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Replacing Bathroom Subfloor: Step By Step Instructions

Is there a musty smell lingering in your bathroom? Recurring mold or mildew? When you walk across the floor, do you feel it giving way? If so, you may be looking at replacing the bathroom subfloor.

The subfloor is the wood base upon which the finished floor — tile, vinyl, whatever — rests. Subflooring may be topped with underlayment, special material required for installing some finished floors.

If water gets under the finished floor — through cracks, gaps, or other imperfections — the subfloor and underlayment are subject to damage. Rotting is a possibility, creating a dangerously unstable floor. It’s important to quickly investigate any warning signs of subfloor damage.

If the subflooring is too far gone or the damage extends under a bathtub/shower, pedestal sink, toilet, or wall, you’ll likely be better off hiring a professional. But if you catch it early, replacing bathroom subfloor is possible for most DIYers. Read on to learn how to tackle the job.

Person using a drill to install screws when replacing bathroom subfloor

Fix the Underlying Problem

Unless you want to have replacing bathroom subfloor on your to-do list more than once, your first step in repairing it is to find and address the underlying problem.

First, look at the water pipes in and near your bathroom to see if there are any leaks. Check around the toilet, bathtub, and shower to determine whether water might be leaking from any of those places. If your toilet is rocking or loose, that’s also a good indicator of a serious subfloor issue

Also, check around your clothes washer and dishwasher, if they’re near the bathroom, for signs of water leaks. Finally, check around vents on the exterior of your home to see if water is getting into them and from there into your subfloor.

Once you’ve identified and addressed the source of the water affecting your bathroom subfloor, you’re ready to start repairing the damaged area.

There is, however, an exception to this rule for replacing subfloor in bathroom. If water damage has extended through your subfloor into the wooden joists below it, it’s time to call in the professionals. To check the condition of your floor joists, you’ll need to look at them from the underside in your basement or crawl space.

For upstairs spaces, serious moisture or mold issues on ceiling drywall could be an indication of joist damage.

As a general rule, anywhere that subfloor or joist material is flaking or moldy, it needs to be replaced.

Remove the Finished Bathroom Floor

The first step in replacing bathroom subfloor is to remove the existing finished floor. It’s OK to take extra care with this step if you’d like to salvage as much of the existing finished floor as possible.

Your best option, however, is to get rid of the old flooring, in part because it will speed up subfloor replacement.

Remove Toilet, Bathtub/Shower, or Pedestal Sink

As already mentioned, if replacing bathroom subfloor will involve removing your toilet, bathtub/shower, or pedestal-mounted sink, your best bet will be calling in a professional contractor.

One exception for removing bathroom fixtures is your vanity cabinet, a relatively easy DIY job. To remove the vanity cabinet for replacing subfloor in bathroom, start by turning off the water valves and disconnecting the water and drain lines.

Unscrew the vanity cabinet from the wall and use a utility knife to remove the caulk holding the cabinet to the floor. Once that’s done, you can simply slide the vanity cabinet out of the way.


If you insist on removing the bathtub/shower yourself for replacing subfloor in bathroom, be prepared for some serious work. Once you’ve turned off the bathroom water supply, you’ll need to cut into the wall space behind the faucet to access the water cutoff.

Next, you’ll need to use a utility knife to slice the caulk around the bottom of the bathtub/shower. Use a pry bar to loosen the bathtub/shower from the floor, and then maneuver it out of the way.

Bathtubs/showers can be bulky and heavy, so this part of replacing subfloor in bathroom may require more than one person.


To remove a toilet, turn off the water supply valve and flush the toilet repeatedly to empty the water already in it. If your toilet has a separate tank, remove the mounting bolts on its underside and gently lift it away from the toilet.

Once that’s done, rock the toilet gently back and forth to break the wax seal holding it to the floor. Stuff a rag into the now-open drain hole in the floor to prevent sewer gas from entering your home.

Pedestal Sink

To remove a pedestal sink, turn off and disconnect the water supply lines, and disconnect the drain line coming out of it. Next, use a utility knife to cut through the sealant holding the sink to the wall and floor.

Finally, unscrew the sink’s mounting bolts and lift it off the floor. You’ll need a second pair of hands for this part of the job, to hold the sink steady while the mounting bolts are loosened.

Cut Out the Damaged Subfloor and Underlayment

Once you’ve removed any bathroom fixtures as needed, your next step in replacing subfloor in bathroom is to cut out any damaged areas of subfloor and underlayment.

The best tool for this is a circular saw. Set it to cut at a depth that will get through the subfloor and any underlayment, but won’t cut into the floor joists. Cut out the damaged subfloor to a point where no damage is visible, and that is located on top of a floor joist on both ends.

Cutting out a section of floor that has been damaged by water

Strengthen the Floor Joists

If your observation of the floor joists revealed that they had also sustained water damage, you’ll need to address that before installing the new subfloor.

