How to Replace Subfloor in Bathroom: Step-by-Step Guide

Your bathroom is an essential space in your home, providing comfort and functionality day in and day out.

However, over time, the bathroom subfloor can deteriorate due to water damage, wear, or other factors.

A weakened subfloor not only compromises the structural integrity of your bathroom but can also lead to costly repairs if addressed.

But worry not!

In this blog, I will walk you through how to replace subfloor in bathroom, starting from the initial preparation and assessment to the final installation of your new bathroom flooring.

Replacing a Bathroom Subfloor

Whether you’re dealing with water damage, rot, or simply upgrading your bathroom, a well-installed subfloor is crucial for the stability and longevity of your flooring.

In this section, I will take you through the step-by-step process of replacing a bathroom subfloor, ensuring a successful and safe renovation.

Step 1: Remove Old Flooring and Plumbing

  • Use a pry bar and a hammer to lift the edges of the old flooring carefully.
  • If it’s carpet, you may need to peel it back gently.
  • For tiles or vinyl, score along the edges using a utility knife.
  • Then, use the pry bar and hammer to loosen the flooring and lift it off the subfloor gently.
  • Turn off the water supply to the bathroom.
  • Disconnect the water supply line at the base of the toilet tank.
  • Drain the water in the bowl by flushing the toilet.
  • Use a wrench to loosen and remove the nuts securing the toilet to the floor.
  • Pull the toilet off the flange and set it aside.
  • Use a wrench for the shower and vanity to loosen and disconnect the water supply lines.
  • Remove any screws or fasteners holding the fixtures in place with the water supply lines disconnected.
  • For the shower, you may need to remove the showerhead and handles.

Required Tools

  • Pry bar
  • Hammer
  • Utility knife
  • Wrench

Step 2: Check Joists and Support Beams

  • Take a close look at the floor joists, the sturdy beams that support the subfloor.
  • Check whether they are damaged, cracked, or showing signs of rot.
  • Measure the distance between the existing joists to determine the length needed for the support pieces.
  • Cut the 6-inch pieces of wood to the appropriate length using a saw.
  • Secure the support pieces to the support beams by drilling screws through the wood pieces into the beams.
  • Ensure the support pieces are placed 36 inches apart to ensure even and adequate support across the entire subfloor area.

Required Tools

  • Measuring tape
  • Saw
  • Drill
  • Screws

Step 3: Remove Subfloor & Joists

  • Locate the screws that secure the subfloor to the joists and remove them carefully using a screwdriver.
  • Use a crowbar to pry up the damaged section of the subfloor gently.
  • Be patient and work gradually to avoid splintering or breaking the wood.
  • Use a saw to cut into the wood along the damaged area carefully if necessary.
  • Take caution not to cut too deep and damage the joists underneath.
  • If any floor joists are damaged beyond repair, it’s essential to replace them to ensure the structural integrity of the bathroom floor.

Required Tools

  • Screwdriver
  • Crowbar
  • Saw

Step 4: Install Joists

  • Take 2×4 pieces of lumber and position them parallel to each other, spaced about 36 inches apart.
  • These will serve as the new joists to support the subfloor.
  • Place the 2x4s perpendicular to the floor joists underneath, ensuring they run across the width of the bathroom.
  • The 2x4s should align with the direction you plan to install the new subfloor.
  • To anchor the 2x4s securely, use L-brackets and concrete screws.
  • Position the L-brackets on both sides of each 2×4 where it meets the concrete floor.
  • Use a drill in “hammer mode” to drive concrete screws through the L-brackets and into the concrete, firmly securing the 2x4s in place.
  • Add additional support by installing braces between the ends of each 2×4 at intervals of about 36 inches.
  • The braces will add stability and prevent the 2x4s from flexing.
  • Secure the braces to both the 2×4 and the floor joists underneath using wood screws.

Required Tools

  • 2×4 lumber pieces
  • L-brackets
  • Concrete screws
  • Drill
  • Wood screws

Step 5: Measure and Cut New Subfloor

  • Use a measuring tape to carefully measure the dimensions of the bathroom floor where the new subfloor will be installed.
  • Include any areas around the edges where the subfloor will extend to the walls.
  • Identify the locations of pipes that come up through the floor, such as those for the toilet, shower, or vanity.
  • Mark the positions of these pipes on the new subfloor material, as you’ll need to cut holes to accommodate them.
  • Transfer the measurements and pipe hole locations from the floor onto the new plywood or cement board using a carpenter’s pencil.
  • Use a circular saw to cut the new subfloor material along the marked lines.
  • For pipes coming up through the floor, use a hole saw to cut holes in the subfloor material carefully.
  • The holes should be slightly larger than the pipes to allow for proper fitting.

Required Tools

  • Measuring tape
  • Carpenter’s pencil
  • Circular saw
  • Hole saw

Step 6: Install New Subfloor

  • Before permanently attaching the subfloor, place the plywood pieces on the joists to ensure they fit snugly together.
  • Check that the edges align properly and that there are no gaps between the pieces.
  • Once the dry fit is successful, begin laying the new subfloor on the joists.
  • Start at one corner and work your way across the bathroom, ensuring the pieces fit tightly together.
  • Use screws to attach the subfloor to the joists securely.
  • Space the screws about 6-8 inches apart along the edges and 12 inches apart in the center of the plywood pieces.
  • Ensure that the screws are driven straight down into the joists for a solid connection.

