How To Fix Rotten Floor Joist Under Bathtub In 7 Easy Steps

There is hardly anything better than taking a bath to unwind after a rough day. However, you can’t expect to keep your floors completely dry and pristine while you’re enjoying the hot stream. Splashes happen, and the area around and under the tub is the most exposed to humidity problems. Water can easily leak through the hardwood or tiles and damage the floor joists. When this happens, how to fix them?

The only way to fix rotten floor joists under the bathtub is by replacing them. To do that, you have to remove the bathtub and expose the subfloor. Cut out the damaged joists and replace them with new ones. You can then install a new subfloor and put the tub back.

How To Repair Rotted Floor Joists

Floor joists are daunting to repair because of the mess you’re facing, but the process isn’t overly complicated. The hardest part will be to remove the tub.

Things You Will Need

  • Replacement wood joists
  • 5/8-inch plywood
  • Pry bar
  • Hand saw
  • Circular saw
  • Jigsaw
  • Hammer
  • 2-inch wood screws
  • Belt sander
  • Floor leveling compound
  • Electric drill driver
  • Drill bits
  • Galvanized nails
  • Carpenter’s square
  • Pliers
  • Protective equipment
  • Belt sander
  • Construction adhesive
  • Tape measure
  • Paintbrush

Step 1 – Remove the tub and prep the area

Before you start working, put on your work gloves and goggles, then turn off the water at the main. Use the pliers to remove the water faucet and tub drain, then remove the tub from its place.

This step is easy if you have a freestanding tub, but you might have to use heavy-duty tools to remove a built-in fixture. If you have any bathroom cabinets or a vanity near the tub, remove it too, as you’ll need ample space.

Once you’ve moved all items, use the pry bar to remove any protruding nails, as well as the baseboards and floor area that has to be replaced. If the damage is extensive, you might have to remove the entire bathroom floor.

Step 2 – Inspect the area 

Look at the floor joists under your tub and determine which ones need replacement and which are undamaged. Use a carpenter square and pencil to draw a rectangle around the area that you have to replace.

Step 3 – Remove the damaged joists 

Once you’ve delineated the damaged area, work within it and remove all damaged joists. Use a pry bar or hammer to pull out any fasteners. If the joists are fixed with screws, you may find it easier to use an electric screwdriver and the appropriate screw bit to remove them.

After you’ve removed all fasteners within your work area, use a circular saw or handsaw to cut out the entire damaged section.

Step 4 – Let the area dry

While you should remove all damp and rotten joists, it is best to leave the entire subfloor to dry out for a day or two.

Step 5 – Cut the new joists and subfloor to dimension 

Measure the size of the gap you just cut out and use a saw to cut the new joists to dimension. Once you have the joists, cut an appropriately sized piece of plywood to use as a subfloor.

Smoothen all ends and edges with 150-grit sandpaper and treat the joists and plywood subfloor with a waterproof coating before installing them.

Step 6 – Install the new joists and subfloor

Place each joist in its position and fasten the area with nails or screws. Reinforce each joint with a piece of wood screwed to the joists, then install the plywood subfloor over the joists.

Fasten the subfloor with wood nails. Use a jigsaw to cut out a new bathtub drain hole into the subfloor, then use a piece of sandpaper to smoothen the edges.

Apply a new coat of waterproof sealant and let it dry for at least 24 hours before installing the floor and tub.

Step 7 – Install the floor and bathtub

Bathtubs need a level floor for stability purposes; thus, the first thing to do before getting started is leveling the floor with an appropriate compound. Use a level to make sure the surface is straight but that it still keeps a slope of 1/4 inch per foot toward the drain.

When the compound is dry, proceed to install the new tiles or hardwood floors (or any other type of floor you have in your bathroom), then put the tub back in its place.

If you have a freestanding tub, simply hook the drain and faucet, and you’re ready to go. For a fitted tub, use construction adhesive to fix it in place, then install the drain and faucet. Don’t forget to turn on the water, and you can start enjoying your bathroom once again.

8 Signs That You Need To Repair Floor Joists With Water Damage

Water damage under the tub is often hard to notice. Nevertheless, you can look out for signs that indicate a damaged subfloor.

1. Misaligned Tub

A misaligned tub is one of the main indicators that you have rotten floor joists. While you may not be able to tell whether the tub is aligned or not just by looking at it, you can check it out with a level.

Alternatively, get into the empty tub and make slight hopping or sliding movements. An unbalanced tub will move, whereas you may feel it sinking if the floor underneath it is rotten. 

2. Musty Smell

Whether your tub is aligned or not, another common sign of floor problems is a heavy, musty smell. The smell comes from the decaying wood, and you won’t be able to get rid of it even if you keep the window open or the extractor fan on.

This odor can also signal a mold or mildew issue. These microorganisms can cause allergies and a variety of respiratory illnesses, so you must deal with them immediately.

