How Much Support Does a Cast Iron Tub Need in a Second Floor Bathroom?

Floor Joist System

• Standard floor joists sit on 16-inch centers, meaning the distance from the center of one joist to the center of the next joist is 16 inches. Large 2-by-10 or 2-by-12 dimensional boards are used to construct the floor; and while 2-by-12s will hold a greater weight load, 2-by-10s are adequate for supporting a cast iron tub. Local building codes in nearly every community require one of these two lumber dimensions for floor joists. Homes built prior to these codes, however, might contain substandard floor joists. If the floor joists are 2-by-8 or smaller or if the joists are spaced wider than 16 inches apart, there is a good chance that you’ll have to reinforce the floor before setting the tub in place.

Subfloor

• On top of the joists is the subflooring. The standard here is 3/4-inch, tongue and groove plywood or oriented strand board (OSB). If the existing subfloor is thinner than 3/4-inch, it may not safely support the weight of a cast iron tub. Collectors love big antique tubs with claw feet, but the weight of those tubs is concentrated on four small feet. If the subfloor isn’t thick enough, there’s a chance that a tub foot could poke through.

Underlayment and Floor Finish

• Installing 1/4-inch underlayment will add surface strength to the flooring without adding a lot of height. The type of floor finish you install can also increase the floor’s surface strength and reduce the risk of a tub foot breaking through to the subfloor. Ceramic tile provides a strong surface to support the tub feet. If you have thin or soft linoleum, however, it won't add support, and the tub feet may tear or dent the linoleum's surface.

Considerations

• If your bathroom floor has an adequate joist system and subfloor, the next biggest risk comes from rotted joists or termite damage. It doesn’t matter how large the floor joists are if they’ve rotted away. A sloping bathroom floor, a spongy feel underfoot when walking or a bulge or depression in the floor can indicate a damaged joist system. Consult with a contractor if you have any doubts about the condition of the bathroom floor.