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How to Install a Bathtub (make it ROCK solid)

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by Jeff Patterson in Bathroom Plumbing, DIY Bathroom Remodel
How to Install a Bathtub

Here’s our promise:

Today you’ll learn how to install a bathtub and make it solid as a rock.

The reason for this tutorial is simple. Initially I thought I was keeping the bathtub in my rental home.

But I decided that was a TERRIBLE idea. So Steve came over to help me put in an acrylic tub.

If you don’t know Steve, he’s my buddy who remodels bathrooms here in Pittsburgh.  And when he does anything he does it the right way.

So if you’re doing a bathroom remodel and a new bathtub installation you’ll love this tutorial.

I’m super EXCITED to share this with you. It’s step-by-step and has a ton of tips that you won’t see in any books 🙂

Bathtub Installation Subfloor Prep

This house is one of my rentals and it’s over 100 years old.

As such, the floor is a mess. And I say that in the most polite way because it was a patchwork of wood. Primarily because this bathroom had a ton of water damage.

Demo Bathroom Floor

Use either a hammer or super bar to pull up the old subfloor. I’ve had my Super Bar for years and love that thing to death.

Pry Up Old Floor

Remove old nails or pound them into the joists

Remove Old Nails

Apply liquid nail to the top of joists.

Apply Liquid Nails

Nail the new subfloor to the joists. In this case we used 3/4″ plywood since it was the same height as the existing tongue and groove wood floor. Doesn’t this look so much better than the old flooring!!

Nail Down New Subfloor

Just make sure you’re nailing or screwing the new subfloor into the joists and not thin air. Also, know where your pipes or electrical lines are located to avoid nailing into them. Notice how Steve drew a line along the subfloor. This indicated the rough location of the tub waste pipe. Shooting a nail through that would totally ruin our day!!

We needed to add a second layer of subfloor. This was 1/2″ plywood. But in order to do that we spread more Liquid Nail on top of the first layer of subfloor.

Apply Liquid Nail to Subfloor

The 1/2″ plywood was then nailed into the first subfloor and joists. By the way, nail guns make this so much EASIER.

Nail Down 1/2" Subfloor

Once the subfloor is prepped, dry fit your new bathtub and check the walls for plumbness. We had to build both a knee wall and the main wall. That was a bit tricky in this old wonky house but we’ll walk you through it.


Building Walls in a Wonky House (what a joy…)

This old bathroom was a pain in the butt. None of the walls were plumb or straight. So we dry fit the tub and centered it on the plumbing wall.

Dry Fit Tub

After making the tub square with the adjacent walls we made a mark on the plumbing wall and on the plywood behind the tub.

Front Wall

Back Wall

Steve then added a stud to the plumbing wall.

Add First Stud

He measured from the front of that stud to the mark he made on the plywood behind the tub.

Steve cut a 2×4 to this dimension and nailed it to the floor, again making sure it was square with the front stud location and mark on the plywood.

Once this is in place you can run your studs from it to the existing framing. Studs should be at least 16 inches on-center and plumb since the tub will be attached to them.  The 16 o.c. spacing is particularly important in this case since we’re using Schluter KERDI-BOARD for the waterproofing.

Plumb Studs

Our video tutorial shows all the steps for building the main wall and for building a knee wall.

Knee Wall

Once your framing is in place you can dry fit the tub again and mark the position of the drain.

Drain Location

Cut an elliptical shape out of the subfloor for the drain. Use either a jigsaw or reciprocating saw. We had a bit of an issue in this project as a portion of the joist was below the drain. You’ll see what I’m talking about in the video!!!

Cut Drain Opening

Now it’s time to assemble the tub drain. This can make or break your project. So it’s a good idea to read on 🙂


Bathtub Drain Assembly…Nobody Wants LEAKS!!!

The Kohler Archer bathtub requires a special overflow drain kit. It actually makes the tub look pretty cool.

We recommend putting the drain assembly together first before securing your tub to the studs. This makes the installation easier…and lessens the likelihood of you tearing out your hair (just keeping it real).

Steve likes to apply 100% clear silicone to all the parts of the Kohler Clearflo overflow kit that will touch the tub. He added the silicone to the overflow then bonded the rubber gasket to it.

Silicone Overflow

Add Gasket to Overflow

He then adds another bead of silicone to the tub where the overflow will go.  And a silicone bead directly on the part of the gasket that will attach to the tub.

Add Silicone to Tub


Add Silicone to Gasket

This process makes the installation of the overflow 100% waterproof. And yes, Steve loves clear silicone almost as much as coffee.

Kohler provides a bracket to hold the overflow in place. Use a screwdriver or impact driver to secure the bracket to the overflow. If you use an impact driver just be careful not to over tighten as this can crack the tub (which would be super BAD).

Install Bracket

Place the slip nut onto overflow pipe and position the rubber gasket behind it.

Position Slip Nut

Then screw on the PVC pipe fitting. The nice part about this Kohler Clearflo kit is the flexibility of the pipe fitting to move.

Moveable PVC Fitting

Steve does the same silicone procedure with the tub shoe and drain. All parts get a nice bead of silicone.

Drain Parts

To see the detailed plumbing installation start watching the video at the 10:00 mark. There are a lot of great tips.

My favorite one is to AVOID getting the PVC primer on the inside of the tub.

