How To Build a Frame for A Drop-in Bathtub: 6 Easy Steps


Drop-in bathtubs are beautiful and fit into corners but building a frame can be a challenge. In fact, most bathtub frames have to support thousands of pounds of weight at maximum capacity. 

You can choose to build a drop-in bathtub frame out of wood, concrete, or metal. Whatever you choose, it’s mostly important to get the outer dimensions of the bathtub right. You’ll also want to ensure your corners and seams are sturdy to support the weight. 

What Can You Use for a Tub Surround? 

The tub surround is the wall and surface area around the tub. Luckily, almost any waterproof substance is suitable for a tub surround. In addition, most tub surrounds will go on any tub frame you could choose to buy. For example, one of the most popular solutions is to use cement or concrete board with roll -on waterproofing membrane or a plastic sheeting between the board and the wall studs. This ensures waterproofing and helps to protect your wall. 

Most people eventually put tile, fiberglass, stone, granite, or even acrylic surfacing over this space. Eventually, any water-resistant surfacing will do. However, tile and slate or granite are definitely the most popular. 

Under that, you can use any sturdy material to support your tube. Wood, cast iron, stainless steel, and cement are all popular. Each has different perks, mostly relating to how durable they are and how easily you can work with them. 

Before You Start Building a Tub Surround 

Your tub surround has to meet the triple purposes of aesthetics, water resistance, and durability.  That means paying special attention to weight, materials, and size. 

Drop-in Bathtub Framing Dimensions 

Your tub frame has to fit the full tub and fit inside the surround. In most cases, the tub frame can simply be the size of the tub, under the lip or edge. Checking this means measuring the dimensions of the tub around the lip, all the way around. 

In some cases, the tub might also bow outwards. This means the frame has to be larger than the support for the edge of the tub. However, you normally only see this on old tubs. Instead, chances are, you’ll be able to build a rectangle and slot your tub into it. 

Drop-in tub frames are normally 60” x 30” x 16”. They might be as small as 45 inches long or as large as 72 inches long and might extend as far as 32 inches wide. However, the standard size is 60” x 30”. Therefore, it’s crucial to check the size of your tub before planning the frame. 

Space Needed for Drop-in Tub 

In most cases, you need to plan space based on the size of the tub, plus room for the frame. Other space requirements depend on what kind of finished look you want: 

  • For example, if you want a deck or finished space around the tub, allow at least 6 inches around the tub for this. Most people want at least a small deck between the tub and the wall. 
  • The frame takes up space. If you’re using a wooden frame, you should calculate at least 2 inches larger than the tub on each side. 
  • Finishing takes up space as well. For example, tiles are often as thick as half an inch
  • Most people mortar tubs into place. This means you’ll likely need your tub’s height + 3 inches for mortar. 
  • In addition, you need to have space for plumbing and pipes under or next to the tub. Assess your bathroom and its layout and plan where pipes need to be before laying the tub. 

Essentially, space varies quite a bit. In most cases, it’s the length plus width of your tub plus 2-4 inches on each side for the frame, plus any extras. Let’s take a look at an example. 

If we want a long tub, 72” x 32” installed against the left wall, with a deck against the wall but not in the room, we can start making plans. Let’s also assume that the plumbing has to run under the tub itself because the drain is against the opposite wall, but for aesthetic reasons, we want the headrest in the tub to face the room, not the wall. So, we could calculate: 

72” + 4” (for two 2”x’4” blocks with the 2” side facing outwards), with an additional 2.5 inches of sealant, tile, and mortar on one side + 6” for a deck. Then, 32” + (2×4”) + 2.5” for the sides. Then, if the tub is 32 inches tall, we could add 32” + 3” to allow space for the mortar and plumbing. The result? The frame has to be 84.5” x 42.5” x 35”. 

Drop-in Types 

Different tub sizes have different dimensions. Unfortunately, these are only somewhat standardized. You may find minute differences between brands, especially when you get out of the “standard” tub size of 60” x 30”. Therefore, it’s always a good idea to measure your tube to be sure. 

Long Drop-in Tub 

Long drop-in tubs might be up to 72” long. It’s rare to find one longer than 6’. However, “long” tubs might be the normal 30” width.  

Small Drop-in Tub 

Small drop-in tubs are as short as 45” and as narrow as 24”, although 28-30″ is still more common. 

Medium Drop-in Tub 

Medium drop-in tubs will usually fit the “standard” size range of 60” to 65” long and 30-32″ wide. 

Large Drop-in Tub 

A large drop-in tub is up to 72” long and up to 34” wide. These tubs are similar to long tubs, except they are also wider. 

