What Floor Should Be Installed Under a Cast Iron Tub?

Flooring Discrepancy

• There is disagreement regarding the need to use special flooring or reinforce flooring under a cast iron bathtub. Sunrise Specialty states that the weight of their cast-iron tubs is between 255 and 340 pounds and notes that 340 pounds is much less than the maximum weight allowance for floors. However, other cast iron retailers and installers warn that the weight of the tub is not the sole determining factor for reinforcing the floor.

Total Floor Weight

• As its name implies, a “cast iron” bathtub is extremely heavy. Some tubs can weigh as much as 300 to 400 pounds, and this does not include the weight of water and human bodies. As an example, a 60-foot cast iron tub can weigh 325 pounds; when completely filled, it can also hold more than 332 gallons of water. If you put a 200-pound man in the tub of water, the total weight of the cast iron tub exceeds 900 pounds.

Flooring Material

• Tile is a popular choice for bathroom flooring material because it resists mold and mildew and is easily cleaned. However, neither tile, vinyl, laminate nor any other type of bathroom flooring is strong enough by itself to support the weight of the cast iron tub. Regardless of the flooring choice, it will need to be reinforced.

Flooring Reinforcement

• For a tile floor, three-quarter-inch plywood subfloor is advisable as reinforcement. Natural stone tiles require 1.25 plywood subfloor reinforcement. In addition, if the cast iron bathtub is on joists, double the joists and make sure that each one of the tub’s legs is positioned directly over a joist. Cast iron tubs should also be positioned near a load bearing wall, if possible, to prevent the floor joists from sagging.