To do that work, first let the damaged joists dry out for a few days. Then, get some boards of the same dimension, or near the same dimension, of the damaged joists. Cut the new joist material to a length that will allow it to be attached to undamaged wood on both sides of the damaged joists.

If the damaged joists are too wet, or show signs of mold or rotting, replace them completely before proceeding with replacing your bathroom subfloor.

Measure and Install New Sections of Subfloor and Underlayment

Once any joist damage is repaired, your next job is to measure and install new sections of the subfloor, and — if required — underlayment. Remember to cut your subfloor and underlayment to a size that will put the edges atop two sturdy joists on either side of the damaged area.

When the new subfloor and underlayment are in place, secure them to the joists with nails placed every few inches. If the subflooring sits atop more than the two joists at either end of the repair, use nails to secure them to the other joists.

If there are any gaps along the edges of the new subfloor, use wood filler or wood shims to fill them. Then, sand the subfloor smooth.

If you’re sanding a large area, an electric sander will be an invaluable tool. Use 100-grit to 120-grit sandpaper for this part of the job.

Install a New Finished Floor

Once you’re done replacing your bathroom subfloor, and any required underlayment is installed on top of it, it’s time to install a new finished floor. If your previous flooring isn’t in bad shape from moisture or from being removed to get to the damaged subfloor, you could reuse it.

Installing new flooring over bathroom subfloor

However, you really should start fresh. In addition to being sure there’s no possibility of lingering moisture or other damage, installing new flooring is a great way to update your bathroom.

There are tons of options for bathroom flooring, from wood to laminate to tile to stone. After the work of replacing your bathroom subfloor, why not have some fun in picking out a new floor, and maybe even installing it yourself?

Replacing Bathroom Subfloor Frequently Asked Questions

Because replacing bathroom subfloor is a serious DIY project, you’ll likely have questions beyond the guidance you’ve just read for tackling the job. Read on for answers to a few of the frequently asked questions about replacing bathroom subfloor.

Are there alternatives to replacing water-damaged subfloor?

In some limited situations, it may be possible to dry out water-damaged bathroom subfloor rather than replace it. If you haven’t noticed any mold forming in your bathroom, and if you’re satisfied after investigating that any water damage is limited, you can try drying it.

That won’t save you, though, from having to remove the finished floor. And once the subfloor is exposed, drying it out is a tedious and lengthy process.

Drying plywood subflooring

For plywood subfloors, the first step is to wipe any water off the subfloor. Next, set up a dehumidifier, operating at its highest setting, in the bathroom.

Along with the dehumidifier, set up as many fans as possible, operating at their highest setting. Point the fans directly at the wet areas of the subfloor. Even when you’ve set up the dehumidifier and fans as efficiently as possible, it will take 24 hours to 96 hours for the subfloor to dry.

Drying oriented-strand board (OSB) subflooring

If you have oriented-strand board (OSB) subflooring, you’ll use the same process as for drying out plywood subflooring. OSB subflooring looks something like plywood, but its surface has a very flaky appearance.

OSB is, however, more likely than plywood to be significantly damaged from water infiltration. If you try to dry out OSB subflooring, you should take steps to be sure that it is, in fact, drying out. One way to do that is to periodically test different areas of the OSB subfloor with a moisture meter.

What is the most durable subfloor material?

As you might have gathered from the first of the frequently asked questions here, plywood and OSB are the most commonly used bathroom subflooring materials. Both, however, are susceptible to moisture-related issues.

In general, though, OSB is seen as the more water-resistant of the two options. OSB does, however, have a tendency to swell along its edges when exposed to moisture. For that reason, OSB is not recommended as subflooring for tile floors.

There are, however, so-called enhanced versions of both plywood and OSB that are coated or made with moisture-resistant materials. Both options should offer improved performance for replacing subfloor in bathroom.

In the end, either plywood or OSB are decent choices for replacing bathroom subfloor. So, it’s likely OK to opt for the least expensive option at building supply stores in your area.

Using a shopvac to clean up after replacing bathroom subfloor

What is the best bathroom flooring underlayment?

Underlayment, which denotes any material placed between the subfloor and the finished floor in a bathroom or elsewhere, performs a number of functions. For one, it smoothes out any imperfections in the subflooring so that the finished floor is level.

Underlayment can also protect against moisture infiltration, and has some sound-deadening qualities as well. The type of underlayment used in bathroom flooring projects will depend on the type of flooring being installed.

Flooring manufacturers will routinely provide guidance on the best underlayment to use with their products. You can also check with your local home improvement or building supply store for guidance.

Wrapping up Replacing Bathroom Subfloor

For both safety and health reasons, replacing bathroom subfloor weakened by water infiltration or other damage is an important DIY skill to develop. Now that you know how to do it, you can be confident in your ability to keep your family protected.

Happily, DIY Painting Tips also offers tips and tricks for a full array of other, more pleasant bathroom DIY projects. Take a look at posts on choosing bathroom paint colors, the easiest way to paint behind a toilet, and the best paint sheen for bathrooms.