How to Install a Cast Iron Bathtub on Plywood Subfloor

Test the floor's structural support unless the bathroom rests above a concrete slab foundation. Bounce up and down on the floor to test for sponginess, and look at the floor joists in the ceiling above if possible. Cast iron tubs can easily weigh hundreds of pounds, even without water or occupants, so floors above a basement, additional stories or elevated homes may need additional floor bracing beyond the existing joists.

Sister joists in, next to the present joists, if the floor seems springy or a visual inspection shows joist spacing exceeding 16 inches. Select boards the same dimension as the current joists -- typically 2-by-10-inch or 2-by-12-inch -- and cut to measure the same length as the existing floor joists. Place each sister joist next to an original joist, spanning the length of the ceiling above, and drive nails or screws through to attach every 8 inches. Secure to the wood framing at the wall edges additionally.

Cover the existing subfloor with additional plywood if the old subfloor is weak or less than 1 inch thick. Measure and cut 1-inch-thick plywood to fit; unless you plan to divide the floor around the bathtub to allow for a difference in floor thickness, cut enough plywood for the entire bathroom. Nail or screw the plywood into place every 8 to 10 inches.

Install your choice of floor coverings if installing a freestanding cast iron tub, such as a claw foot tub. Tubs that rest on the floor do not need floor covering beneath. Follow installation directions for the flooring desired.

Maneuver the new bathtub into the bathroom, getting it as close to its final installation location as possible. Check to ensure you have it pointed correctly so the drain and plumbing pipes correspond to the direction needed on the tub. The last thing you want is to have to pull it back out and flip the tub around.

Attach any legs, drain gaskets inside the tub, overflow assemblies, faucets or other parts as instructed by the manufacturer. Read the product instructions carefully before adding these parts to ensure a leak-free installation.

Slide the bathtub in place. Since cast iron is so heavy, try placing it on cardboard or thin wood runners if you are working in a small area without room to maneuver. Once the tub is in place, pry it up gently with a board placed under the bottom of the tub. Slip the runners out from under the tub and lower to position.

Connect the drain pipe to the bathtub drain and attach the water faucets to the hot and cold water lines. Consult a professional plumber if you need help at this point. Each plumbing installation is slightly different, and failing to connect plumbing properly can damage your floor.

Test the plumbing once complete. Fill your new cast iron tub with water slowly, watching for any leaks or sign of structural stress in the floor. If everything looks good, feel free to test your tub with a long, hot bath.