Required Tools

  • Screws
  • Screwdriver or drill

Step 7: Seal Joints (If Using Cement Boards)

  • Use a notched trowel to spread the joint compound over the seams between the cement board pieces.
  • The notched trowel will help create an even and consistent layer of compound.
  • The joint compound will fill in any gaps or uneven areas between the cement board pieces, providing a seamless surface for the tile installation.
  • Immediately after applying the joint compound, place adhesive tape over the wet compound along the seams.
  • Press the tape down firmly to ensure it adheres well to the joint compound and the cement board.
  • Use the flat edge of the notched trowel or a putty knife to smooth the tape and remove any excess joint compound.
  • The goal is to create a smooth and level surface without any bumps or ridges.
  • Let the joint compound dry entirely for a few hours.

Required Tools

  • Notched trowel
  • Joint compound
  • Adhesive tape
  • Putty knife

Step 8: Check for Slope

  • Position the spirit level on various areas of the bathroom floor, starting from different corners and moving toward the center.
  • Observe the bubble in the level. A slight slope is necessary to ensure water flows towards the drain.
  • The bubble should move slightly towards the drain end, indicating a gentle slope.
  • If the bubble stays centered or moves away from the drain, the floor is not sloping correctly.
  • To make adjustments, add shims under the subfloor where needed to achieve the desired slope.
  • After making adjustments, recheck the slope with the level to ensure it directs water toward the drain.

Required Tools

  • Spirit level
  • Shims

Step 9: Install Bathroom Plumbing Fixtures

  • Position the toilet flange over the pipe that comes up through the floor.
  • Carefully lower the toilet bowl onto the flange, ensuring the wax ring creates a watertight seal between the toilet and the flange.
  • Press down on the toilet bowl to compress the wax ring and create a secure connection.
  • Reattach the nuts to the bolts at the base of the toilet bowl and tighten them evenly.
  • Avoid overtightening to prevent damage.
  • Install the shower pan or base using a recommended adhesive.
  • Attach the shower drain to the drainpipe underneath, ensuring a watertight connection.
  • Assemble and install the shower walls or enclosure, following the provided guidelines.
  • Position the vanity cabinet in its new location and check that it is level and even.
  • Secure the vanity to the wall using screws and anchors, ensuring it is stable and won’t tip over.
  • Reconnect the plumbing fixtures, such as the sink and faucet, making sure the connections are tight and leak-free.

Required Tools

  • Wrench
  • Screws and anchors

Step 10: Install Bathroom Flooring

  • Choose the type of flooring that suits your bathroom’s style, preferences, and needs.
  • Common options include ceramic or porcelain tiles, luxury vinyl, laminate, or waterproof engineered wood.
  • Begin installing the flooring material from the center of the room.
  • This ensures a balanced layout and reduces the chance of ending up with small, uneven pieces at the edges.
  • For tile installation, apply the adhesive or mortar on the subfloor using a trowel.
  • Place the tiles one by one, pressing them firmly into the adhesive.
  • Use tile spacers to maintain even gaps between the tiles.
  • This will ensure a professional-looking finish.
  • Continue laying the tiles until the entire bathroom floor is covered.
  • Typically, these types of flooring come in interlocking planks or tiles, making installation straightforward.
  • Once the flooring is fully installed, remove any excess adhesive or mortar and clean the surface as needed.
  • Install baseboards or molding around the edges of the bathroom to create a finished look.

Required Tools

  • Flooring material
  • Trowel
  • Tile spacers
  • Saw
  • Baseboards or molding

Step 11: Clean Up

  • Properly dispose of the old flooring materials according to local waste disposal guidelines.
  • If possible, recycle or donate any reusable items.
  • Use a broom to sweep the subfloor thoroughly.
  • Use an air compressor or a vacuum cleaner with a suitable attachment to blow away dust and dirt from hard-to-reach areas and crevices.
  • Give the installed plumbing fixtures a final cleaning using mild soap and water.

Required Tools

  • Broom
  • Air compressor or vacuum cleaner

Remember, replacing a bathroom subfloor is a significant task, so take your time, follow the steps carefully, and seek help from a professional if needed.

Bottom Line

Replacing the subfloor in your bathroom may seem daunting, but armed with the knowledge and step-by-step guidance from this blog, you’re now well-prepared to tackle this renovation.

Remember, proper preparation, careful installation, and attention to detail are the keys to success.

With determination and patience, you’ll soon witness the transformation of your bathroom, boasting a fresh, reliable, and appealing subfloor that will stand the test of time.


How much does it cost to replace a subfloor in a bathroom?

The total cost of replacing a bathroom subfloor can also vary based on the size of the bathroom. Here are the estimated cost ranges for small and large bathrooms:

  • Small bathroom: The cost may range from $100 to $380, depending on the square footage and the chosen subfloor material.
  • Large bathroom: For a larger bathroom, the cost can be $380 to $570, considering the increased square footage and potential material requirements.

What are 3 common subfloor materials?

Particle board, OSB (Oriented Strand Board), and plywood are three typical subfloor materials.

Plywood is a sturdy and durable sheet material made from thin layers of wood veneer, making it an excellent choice for subflooring.

OSB, composed of wood strands and adhesive resins, offers strength and moisture resistance at a more budget-friendly cost.

Particle board, made from wood particles bonded with resin, is a cost-effective option but may be less durable and moisture-resistant than plywood and OSB, making it more suitable for low-traffic areas.

How thick should bathroom subfloor be?

The ideal thickness for a bathroom subfloor should be at least 1 ⅛ inches (approximately 29 mm).

This thickness ensures that the subfloor is strong, stable, and capable of supporting the weight of the flooring material, especially when using tiles.