Q: My son's shower has developed rusty spots where the tile sits under the shower frame. What can we do to fix this?

Annandale, Va.

A: There is nothing wrong with the glass and the aluminum framing around it. That couldn’t be the culprit anyway, because aluminum doesn’t rust.

The shower door was clearly installed incorrectly. Typically, the bottom of the framing rests on a threshold made of tile or stone that is deep enough to extend beyond the frame inside and outside the shower. On the sides, the tile also extends beyond the door frame and has cement board or other material suitable for wet environments behind it. To cover the edge of the tile where it meets drywall, there is bullnose trim.

In your son’s shower, the door framing looks as though it was set against the edge of the tile, leaving a thin strip of cement board backing visible along the sides. (This would explain the gray strip next to the door frame.) Then the outside of the framing was covered with drywall, with metal flashing — the source of the rust — to cover the drywall edges. Then that was smoothed over with caulk to serve as a sort of pseudo-molding. At the bottom, along the shower door, there is the same treatment, topped by baseboard. No waterproof layer keeps drips from soaking in.

“This is the first time I’ve ever seen baseboard right next to a shower,” said Kamal Tadlaoui, owner of Mirror & Glass Center in Alexandria (703-212-7171; mirrorandglasscenter.com). “Wood shouldn’t be right next to a shower — it will rot.”

If you need a quick fix and care only about making the room look better, you could probably scrape off the rust and smooth on new caulk, Tadlaoui said. But that would be a temporary solution.

The real fix, he said, is to remove the door and framing and then redo the surfaces underneath and on the sides. Unless you’re especially handy, that means hiring a contractor or a home handyman. Tadlaoui recommends Isau Rhodes (301-257-9895), who estimates he could do the work for $1,500, not including materials.

You will need to cut back or carefully pry off the baseboard, then cut back the drywall below and on the sides of the shower door. Replace the drywall with strips of cement board. Ideally, there will be enough room to cover the seam between the existing cement board and the new strips with fiberglass mesh tape. When smoothed over with thin-set mortar, that makes a waterproof seam.

Along the walls, you will need to install narrow tiles and pieces of bullnose tiles, with the curve of the bullnose meeting the edge of the adjoining drywall.

At the bottom, you’ll probably also want tile to replace the baseboard, plus a threshold — a horizontal piece that spans the width of the shower door and is deep enough to extend about two inches into the shower and at least one inch beyond the tile or baseboard on the room side. (Home Depot sells engineered marble shower thresholds six inches wide and 36 inches long for $21.98. Tile shops should have a wider selection, with more colors of stone and man-made materials.)

Install the threshold on a bed of thin-set mortar, with the underside of the threshold coated with a thicker layer of mortar near the outside edge than on the inside edge. This way, when it’s flipped over and pressed into place, the threshold will angle into the shower, allowing water that collects there to drain. To check for the proper tilt, hold a carpenter’s level across the threshold. When the level is at the level mark, there should be a gap of about a quarter of an inch underneath on the shower side.

Ideally you could use leftover tile for the job. But because the shower was put in without bullnose tile, it’s unlikely you will have everything you need, even if you have some leftovers.

If you have even a single piece, take it with you to tile stores. If not, and if you don’t have receipts that identify the color, bring cellphone pictures and compare that way. On-screen colors aren’t always accurate, so get a sample or two to take home with you to verify the match. If you can’t find a good match, go for a contrasting trim color. You might want to match the off-white of the baseboard. Or pick up one of the dark colors in the tile. Make sure that bullnose trim is available for whatever tile you select.

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