Cast-Iron Bathtubs vs. Fiberglass Coating

Soaking tub below window in luxury bathroom
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Fiberglass and cast-iron bathtubs in some ways represent both ends of the spectrum for bathtub options. Fiberglass (and other plastic tubs) are modern, moldable, versatile, and lightweight. Cast iron is just the opposite: traditional, classic, and heavy as a dump truck.

But as different as they seem to be, considering the pros and cons of both of these popular tub materials can help you decide what you really want in a bathtub. (And you might be surprised at some of their similarities.)

How They’re Made

Fiberglass and cast-iron bathtubs couldn’t be more different than in their construction. Fiberglass bathtubs start as a molded fiberglass form that gets coated with a layer of acrylic, called a gel coat.

The gel coat provides the color and protective finish. With an overall thickness of about 1/4 inch, fiberglass tub material is relatively flimsy stuff, which also explains its light weight.

Cast-iron bathtubs have a molded (cast) iron base that’s about 3/8 inch thick or thicker. The tub interior is then coated with three layers of high-grade overcoat enamel that are fired at about 900 degrees Celsius for a rock-hard finish with a high sheen.

The exterior of the tub is painted with several layers of primer and paint to match the looks of the interior but without the enamel. If you’ve ever used a classic Le Creuset enameled cast-iron pan or Dutch oven, you have a sense of how cast-iron tubs are made, not to mention of the overall quality.

Interestingly, cast-iron tubs can be surprisingly resource-efficient, with new tubs containing up to 80 percent recycled materials. And if you find a nice vintage cast-iron tub with a good finish, it’s 100 percent recycled.

Form Factors

Here’s where fiberglass and cast-iron tubs may be more alike than you might expect. Cast-iron tubs are available in many more sizes and shapes than the familiar clawfoot tubs of Victorian vintage.

You can find rounded, squared, and slipper shapes as well as an alcove, drop-in, and pedestal styles. Of course, the classic freestanding clawfoot tub reigns supreme (like Victoria herself).

Even so, the shape and style options with cast iron are nothing compared to the variety of fiberglass selections. For every bathtub shape in production, there’s probably a version in fiberglass. This even includes a “couples tub” complete with two nesting tub basins that form the shape of a yin-yang symbol.

Thud Factor

You can’t beat cast iron in this category. A tub with a solid-iron core, weighing in at 350 pounds or more, feels considerably more solid than a tub made with synthetic fabric mesh, resin, and a thin layer of plastic. To get a feel for the thud factor of a cast-iron tub, hit the bottom of an old-fashioned cast-iron skillet. To get a feel for a fiberglass tub, tap on the body of a ‘70s-era Corvette (be sure to ask the owner first).

Shapes and Features

If you want special shapes, molded features, or any kind of jets, your choice of material is a no-brainer: fiberglass. Special features on fiberglass tubs include body-cradling shapes, molded seats, armrests, headrests, and (why not?) drink holders. It used to be that cast-iron tubs simply didn’t offer these options. Today, you can find cast-iron tubs with jets, handles, and some molded features, but you’ll pay dearly for these extras.

Fiberglass (or other plastic/moldable material) is also the material of choice if you want a tub with matching wall panels or a tub-shower combo. Another style you won’t find in cast iron is a walk-in, or accessible, tub.

Supporting a Cast-Iron Bathtub

Weight is an important consideration, but it’s not a deal-breaker in most cases. The floor structure in a well-built home usually can handle the weight of a cast-iron tub. But if you suspect your floor framing might not be what it should—perhaps there are dips and soft spots in the floor, or it simply doesn’t feel solid underfoot—have an experienced builder or an engineer check it out. The floor joists (the horizontal structural supports) may be undersized for the floor’s span.

The subfloor is another issue, and this relates to some tub styles more than others. Drop-in and apron-style cast-iron tubs are supported by a large framed structure that distributes the weight of the tub over a broad area.

Freestanding tubs, and particularly footed tubs, bear all of their weight on four small points of contact, and those bear on the subfloor as well as the framing below. If you plan to have tile under the tub, make sure the subfloor is stiff enough to resist deflection (flexing) adequately for the tile. Otherwise, the tile will crack.

Calculating the actual weight of a cast-iron tub is simple: start with the tub’s dry weight (without water), add the weight of the water when the tub is full (water weighs 8.3 pounds per gallon), then add the body weight of the bather. For example, if the tub weighs 350 pounds, is filled with 40 gallons, and is used by a 200-pound person:

40 gallons x 8.3 pounds = 332 pounds

350 + 332 + 200 = 882 pounds

Cost Comparison

As of 2018, cast-iron bathtubs start at about $400 for a basic, alcove-style unit. Classic clawfoot or freestanding pedestal tubs start at $1,200 to $1,600. Large cast-iron jetted tubs can cost $3,500 and more.

Fiberglass tubs start at under $200 at the low end and go up to over $6,000 for fancy whirlpool walk-in units at the high end. In between, there are drop-in tubs at around $400 to $500, jetted and whirlpool tubs for $900 and up, and freestanding units for $800 to $1,200.

Close Cousins of Fiberglass and Cast Iron

While there really is no similar alternative to a cast-iron bathtub, if you like the look and feel of an enameled surface, a much cheaper alternative is enameled steel.

With prices starting at under $200, enameled steel was long the standard for builder-grade tubs, although it’s largely been replaced by fiberglass and other plastic tubs in this category. Enameled steel essentially is a flimsy version of cast iron, with a much thinner enamel coating (but they’re still pretty tough and can last many years).

The most common alternative to fiberglass bathtub material is acrylic. In fact, many sources refer to fiberglass and acrylic interchangeably. Acrylic tubs are made with vacuum-formed acrylic sheets reinforced with fiberglass.

Acrylic tubs tend to be more expensive than fiberglass and have a deeper color layer that can be more durable than the relatively thin gel coat used on fiberglass tubs. If you’re looking for an acrylic tub rather than fiberglass, check into the tub construction, as many manufacturers and retailers use “acrylic” loosely.