3. High Humidity Level

Water soaked into wooden subfloors or joists doesn’t dry out fast, but it still evaporates. As it does, the moisture rises and gets right back into your bathroom. Thus, if you notice abnormal humidity levels in your bathroom long after you’ve bathed or showered, you could have rotten floor joists or subfloors.

To prevent issues (because you can’t always tell whether the humidity level is high or not), it is best to keep an indoor thermometer/hygrometer in your bathroom.

4. Recurring Mold and Mildew

We mentioned the musty odors above and told you they are often the result of mold and mildew. Thus, if you’re dealing with recurrent mold and mildew in your bathroom (you’re cleaning it just to see black spots reemerging after a couple of days), it could be because of a soaked subfloor.

5. Gaps in Tile Grout

To have rotten joists, water must first find its way to them. And the only way to do that is through your floors. Thus, it doesn’t come as a surprise that rotten joists go hand in hand with gaps in your tile grout or hardwood caulk. 

Inspect your floors and look for small holes or cracks in the grout. If they’re present, it is likely that your joists are damaged.

6. Spongy Floors

If you have hardwood floors in your bathroom, also look for signs of floor damage. Water-damaged wood is easy to spot as the area becomes swollen and spongy. You may also feel it wobbly as you’re stepping on it or as you lay in your bathtub – in this case, the tub is also likely misaligned. 

7. Water Stains

If you have a freestanding tub, you should also inspect for water stains under it. Damp spots under the tub indicate that water pools there for long periods, and you can rest assured it will eventually start to leak through the floor.

8. Slow Leaks

Spills and splashes aren’t the only way the joists under the tub can get damaged. All fixtures in your bathroom can leak, including the tub’s drain or faucet. 

Sometimes, these leaks are so slow that it might be hard to notice them unless you’re specifically looking for them.

Make a habit of checking all fixtures when you’re cleaning the bathroom. Pat the faucets and look under the tub – if possible – to make sure the area is dry before you start cleaning. If the faucet is wet and the floor under the tub is damp, you may have a leak.

In addition to repairing the rotten floor joists, you should also fix the damaged fixture. Otherwise, your new joists will also rot sooner or later.

How To Replace Floor Joist Without Removing Floor

Removing the tub and floor when repairing damaged joists is indicated because it gives you the possibility to also repair the floor and subfloor. However, if you don’t want to make such a mess, you could replace the joists only. Here’s how to do it.

Step 1 – Go to the area below your bathtub 

In most homes, you can access the area below a ground floor bathroom through a crawl space at the side of the house or through the basement. Go to the area under the subfloor and identify the joists under the bathtub.

Step 2 – Mark the damaged area and remove the joists 

Follow the same steps above to determine which joists have to go and which are in good condition. Use a carpenter square and pencil to mark a rectangle, then use a saw to remove the damaged joists. 

Step 3 – Replace the rotten joists with new ones 

Cut the new joists to size and fasten them to the floor joists you’ve left untouched. Use sill plates to reinforce the joists and enhance structural support.

Warning: While this method allows you to replace the joists without removing the floor and subfloor, keep in mind that they might also be damaged and weak. You should only access the joists from underneath the subfloor if you’re sure the floor and subfloor are solid enough to support the tub’s weight as you’re working.

Frequently Asked Questions

Although replacing the rotten floor joists isn’t complicated per se, the project might still intimidate the not-so-confident DIYers. If you still have questions, check out the answers below. 

How much does it cost to repair a rotten floor joist?

Joist replacement can cost you between $10 and $40 per square foot, so the total cost of the project depends on how big the damaged area is. You should set aside at least $300 for minimal joist sistering and between $2,000 and $5,000 for minimal joist replacement.

These prices include labor and materials, but they don’t include the costs of opening a floor or ceiling and repairing them afterward, nor the costs incurred to remove and reinstall the bathtub.

Is there a subfloor under the bathtub?

Yes, most houses have a plywood subfloor under the bathtub. However, this isn’t a rule. Your house may also have concrete subfloors. 

A plywood subfloor is easy to remove and replace, but it is also more likely to rot due to water damage. Concrete subfloors last longer, but the material often cracks. While these cracks don’t alter their structural stability, they give water a perfect spot for leaking and damaging the floor joists. 

As you can imagine, concrete subfloors are also harder to remove and replace compared to plywood.

Can you drill holes in floor joists for plumbing?

You can drill holes in floor joists for plumbing as long as the diameter of such holes isn’t larger than one-third of the joist depth and if it is at least two inches away from either edge. 

Considering that standard joists are spaced 16 inches apart, it is often better to run plumbing pipes through these spaces rather than cutting holes in the joists.


Rotten floor joists can cause a lot of problems in your bathroom, from bad smells to high humidity and mold problems. But the main issue is that they can become a hazard if they lose structural support. Unless you want your floor and tub to collapse, you have to repair them.

Have you ever repaired floor joists and subfloors yourself, or would you rather hire a specialist? Tell us in a comment.

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