How to Fix Rotten Floor Joist Under Bathtub

Do you have a rotten floor joist under a bathtub? If so, you need to fix it. Because of the nature of the use of our bathrooms, the floors’ frequent contact with water is inevitable.

Bathroom repairing or remodeling is a must if you need to upgrade the old decor.

Wiping out spilled water on your bathroom floor may be simple, but if not done correctly, it may penetrate to the joist below your bathtub.

As a result, you may end up repairing a severely rotted subfloor. To reduce wiping out spilled water, you can use the best suitable air bathtub as the bathtubs are so deep.

How do you fix the rotten floor joist? Or, how to repair floor joists with water damage?

You could be fixing the entire floor joist under your bathtub or just a part of it. It depends on how far the damage or rotting has gone.

Steps to Fix a Rotten Floor Joist under a Bathtub:

Your home’s subfloor is an essential architectural component because it supports its first floor. The subfloor secures the floor joists and acts as a leveled foundation for the house’s finished floors.

When the subfloor gets soaked, you should start the repair and drying process right away.

If you are wondering why you should note that the damage caused by water spread up quickly. Use the following steps to fix a rotten floor joist under a bathtub.

Step 1: Stop the Water

Before repairing your severely rotted subfloor, stop any water source in your bathroom. It would be wise to close the primary water source to your bathroom other than just the tubs.

Step 2: Expose the Subfloor

Depending on your bathroom’s floor type, use the appropriate tools to remove the baseboards found around the affected areas.

Pull back the carpeting to your bathroom’s nearest wall. If your bathroom floor gets made of vinyl, cut out only the sections over the rotten floor joist.

You will have to replace them after the repair. It is also advisable to inspect the adjacent bathroom walls as you remove your bathroom’s baseboard.

As a result, you will be able to identify any sign of damage from the water.

Step 3: Marking the Area

After opening the subfloor, you need to mark all the damaged areas using a proper marking object like chalk or a marking pen.

You may wonder why? Because the marks will help you make accurate cuts of the damaged subfloor. You could get concerned about how to find the floor joist with water damage under the bathtub.

Fortunately, there exist several ways, but the best accurate and easy-to-use method is by using a stud finder.

Step 4: Cut the Subfloor Carefully

After marking the severely rotted subfloor, put on protective gear like a dusk mask, gloves, and eye protection.

Then take a circular saw and cut out all the damaged areas carefully following your mark guides.

Ensure you set your circular saw to a depth of 1 – inch. Also, cut as near as you can to the floor joists.

Step 5: Remove the Damaged Materials

Use a pry bar to remove all the damaged subflooring and joist. You may need to work slowly to avoid much splintering around the area.

Loose nails may harm you or form weak joints. Ensure that you get rid of them too. Use a heavy-duty trash bag to dispose of everything. For the remaining debris and dust, clean them up using a shop vac.

Step 6: Quicken the Drying Process

Before finishing your repair, give the damaged area enough time to dry.

Allow it to air and dry for several days to avoid damp materials, which is an ideal environment for mold and bacteria.

But you can use a dehumidifier or several large fans to make the area dry faster.

If, after opening the subfloor, you find some mold, you may need to take care of them before going ahead with the subfloor work.

Step 7: Reinforcing the Floor Joists

When the area with the severely rotted subfloor and floor joist under the bathtub dries completely, use 2 x 6 – inch boards to side the joists.

You can also use lumber blocking to span the spaces. In doing this, you will strengthen the joists damaged by water.

Step 8: Installing a New Subfloor

It is the last step to fix a rotten floor joist. After strengthening the floor joists under your bathroom, you need to cut a fitting subfloor material to the area removed.

It is essential to take measurements of the area removed to get the exact subfloor material size ideal for replacement.

Mostly, the new subfloor material may expand. You, therefore, need to allow a gap of 1/8 – inch between the new and the old materials.

8d galvanized nails or deck screws will work out great in fasting down your new subflooring. But if you want to finish the installation faster, a nail gun or rapid-load screw gun would be ideal to use.

You need not worry if you don’t have either of them or a budget to buy one. You can rent one from your nearest home improvement center.

Repairing a severely rotted subfloor will take the above steps to the latter.

But you may wonder how to know if the floor joist under the bathtub requires fixing when your bathroom floor gets made of tiles.

Generally, it is quite a challenge to know if the subfloor is damp if the floor gets made with tiles. Also, it becomes difficult to dry a subfloor under a tiled floor.

For your insight, here is how you deal with a severely rotted subfloor on a tiled bathroom floor.

Fixing Rotten Floor Joist under Bathtub in a Tiled Bathroom Floor

Damp areas are a breeding ground for molds and bacteria. If you recognize either of them anywhere in your bathroom, the possibility of you having a severely rotted subfloor is high.

The only way to confirm is by pulling up the tiles around the area that frequently gets water or where you notice molds or humidity. But it’s not a smooth ride, as it will require much of your time and energy.

But if you don’t want to undo your bathroom floor blindly, you should seek the services of a restoration contractor.

He or she will use specialized equipment designed for checking dampness below floors.

Though you may part with some money, the contractor will save you time and damage your tiles unnecessarily.

In case the subfloor or joists requires repair, you get to use the above eight steps to repair your rotten floor joist.

Sometimes it’s hard to know if your floor joist requires fixing. To enlighten you, here are some tips to indicate that your subfloor is severely rotted and needs repair.

Tips on Whether Your Floor Joist under Bathtub is Rotten and Requires Fixing

a) Tub Level

If you have no access to under your bathtub like an alcove bathtub, the best way to check if its floor joist is rotten and requires repairing is by checking its level.

A cracked grout or caulk can indicate that your tub has sunk. Also, try hopping up and down while inside the bathtub.

If the bathtub moves within its location, it shows that the area beneath has become weak. It could be because the joists or subfloor have rotted.

b) Water Stains

If a surface has a water stain, it may indicate that the place is always getting wet.

As a result, the surface may get damp up to below the floor. It is, therefore, crucial to check such stains out.

c) Slow Leak

If you notice a slow leak in your bathroom, and especially under the drain and faucet, you may need to check out your bathtub underneath.

The leakage may have happened for an extended period causing the floor joist under tub or subfloor to rot.

Also Read: How to Measure Bathtub Drain Size.


As explained above, it is inevitable for our bathrooms not to have rotten floor joists under a bathtub due to their constant encounter with water.

But repairing a severely rotted subfloor of your bathroom requires removing the bathroom tub, and it’s not an easy task. Fortunately, using the guide explained above will make it easy.

But if you don’t trust yourself to do it right or haven’t enough time to carry out the process, you can employ the services of an expert in home renovations.

Also, if you have no idea whether your subfloor or floor joist under the bathtub is rotten and requires fixing, use the tips discussed above to find out.

If they are rotten and need repair, you may have to do so immediately. Rotten floor joist under bathtub and severely rotted subfloor may interfere with your bathroom underneath the structure and its strength.