Bathtub materials: what tubs are made of

The four materials compared

From each maker’s spec sheet or data sheet; water depth as each sheet gives it. The residential code’s table of fixture standards (IRC Table P2701.1) lists CSA B45.5/IAPMO Z124 for plastic bathtub units, and ASME A112.19.1/CSA B45.2 for both enameled cast iron and porcelain-enameled formed steel fixtures. Diamond certifies to ANSI Z124.1, an earlier name in the Z124 series.

Acrylic

In an acrylic tub, the surface you bathe on is acrylic. Aquatic, part of American Bath Group, describes two ways of making it: heating a cast acrylic sheet and forming it over a mold with a vacuum, or applying the acrylic onto a mold and letting it cure, which is how its AcrylX tubs are made. Behind that surface, in Aquatic’s words, resin “saturates and locks together fiberglass strands to build the structure of the bath.”

• Weight: light. Kohler’s Archer weighs 80 lb and its Underscore 100 lb, both 60 × 32 in alcove tubs.
• Depth: acrylic can be molded deep. The Underscore holds 15½ in of water and 68 gal with a 19 in step-over; Kohler’s cast iron alcove tubs hold 8⅝ to 10⅞ in.
• Hot water: Kohler asks for a hot water supply of at least 70% of the tub’s capacity, so a 68 gal tub needs about 48 gal of hot water to fill.

Fiberglass

A fiberglass tub has a gelcoat surface over fiberglass. Diamond Tub & Showers, made by Frontline Mfg. in Warsaw, Indiana, lists its tubs as “fiberglass/gelcoat” and certifies them to ANSI Z124.1; its 60 in TOA 603319 holds 52.9 gal to the overflow. Diamond also makes one-piece fiberglass tub-shower units, such as the TZ 603377, which come with the walls molded on.

Acrylic or fiberglass? Both are plastic tubs, and the code treats them alike (IRC Table P2701.1); the difference you touch is the surface. Aquatic, which makes acrylic tubs, says its applied acrylic gives “fewer scratches, cracks and chips” and backs it with a lifetime residential warranty. Compare the warranties on the spec sheets of the two tubs you’re choosing between.

Enameled cast iron

Cast iron tubs have an enamel surface over a cast iron body. They are the heavyweights: Kohler’s 54 in Seaforth weighs 277 lb, its 60 in Villager 316 lb and the 60 × 32 in Mendota 351 lb, before any water goes in. They are also shallower: the Villager and Seaforth hold 8⅝ in of water, the Mendota 10⅞ in.

• Getting it in: plan the route and the helpers before delivery. More on weights, with water, in standard bathtub size.
• The floor: Kohler gives a minimum floor load on each sheet: 49 lb per sq ft for the Villager and Seaforth, 56 for the Mendota. The acrylic Archer asks 45 and the deep Underscore 56, so a deep acrylic tub full of water asks as much of the floor as a cast iron one.

Enameled steel

Steel tubs are formed steel with a porcelain enamel finish; the code lists them as porcelain-enameled formed steel fixtures. Bootz builds its tubs this way: its 60 × 30 in Aloha is 14¼ in high with an 8½ in soaking depth and lists the same ASME A112.19.1/CSA B45.2 standard as Kohler’s cast iron. Its BootzCast and SynIron 2 tubs add an insulating backing; Bootz calls them an affordable alternative to cast iron that weighs about 90 lb, and says the backing reduces noise and keeps the water warm longer.

• Shower doors: on a Bootz steel tub, a door may be attached to the tub only with adhesive, never drilled or nailed into it, its data sheet says.
• Chips: enamel can chip if something heavy drops on it. Bootz points to porcelain-enamel repair kits for small chips.

Which material, for which bathroom

• An upstairs bathroom or a tight stairway: an acrylic or insulated steel tub weighs about a third as much as a cast iron one, by the figures above.
• A deep soak in a 60 in alcove: the deepest standard-size tubs we found are acrylic, 15 to 15½ in of water.
• A tub-shower with no tile: one-piece tub-shower units, like Diamond’s fiberglass ones, come with the walls molded on.
• Replacing a tub with a shower instead? Our tub to shower planner shows which shower bases fit the tub’s space, and the remodel cost calculator prices the job.

Every material comes in the standard alcove size, 60 × 30 to 32 in. Sizes for every kind of tub, with the space each needs, are in standard bathtub size.

Questions people ask