Eradicating Shower Mold

Through the years I have harped at homeowners about routinely sealing the grout in their tile showers. Not because I enjoy nagging, but because it is important to the health of the shower, the house and its occupants.

The Problem

Grout is a cement-based material, and like all cement-based materials, is not completely waterproof. Yes, I agree, it should be waterproof, being installed in a shower and all, but that is water under the metaphorical bridge, and if you don’t like it, well, use a waterproof epoxy grout or enjoy a fiberglass shower.

Since grout is not waterproof, and since we tend to, ahem, use water in showers, that moisture is absorbed into the grout when it is not sealed. Moisture being held in suspension first breeds mold, one very big reason we are forced to clean our grout and caulking with harsh chemicals. But more important, that moisture gets past the surface and into the wall.

If the house was built from roughly 1965-90, the wall behind the tile is covered with drywall (greenboard). When drywall gets moist, it expands and swells, the grout cracks as a result, more moisture gets in, the tiles loosen, the wall leaks and you need a new shower.

Most of these drywall-backed showers have already been replaced, unless equipped with high-quality sanded or epoxy grout, routinely sealed or rarely used.

Newer showers have cement board behind the tile instead of drywall. Trade names such as Wonder Board or Hardi-Backer may be familiar. The cement board does not swell when moist, does not flex or easily puncture like drywall and can be coated with a waterproof covering.

But the moisture still must go somewhere. Gravity takes over. As the moisture reaches the base of the shower pan, it becomes trapped. Caulking along the shower pan base prevents moisture from draining back into the shower, and on the back side, the hidden shower pan lip prevents it from going onto the floor behind.

So it must travel laterally. Weepholes were developed to allow that moisture to drain back into the shower before exiting the enclosure and causing damage outside.

Pictured is a shower-pan weephole. Sometimes the weephole will be a high spot, a low spot, an indentation or a bump, depending on the whim of the manufacturer. Either way, no caulking should be plugging it.

The weephole shown here was plugged with grout and layers of caulking. Without a functioning weephole, moisture behind this traveled outside the shower, behind the baseboard trim and onto the floor. That water then got beneath the carpet, spread throughout the bathroom floor and created the pictured mold problem beneath the carpet pad.

Sealing the grout would have prevented the entire problem. Same with leaving the weepholes open. Pick one or, better yet, do both.

Exacerbating the issue is the fact that both sides of the base of the shower door were caulked, again holding water and funneling it into the wall. The inner side of the shower-door frame should be open to allow water to drain back into the shower.

The Solution

All tiles inside this shower were still securely fastened to the cement board (determined by tapping on the tiles with knuckles and observing the intact grout), making it completely salvageable. If it had been drywall rather than cement board behind these tiles, it would have required a complete tear-out and rebuild.

This repair involves only removing the caulking, thoroughly cleaning the grout, sealing the grout and replacing the caulking. The weepholes need to be opened, and the sealant at the inner side of the door track needs to be removed. The carpet and pad need to be removed. The mold on the floor needs to be remediated, new flooring installed and the damaged baseboard replaced.

Expensive, yes, but it could have been much worse on a home only 9 years old.