Glue Pipes

Yep, that’s a pretty good tip since this Kohler Archer cost me about $800.

One of the biggest mistakes to make when installing this kind of tub is to not set it in mortar. So we’ll show you how to do that.

You’ll be surprised how easy this can be…well, after watching Steve of course.


Embed the Kohler Archer Tub in Mortar (this is IMPORTANT)

Steve always has the best material suggestions. And this tutorial is no exception. He likes using Mapei’s 4-to-1 Mud Bed Mix.

Mapei Mud Mix

Usually you need one 55 lb of mortar for one tub. But as you’ll see in our video we had to use two bags, but it’s not like the Mud Bed Mix is breaking the bank.

Mix the mortar so that it can support the tub. It should not be runny but rather stand up on it’s own.

Mud Bed

Pour the mortar on the subfloor so that it’s a few inches from the drain opening. The mortar should be evened out and about 2 inches thick on all sides.

Then embed the tub into the mortar.

Embed Tub

Press the tub down into the mortar and place a level on it to ensure it’s level. Pre-drill holes in the tub lip. The holes should correspond with the stud locations.

Drill Holes in Tub Lip

Once the tub is level you can attach it to the studs using 2 inch galvanized screws. Place your foot on the inside edge of the tub to hold it steady while securing one screw in the center of the tub on the main wall.

Screw Tub to Studs

Check the tub is level again on the main wall then transfer your level to the front of the tub. Push the tub down into the mortar to get a level position and secure one screw through the tub lip and into the stud. Do this for the back wall as well.

Level and Screw Tub to Wall

Once you’re satisfied the tub is level walk on it to embed it into the mortar. Also, if there’s a gap between the tub flange and studs use a shim. Otherwise you could break the tub lip when you secure it to the stud.

We use a similar method for installing bathtubs in basement bathrooms, too.

Watch our video to see all the awesome tips


What’s Next

After you’re done adding your bathtub you can start the waterproofing process and tiling.

We have several tutorials that will shed light on this process.

If you’re doing a bathroom remodel and want to simplify the process enroll into the Bathroom Repair Tutor Video Library

We show how to build showers (Schluter, Wedi, Curbless, etc.), install tile, upgrade basement bathrooms, and more.

The extra guidance is invaluable

Enroll Today



Jeff Patterson





  1. mitch says:

    Great tutorial Jeff – thanks. Is it much different to install a steel tub? Do you need to use the mud bed for something like a steel tub?

    1. Depends Mitch, often with steel tubs the answer is no. But if the directions call for a mortar bed as being option we’d recommend using one anyway. It makes the tub more solid and prevents shifting over time.

  2. Mike says:

    Great info and video. I’m about ready to install a Kohler Bancroft tub (60×32) with Kohlers pureflo drain system. I have no access to tie in my drain to the 2″ p trap. However I may be able to cut a hole in my laundry room ceiling to make the final drain tie in. It looked like Steve did something similar to tie drain into the p trap? Also, the 2″ p trap I put in has a threaded seal fitting at the 2″ line. Do I just make sure it’s tight or do I need to use a certain sealant to ensure no leaks? Or should I install a regular 2″ p trap? I will also need to put a reducer at the 2″ p trap inlet to receive the 1 1/2″ drain then glue it all up after dry fits are good. I just want to be sure I’m on the right track and your inputs on my drain.
    I’m also putting a knee wall at the end of my tub like you did in the video. It’ll have clear glass installed on it. I’m unsure how tall I should make it. How tall was the one Steve put in?
    You’ve sold me on Schluter products and I’m going with the ditra floor heat system and kerdi on the tub/shower walls.
    Thanks for any input you have to offer and keep up the great videos!

  3. Nicki says:

    How long do you need to let the mortar bed set up before finishing the plumbing and shower assembly? 24 hours? 72 hours? Longer? We can’t seem to find an answer to this question any where.

  4. Jeff Knecht says:

    Great Video
    1. Stringers-I’m installing Am Std. Studio tub and requires stringers and bedding. Noticed no stringers in video.
    2. Some installers rec. tar paper between floor & mortar and plastic between mortar & tub.
    3. If tub is on outside wall, need vapor barrier below tub rim ?

  5. utzo says:

    Really, good post .

  6. jacob kauffman says:

    I’m installing a new tub that I didnt build a frame for like in my previous house. I put the tub in and it’s got a mild wobble. Had to do a search to see what I’m missing.

    Your video was a perfect refresher to what I did many moons ago but totally forgot.

    Your video was very informational and I learned a few tricks I didnt know before.


  7. John says:

    Thank you! Helpful and relevant during a bathroom remodel in our 97 year old home.

  8. Joseph McCarthy says:

    Do I need to bed mortar a cast iron tub

  9. Rick says:

    The directions on the Acrylic hot tub I’m installing say to use roofing nails to attach the lip of the tub to the wooden stubs by hammering them over the lip. You use screws into holes drilled into the lip, which I would prefer. Besides going against the manufacturers installation instructions, is there a concern that the holes would lead to cracking?

  10. Benjamin Rogito says:

    Hi Jeff, I just got a Delta classic bathtub and would like to do this as a DIY project. We have water pipes running on the bathtub bed, how best can I tackle this keeping in mind that I need To use mortar to sit the tub in place.

  11. Lycos ceramic says:

    Wow!! Really appreciate you sharing this blog post.

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