Cast iron tubs weigh substantially more than modern fiberglass bathtubs. Regardless of this weight difference though, most standard floors have the strength to support a cast iron tub. While floors do need extra support when installing a cast iron tub, in some instances, such as in older homes, you may choose to reinforce the floor prior to a tub's installation for safety.
Floor Maximum Load
With the standard joist size of around 2-by-6 to 2-by-8 inches, and the standard joist spacing of around 16 inches, the standard maximum load of the floor in a room in a house is 40 psf, or pounds per square foot. Regardless of the standard of measure, this maximum load does not actually represent the weight that a floor can support per foot, but the total amount of weight safely supported by the floor.
Bathroom Max Load and Tub Weight
If the floor was built using the general 16-inch spacing, you can expect a bathroom floor to safely hold 40 psf. So, a 5-by-10 foot bathroom floor generally supports up to 2,000 pounds. This provides sufficient support for most cast iron bathtubs, which run under 500 pounds, even when full. Since a bathtub is a bathroom staple though, the bathroom in a home is often built with this in mind. This means that the floor may already be reinforced to hold even more weight. If you have the original plans for the home, they come in handy in checking this.
Upgrading the Surface
If you fear that a bathroom floor does not provide enough support for a cast iron tub, you may choose to upgrade the surface of the floor. The older wood gets, the weaker wood gets, especially in a room where the boards had constant exposure to moisture. Replacing old wood boards with new wood boards helps provide additional support. You may also choose to switch out old wood floorboards with a flooring material made to bear more weight, such as Oriented Strand Board, or OSB.
Upgrading the surface of a subfloor is not the only possible solution for reinforcing a floor to support a cast iron tub. A subfloor may also be reinforced from beneath the floorboards. Add extra support to a floor beneath the floorboards by adding additional joists to the floor. Do this by either removing the flooring surface planks all the way across the floor and installing extra joists in the same direction as the original joists or by cutting smaller joists and installing them perpendicular to the original joists in the floor.