DIY Drop-in Tub Framing (6 Steps to Follow) 

If you want to build your own tub frame, you’ll need a few basic tools and basic carpentry skills. We’ve focused on building a wood tub surround for this guide because it’s easy and beginner friendly. 

Collect Tools 

You’ll need a small selection of tools including two types of saws for building a tub frame. These include: 

  • A box of No. 9, 2 ½ inch Phillips’s head screws (hardened steel) 
  • A power drill with a cord long enough to reach everywhere, or a cordless drill with a Phillips tip 
  • A circular saw. Alternatively, you can have your 2×4’s cut at the hardware store 
  • A jigsaw. You may also use a circular saw, but it will require more skill to cut the curved parts of the frame 
  • A tube of silicone or a pot of wood glue 
  • A measuring tape 
  • Chalk or a pencil (or both) 
  • A pen and paper to write measurements down 

Measure the Tub 

You always want to ensure you have your tub on hand before you start building your frame. That’s because different manufacturers use slightly different measurements. You’ll have different lip widths, different widths, and different building styles to take into account. So, the first thing you should always do is measure the full tub. 

  • The length and width of the lip around the tub 
  • The full size (rectangle) of the tub of maximum width to maximum length. Even if the tub is oval and not squared at the front 
  • The height of the tub 

These measurements allow you to calculate the necessary height of the supports, the size of the wood you need, and how you’ll install the tub itself. 

So, your tub frame should be a horizontal beam with vertical supports and another horizontal beam on top, in a rectangle. You are, in essence, building two wooden rectangles and adding vertical supports to connect them together. 

The 2×4 on the floor should lay flat, so the wide part of the wood is on the floor as a support. At the top, the 2×4 should be vertical, so that the thin part of the 2×4 is on the tub. So, when calculating wood, you’ll have to subtract 6 inches from the total height of the frame for the vertical supports. 

So, if you have a 24” tub and want it to be another 6” off the floor, you would want 24” supports. 

In addition, you should place vertical supports at least every 16 inches along the full length of the frame. It’s okay to have them closer together but not further apart. So, a 72” x 30” frame should have 6 vertical supports on each side and one in the middle on each end. 

In addition, you may choose to fortify your frame with additional crossbars, running between the vertical supports under your tub. It’s not recommended to put these in until the tub is in place, because it’s difficult to calculate where they should be without calculating the curve and belly of the tub. 

Buy Wood and Supplies 

You normally need 2”x4” boards in fir, pine, or cedar. Additionally, you might choose another type of wood, but it will cost more and isn’t really necessary. You may choose an impregnated version to resist rot. Or you might also choose to treat the wood yourself, but the instructions are not included in this guide. 

Finally, you want the total length of boards calculated for your tub frame. In most cases, you should also get something like 20% extra, giving yourself room to make mistakes. If you’re on a tight budget, get exactly what you need, but try to cut the vertical supports from ends – which should save you some wood. 

You’ll also need a sheet of ½ inch plywood or composite. In most cases, a single sheet is 4’ x 8’ and you’ll need about 2/3rds of the full sheet. 

If you don’t have other tools, you’ll want to acquire them at this time. 

Cut the Wood

Measure your wood 2×4’s and mark them with the chalk or pencil. If you’re cutting multiple supports from the same board, you want to either label those boards or cut one and then measure the next. In most cases, it’s relatively easy to measure everything at once, double check the measurements, and then cut everything together. 

How? If you have 16’ 2×4’s, you can easily divide them based on supports. If you use the tub size we used as an example above, you’d want 4 x 7’ and 14 x 2’ supports. You could choose to supplement this with cross beams later. 

So, you’d cut two 7’ sections out of one board, leaving a single 2’ section over. Then, you’d repeat that. You’d then have to measure 12 2’ sections out onto 2 boards, leaving 4’ over on each, or one 8’ section depending on your preference. You also need four crossbeams, which in this case would be the width of the tub and minus the long horizontal supports (30’ – 4’ = 26”). Measure these and cut them. 

In every case, you’ll want to focus on creating flat ends. The best way to do that is to measure both sides of the board and to use a ruler or a straightedge to draw a straight line across. Then, use your skill-saw or rotary saw’s level to ensure the lines are straight. 

Finally, you’ll want to turn the tub upside down on the plywood. Trace the outer lip of the tub. Then, remove the tub. From here, you’ll want to measure the inside lip of the tub. For example, if the lip extends 2” out from the tub on all sides, you’ll want to narrow the hole you traced by 2”. Most likely, it’s more complicated than that, but you want the hole to narrow just enough that the tub barely fits in. 

If you want to have a tub deck, you’ll want to add an additional row of horizontal frame and supports for the tub. In this case, extend the horizontal support out to the length you want the deck on each side. You’ll then want to build a supporting frame directly around the tub – so you’ll have two rows of supports on the side with the deck. 

