I hope that this is a relatively simple question.

I am converting a bedroom into a bathroom. I am going to be installing stone or ceramic tile and a combination bathtub/shower.

Because of the tile, I will be installing 3/4" plywood and then 1/4" cement board as underlayment.

I will definitely need to put the plywood under the bathtub, but do I need to install the cement board under the tub also?

Thanks!

Hi frazwood:

No..no need to install the cement board under the tub. In fact, I use the 1/4" cement board (and tile) to kind of LOCK the tub in place.

However, I don't push the concrete board up to the tub... instead I keep the cement board about an 1/8th inch off tub and then tie it all together by filling this 1/8th inch space with thinset mortar or modified thinset mortar.

Which raises point... you should be using a 1/4" notch trowel with thinset mortar or modified thinset mortar to install the cement board to the plywood... use hardibacker and hardibacker screws if available in your area... then want to tape the joints using alkaline resistant tape and thinset mortar.

Check out this site for more information:

James Hardie: HardieBacker 1/4'' backerboard (http://www.jameshardie.com/homeowner/products_backerboard_quarterInch.shtml?openTab=jsn avLink4) Check out under FLOOR INSTALLATION

And what kind of tub Fraz.. If this is going to be a fiberglass or acrylic unit you should plan on installing the tub/shower unit in a bed of mortar or similar... makes a huge difference in terms of creaking, flexibility, etc. (here would set tub in substrate, fill tub 1/2 to 3/4 of the way with water, let dry 24-48 hours and then drain the tub, step in.. and you will be impressed by solidness! ). Let me know if need more information on this.

Let me know what you think... Mark

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Mark:

Thanks for the help. Your answer makes (using the cementboard to help keep the tub in place). I have my hardibacker, hardibacker screws, 1/4" notched trowel, and tape ready-to-go (I'm installing cementboard in another room before the bathroom).

I was definitely planning on installing the tub in a bed of mortar (I was going to follow the installation directions provided with the tub... which say to do just that).

Tim

Mark has given you some good info here. Do you have good access to the area directly below the old bedroom and have you pulled a permit from the city yet? Keep us informed as this project progresses, it's the kind we especially like.

Do you have good access to the area directly below the old bedroom and have you pulled a permit from the city yet? Keep us informed as this project progresses, its the kind we especially like.

Yes, I have access below the old bedroom. It should be a relatively straight-forward project, although I've never done anything like this before...

I have pulled a permit. I have a rough-in electrical inspection tomorrow and then a framing/rough-in plumbing inspection once I complete the plumbing.

I'll try and keep y'all updated.

Thanks!

Tim

And what kind of tub Fraz...? If this is going to be a fiberglass or acrylic unit you should plan on installing the tub/shower unit in a bed of mortar or similar...makes a huge difference in terms of creaking, flexibility, etc. (here would set tub in substrate, fill tub 1/2 to 3/4 of the way with water, let dry 24-48 hours and then drain the tub, step in..and you will be impressed by solidness!!). Let me know if need more information on this.

Let me know what you think...Mark

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I am now in the process of installing the tub but I am not going to use mortar because the installation instructions specifically state not to do this (I was incorrect in my post above). The body of the tub does not actually touch the floor.

I am not 100% sure, but I think it's an American Standard Princeton Recess tub.

Hi Fraz:

Thanks for update... hope all goes smoothly... ;)

Can you tell me what the manufacturer recommends setting the tub in.. Something... nothing?

Thanks.

Hi Fraz:

Thanks for update....hope all goes smoothly...;)

Can you tell me what the manufacturer recommends setting the tub in...? Something...nothing??

Thanks.

Sorry for the delay in responding...

The manufacturer's instructions clearly state to not set the tub into ANYTHING. In fact, there is a large sticker on the tub that tells the installer of the tub that setting the tub into concrete, mortar, or the like will void the warranty.

A brief update on things...

I passed my plumbing/framing inspection earlier this week. My wife installed the tile on the floor yesterday. The next projects are installing the wallboard (non-tub area) and hardibacker (tub area) and then to install the tile around the tub.

It'll take a few more weeks, but hopefully I'll post some pictures when it is done.

I'll tell you... no mortar under tub is a first for me... especially in terms of VOIDING warranty!

Thank you for that information! I will review the literature to see why this is so... ;)

Good luck on the rest!

Mark

Some manufactures have support already installed under the tub but didn't know it voided the warranty to add more support. Mark if you pull the installation information let me see it please and why vwasn't this thread moved to the Plumbing Page? Regards, Tom

May want to ask Bob that one, Tom... ;)

And as soon as Fraz. Confirms the tub name for me I will look it up and let you know!

Hey Guys:

It's been a long time, but I thought that I'd give you the final update from this one.

The tub was definitely an "American Standard Princeton Recess" tub.

I installed it without mortar and so far it's been great. I am attaching pictures of the entire bathroom.

(by the way, I didn't post this in plumbing originally because I didn't ever intend to ask a plumbing question... it was more about tile/flooring).

Anyhow, thanks to everyone for the help.

Tim

Looks great Tim... nice job!

That tub is a cast iron tub... yes?

Thanks for update tim.

MARK