for many decades with no deterioration. It’s possible the grout was defective from day one because a little too much water was used to mix it or the sponge used to smooth the grout joints was too wet.

The grout may have held up for many years but was ever so slowly eroding. When far too much water is used to mix the grout and finish it, the deterioration can happen in months, not years.

Your job is to install new grout in the areas where you see moderate to severe deterioration. The first step in the process is to deep clean the tile grout so you get as close a color match to the grout as possible. I say this assuming you’re not going to re-grout all the tile.

Cleaning tile grout can be a multiple-step project because hard water deposits might be trapping dirt on both the tile and the grout. You remove hard water deposits using regular white vinegar. I prefer to lay paper towels on the lime deposits and saturate them with the white vinegar.

White vinegar is a mild acid, and it chemically breaks down the alkaline hard water deposits. Allow the vinegar to work for hours before trying to scrub. It may take multiple applications of the vinegar to remove all the lime and dirt deposits. Once this is done, deep clean the grout with an oxygen bleach solution.

After the grout is clean and dry, you can try to obtain a color match. The color of the new grout powder is what it will dry to when just using water to mix it. Don’t add any milky additives, as that can cause blotchiness in the new grout.

You need to remove existing grout before you begin to re-grout. Remove a minimum of 1/8-inch of the grout in all areas where you plan to put in new grout. The best tool to use is a modern vibrating multitool that works much like a barber’s clippers. There are different carbide-tipped blades you can put on the multitool that allow you to carefully remove the grout without damaging the tile.

Work slowly and vacuum up the old grout on a regular basis. Wear a high-performance dust mask to prevent inhaling any silica dust.

To get professional results when installing the new sanded grout, please watch the four-part “How to Grout Floor Tile” series at my AsktheBuilder.com website. While it’s not hard to grout, there are some important steps and a few tricks to ensure the new grout lasts for the next 50 or more years. Good luck, and please send me photos of the completed job.

Need an answer? All of Tim’s past columns are archived for free at www.AsktheBuilder.com. You can also watch hundreds of videos, download Quick Start Guides and more.