Introduction: How to Install a Bathtub (make It ROCK SOLID)

By Home Repair Tutor
About: We love home improvement and enjoy sharing tips on YouTube and Home Repair Tutor. Everything we do is self taught. Over the last 12 years we've bought and rehabbed several rental homes in Pittsburgh. Sometim…

Here’s my 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

You'll need these supplies

Here are the supplies you need

  • Kohler Archer Bathtub
  • Kohler Archer Clearflo Overflow Kit
  • Reciprocating Saw
  • Circular Saw
  • Jig Saw
  • Impact Driver
  • Paslode Framing Nailer
  • HammerSuper Bar
  • Liquid Nails Construction Adhesive
  • Caulking Gun
  • Tub Drain RemoverT
  • wo Inch Galvanized Screws
  • 100% Silicone Sealant
  • Chalk Line
  • 4 Foot Level
  • Mapei Mortar
  • 5 Gallon Buckets
  • Hammer Drill
  • Mixing Paddle for Mortar

The first step for any bathtub installation is to make sure the subfloor is solid. So let’s dive into that.

Step 1: Pull Up Old Subfloor

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.

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.

Step 2: Remove Nails

Remove old nails or pound them into the joists.

Step 3: Apply Liquid Nail

Apply liquid nail to the top of joists.

Step 4: Nail New Subfloor to Joists

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!!

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!!

Step 5: Add Liquid Nail to Subfloor

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.

Step 6: Nail Second Layer of Subfloor

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

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.

Step 7: Mark Position of Tub

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.

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

Step 8: Build Main Wall and Pony Wall

Steve then added a stud to the plumbing wall.

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.

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

Step 9: Mark Drain Position

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

Step 10: Cut Subfloor for Drain

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!!!

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

Step 11: Assemble Waste Overflow

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.

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.

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

Step 12: Secure Overflow Bracket

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 overtighten as this can crack the tub (which would be super BAD).

Step 13: Assemble Overflow Drain

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

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

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

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.

Yep, that’s a pretty good tip since this Kohler Archer cost me about $800.
One of the biggest mistakes DIYers can 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.

Step 14: Embed Tub in Mortar

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

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.

Step 15: Secure Tub to Studs

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.

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.

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.

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.

Step 16: Watch the Video for All the Details

Watch our video to see all the awesome tips. Steve and I wanted this to be the best installation video for an acrylic bathtub.

If you want to learn how to install an acrylic bathtub this video and tutorial should help a TON.
As you saw it’s not all that bad and when you have a teacher like Steve it’s even easier. Thanks for reading, watching and adding your thoughts to the comments. Do ask any questions down below. We’d be happy to help Make it a great day,

Jeff