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kind of foundation for cast iron bathtub

We want to install a Kohler cast iron tub. We will have a 1/2-inch plywood subfloor after taking up the existing vinyl, etc. Installation instructions for the tub sound like 1/2-in plywood will be adequate. But, since the tub (316 lbs w/out water and person!) will rest on its 4 little feet on that subfloor, will 1/2-inch plywood actually be enough to support this weight? I do not want the tub to "sink" or flex and cause a crack in the interface to the tile surround above the tub. If anyone has had experience installing a cast iron tub, would appreciate any advice.
Many thanks!

Re: kind of foundation for cast iron bathtub

What are you fixin' to do for flooring outside the tub?
if you had tile in mind 1/2" plywood alone is far too thin for tile. 5/8" is the bare minimum and most pro's prefer 3/4" installed properly. If you were planning on natural stone tiles, you'll need a minimum of 1.25" of plywood properly installed.

Re: kind of foundation for cast iron bathtub

Assuming that your bathroom is on joists, you may want to double them in the area under where the tub is going to rest. This is a job for a framer. Make sure that the 4 tub legs will rest exactly over joists. See old postings for more information and details.

Then lay the 3/4" or thicker subfloor, fasten it as directed and finish your rough plumbing.

Lay the finished floor of your choice, connect the plumbing, fill the tub and enjoy the fruits of your labor.

Re: kind of foundation for cast iron bathtub

I have one of these tubs on 3/4" (5/8" actual) subfloor. The subfloor has 1/4" luan over it and then the vinyl. I ran the vinyl floor wall-to-wall to give some floor protection under the tub and set the tub on the vinyl.

I weigh 400# myself and then add the cast iron tub on that. No problems at all.

You can run extra joists. I could not because below this I have a lot of utilities running through my joists. I did, however, have my contractor connect the joists across to each other with blocking and carried this to at least one additional joist on either side of where the tub sits. We thought it would help distribute the load.


Re: kind of foundation for cast iron bathtub

Your design the load for the tub full of water. SO you need ot know the volume of the tub. A person is about the same density as water, although they could be standing up in the water ... so I'd add another 500lbs to that calculation.

SO using some rough numbers, I calculated 30 cubic feet from a 30" wid, 60" long 24" deep tub. Water weighs 62lb/cuft. SO 1800lb for the water + 320lb tun + 500lbs for people. So you're at around 2600lbs!!!

Best to double check the volume calculations... but it's probably not far off.

Tubs need MAJOR reinforcement. You should located them when possible near a load bearing wall or you will have sagging of the floor joists over time... which can crack tile floors.

On my 90 year old house, both of my tubs not surprisingly are located adjacent to the load bearing walls. Its' also a easier location to run piping to and from the basement up those walls.

Re: kind of foundation for cast iron bathtub

For a standard Kohler 60" cast iron tub you are looking at approx 1,000 lbs total weight, tub 325, water 332 (8.3x40 gal) and one fat body 250

I install lots of cast iron tubs and we always reinforce and/or rebuild the framing because it's rarely adequate for the weight.

1/2" ply sub won't cut it for much of anything. rip it up, evaluate the floor framing and go from there. Don't take any short cuts.

Re: kind of foundation for cast iron bathtub

I have to agree with LHR, 1/2" is not thick enough for any floor, much less a tiled floor with a cast iron clawfoot tub. You didn't say whether it was a clawfoot tub but the "four little feet" indicates that.

My suggestion is that you leave the 1/2" plywood down and glue 3/4" plywood on top, then screw through both layers to the joists.

Now there is a question of the joists. You need to insure that they are adequate for your span. You need to use the tables for the maximum flex for a tile floor, which is less than a regular floor. I know in my case, I have 2x10 on 16" centers for a 14' span, cross braced at the halfway point.

If you know exact location of the feet, you could use two 2x's for cross bracing directly under the feet.

BTW, water weighs exactly 8 lbs per gallon, by definition. Standard contractor grade tubs are 30 gallons but there are some oversized "jacuzzi" types that go up to 80 gallons.

Re: kind of foundation for cast iron bathtub

You didn't clearly state what type of tub this was: a freestanding (clawfoot) tub, an alcove-mount tub, or a deck-mount tub. I'm guessing it's alcove-mount.

(An alcove-mount tub is the "typical" bathtub with a false front and three tiled walls going straight up from the tub's rim. A deck-mount sits in a hole on a raised platform, with the deck of the platform extending outwards on all four sides from under the tub rim.)

The aforementioned advice is good, but seems to be oriented toward freestanding tubs.

If you have an alcove-mount tub, many of them are designed so the rim rests on ledger boards attached to the wall. This implies that the walls -- and the flooring underneath the wall -- must be substantial enough to support the weight of the tub, water, and bather.

If you have a deck-mount tub, of course you'll want to make sure the floor framing is sufficient. Some plumbers will pour concrete on the subfloor and set the tub into the wet concrete in order to provide full support for the tub to prevent flexing (unlikely with cast iron!) or the deck sagging. I think this is more common for fiberglass tubs than cast iron.

Re: kind of foundation for cast iron bathtub

Thanks for all the advice. After talking to our contractor (who will install this heavy tub), we are going to reinforce the joists and add 3/4-inch plywood underlayment. The tub area will be next to a foundation wall, which will help. Again, many thanks for the advice.

Re: kind of foundation for cast iron bathtub

Thanks for the post very inspiring, was searching for ideas a stumbled on these, thanks again

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