Drill Holes

Pre-drilling wood allows you to ensure that everything is measured properly. It also makes it easier to avoid cracking the wood. However, many people prefer not to do it. Go with personal preference here. 

Put Everything Together 

Putting everything together is basically a process of building a box. If you can line corners up and put screws in, you can build your drop-in bathtub frame. 

  1. Start with the horizontal supports. Lay the long boards out with the short boards on the inside. You want the long boards to make up the outer corner on every corner. Use the screws to screw them together. For the strongest fit, use two straight screws on each long side and then put two sideways screws in on the opposite corner. Make sure you embed the screw head into the wood. Repeat for the top and the bottom. 
  2. Use a tape measure and pencil to mark out where the vertical supports should be. Flip the rectangle up and lay the bottom line of supports first. Put screws in from the bottom. Here, you normally want two per board. Additionally, your verticals should be in the same direction as the board they are resting on. So, if you’re going for a narrow surround and have tilted the 2×4 2” side up, they should have the 2” side facing the length of the board. If you want a wider surround and have the 2×4 laying 4” side up, the 4” side of the support should face the length of the board. This means your vertical supports always go edge-to-edge on the horizontal support. Repeat the process of attaching the supports fully around the bottom frame. 
  3. Place the top frame over the supports and use screws to attach them. You may want someone to help you hold things in place. 
  4. Take steps to attach your frame to the floor at this time. If you have a wooden floor, you can use screws. If you have a concrete floor, you may want to rent a nail gun to insert pins into the floor instead. 
  5. Put a bead of silicone or wood glue around the top of the frame and glue the plywood on. You can add screws as well. 
  6. You can now drop your tub in. Make sure you have sufficient help to avoid hurting yourself. 
  7. You can add blocks under the tub or add in extra supports on the frame.
  8. You’ll want to add a layer of waterproofing over this. From there, you can add your tun surround material over the frame. 
  9. It’s also important to keep in mind that you have to make room and calculate water fixtures, plumbing, and drainage pipes into the frame. Best case scenario, both exit the wall under or inside the frame. 

Essentially, if you can do basic corner work and carpentry, you can easily build your own tub frame. 

How Do You Support a Drop-in Tub? 

Chances are, you’ll be loading your tub and you’ll need good support. For example, your standard tub holds 40-60 gallons of water. At 8.34 pounds per gallon of water, your tub holds 333-500 pounds of just water. The tub itself is likely to weigh between 110 and 175 pounds depending on what it’s made of. On the other hand, if you’re using an old cast-iron drop-in tub, it will weigh much more. Combine that with the maximum amount of weight from people inside. Eventually, most drop-in tubs need a frame capable of supporting at least 1000 pounds. 

That means choosing a sturdy material, using the right materials, and using waterproofing. For example, most people eventually use wood 2”x4” for the frame. You may want to impregnate the wood to make it more water resistant. You might also choose to use cast iron or stainless steel or even a cement frame. In this case, we’ll focus on the wooden frame because you can build it with little equipment and little technical skill. On the other hand, building a concrete frame means building a frame for the concrete first. And, building a stainless-steel frame means welding, which normally requires a license. 

Finally, most people put supports under the tub itself. This can be cement. It might also be wood or concrete blocks. Choose something that’s sturdy, the right height, and supports the bottom of your tub. For example, some acrylic tubs actually require cement support along the full bottom of the tub. Most can be installed without these supports, but you might still want to install a frame around the tub to support the bottom, unless it’s resting in the floor. 

Frequently Asked Questions 

Tub frames are simple builds, but they have to support a lot of weight. It’s only natural you’d have more questions. 

What is the Standard Tub Deck Height? 

Tub decks are the platform surrounding the tub. They are never higher than the tub itself. However, you can install your tub as high as you’d like. In most cases, the tub deck is 15-20 inches. A lower tub is easier to get into. A higher one might need steps, or another means of accessibility. 

Why are Drop-in Tubs More Expensive? 

Drop-in tubs range from $900-$1800 in most of the U.S. However, freestanding tubs usually start as low as $500. Despite that, the drop-in tub is normally lighter and smaller. Why the price difference? Most drop-in tubs have to be reinforced around the edges to support the full weight of the tub. That’s true whether you choose acrylic or something fancier like marble or stone. The drop-in tub always needs reinforcement around the lip. 

Conclusion 

Building a frame for a drop-in tub is a relatively simple process of building a box and attaching it to your floor. Complications can arise if you have a very large or heavy tub, if you have to run plumbing under the tub, or if you’d like to drop the tub into the floor to lower the deck. However, in most cases, you can easily build your tub frame in a day. Then, you can waterproof it and finish it with surround material to finish your bathroom.

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