What Should a Cast Iron Bathtub Sit on?

About Cast Iron Tubs

• Cast iron tubs are made from gray cast iron, a material composed of pig iron, carbon and silicon along with other compounds to give it additional characteristics for strength and shock resistance. The metal is glazed with a porcelain coating for a smooth, attractive finish. The advantage of cast iron tubs is that the metal composition holds the heat longer and transfers it back into the water. Acrylic and fiberglass tubs do not have this ability. Holding the water’s heat longer reduces energy costs.

Weight of Cast Iron Tubs

• Cast iron tubs generally weigh between 255 to 340 pounds, depending on size, according to the SunriseSpecialty website. If the average tub holds 42 gallons of water and water weighs about 8.35 pounds per gallon, that would mean that it would hold 350 pounds of water. If you add that to the weight of the cast iron tub, the total weight of the tub plus water can be as much as 700 pounds.

Flooring Under Cast Iron Tubs

• The above calculation makes it clear the flooring beneath a cast iron tub must be able to support a great amount of weight. If there are any structural weaknesses beneath the tub, a flooring failure could collapse the flooring, with the tub falling to the floor below. Older houses are generally built to support such fixtures because they were commonplace in the past. However, water damage and rotting can weaken the wood materials in flooring, so careful inspection of the structure of the floor and the materials below it will ensure a safe base for the cast iron tub. Metal shims may be placed under the claw feet of free-standing tubs to distribute the weight evenly. Inset tubs may require reinforcing with drywall materials to spread the weight evenly over the floor surface on the installation site.

Inspecting Floors Beneath Cast Iron Tubs

• If the cast iron tub is to be installed on the ground floor, go to the basement and check the floor joists above for sagging or signs of stress. If the structural components appear to be sound, it is likely to be sufficient for the cast iron tub. For upper floors, you should check subflooring materials for moisture damage and replace them if necessary. Mortar floor leveler may be necessary to level and support the tub.