Replacing a subfloor under a wall can seem like a daunting task, but with the right preparation and tools, it can be done successfully as a DIY project. A subfloor provides the foundation for your flooring and needs to be strong and level to support the flooring materials. When part of the subfloor is damaged, rotten, or uneven under a wall, replacing it is necessary before installing new flooring. With attention to safety and step-by-step process, you can replace the subfloor section without taking down the entire wall.

Determine if Subfloor Replacement is Necessary

There are a few signs that indicate it’s time to replace part of the subfloor under a wall:

Spongy or uneven areas under the flooring near the wall – This indicates rot or deterioration of the wood subfloor. Pressing down on the flooring and feeling too much give means the subfloor cannot properly support the flooring.
Water damage or signs of moisture – Water leaks, flooding, or excess moisture can cause the subfloor to rot, warp, or grow mold. Stains, soft spots, or a musty smell near the wall are signs of water damage.
Hollow sounds when walking on the floor near the wall – Hollow or popping sounds can mean the subfloor is warping or detaching under the flooring. This indicates it is not securely fastened.
Creaking near the wall – Noise when walking near the wall is often due to loose decking or deterioration of the subfloor.
Planning to update flooring – When installing new flooring, it’s wise to check the subfloor for levelness and damage first. Any problem areas should be addressed before the new floor is installed.

If you notice any of these issues near the wall, it’s best to replace that section of the subfloor before re-installing flooring.

Gather Tools and Materials

Replacing a section of subfloor will require specialized tools. Renting or borrowing some items can save money. You will need:

Pry bar – for removing baseboards and pulling up flooring
Circular saw – for cutting subfloor
Reciprocating saw – for cutting subfloor edges around wall studs
Hammer
Safety equipment – gloves, goggles, mask/respirator
Replacement subfloor panels
Screws or nails
Wood putty or filler
Measuring tape
Level
Chalk line

Other supplies that will make the process easier include a mini-sledgehammer, jigsaw, oscillating multi-tool, and power drill. For safety, be sure to use proper precautions and ventilation when using power tools.

Prepare the Work Area

Replacing a subfloor section while leaving the wall intact takes careful preparation:

Mark the boundaries – Measure the area of the subfloor needing replacement and use a chalk line to mark the edges to guide your cuts. Extend the replacement area 2-3 inches past any water damaged or rotten areas for the best adhesion to solid subfloor.
Clear the area – Remove any furniture, rugs, flooring, debris, or items leaning against the wall to allow full access.
Detach from wall – Remove baseboards and use a pry bar to detach the flooring from the wall. You need to be able to lift the flooring to access the subfloor.
Watch for plumbing or wiring – Turn off water lines in the area and be aware of any electrical wires under the floor. Cut around them carefully or have a professional relocate them if needed.
Contain dust – Use plastic sheeting around the work area to reduce dust spreading through the home. A fan blowing out a window also helps.

Lift and Remove the Damaged Subfloor

With the area prepped, it’s time to remove the subfloor section:

Pry up flooring – Use a pry bar and hammer to detach flooring nails/staples from subfloor. Remove flooring pieces completely.
Cut subfloor – Use a circular saw to cut through the subfloor on your marked lines. Make the cuts about 1/8″ deep to avoid hitting any pipes or wires.
Chisel out subfloor – Use a chisel and hammer to remove the subfloor inside the cut lines in sections. Be careful not to damage wall studs.
Cut around studs – Use a reciprocating saw to cut the subfloor flush with the wall studs. Cutting around each stud frees the subfloor piece.
Remove subfloor section – Once cut free on all sides, pry and pull the damaged subfloor piece out. Discard removed subfloor.
Check plumbing/wiring – Inspect to ensure no pipes or wires were hit when removing subfloor. Address any issues before moving on.

Thoroughly removing all of the damaged subfloor prepares for the new subfloor installation.

Install New Subfloor Pieces

With the subfloor removed, it’s time to install replacement pieces:

Measure opening – Measure the opening to determine what size replacement subfloor panels you need. Account for any unevenness against the wall studs.
Cut subfloor panels – Using a circular saw, cut your replacement subfloor panels 1/4″ smaller than the opening. This allows expansion room. Make pieces snug against wall studs.
Dry fit new subfloor – Place the new subfloor panels into the opening without nailing/screwing to ensure they fit correctly. File edges if needed.
Apply construction adhesive – Use quality wood construction adhesive along the edges of wall studs and on joists to “glue” the subfloor in place.
Attach to wall studs – Hold panels flush to wall studs and use 2″ screws driven into studs to secure. Stagger screws every 8-12 inches.
Attach to joists – Screw replacement subfloor into floor joists, spaced 6-8 inches apart. Stagger screws to adjacent joists.
Fill gaps – Use wood putty to fill any visible gaps against wall studs or at seams. Let putty dry completely.

Attaching the new subfloor securely and sealing edges prevents future movement or moisture issues.

Replace Flooring

With the new subfloor installed, the final step is replacing your flooring:

Sand or grind subfloor – Lightly sand or grind the new subfloor area so it is flush with the existing subfloor. Smooth any uneven edges.
Deep clean floor – Scrub and mop the floor to remove sawdust, adhesive, and dirt from construction. Allow to fully dry.
Reattach flooring – Starting at the wall edge, attach original or new flooring using a power nailer, stapler, or adhesive. Tap flooring into place gently with block.
Replace trim – Reinstall baseboards, quarter round, and any other trim pieces using a nail gun or wood glue. Fill nail holes.
Seal edges – Use silicone caulk to seal flooring edges against the baseboard and wall. This prevents moisture from seeping under.

Once the replacement subfloor section is covered with flooring, you should not be able to tell anything was replaced! With the right prep, tools, and safety measures, this is a DIY project that can give your floors new life.

Frequently Asked Questions About Replacing Subfloor Under a Wall

Replacing part of a subfloor under an existing wall brings up many questions for homeowners doing DIY projects. Here are helpful answers to some of the most common questions:

What type of tools are needed?

You will need standard demolition and construction tools like a pry bar, hammer, circular saw, reciprocating saw, level, tape measure, safety gear, nails/screws, and wood filler. Specialty tools like an oscillating multi-tool or jigsaw can also help with detailed cuts.

How far should the subfloor replacement extend?

It’s best practice to cut back the subfloor replacement 2-3 inches beyond any water damaged or rotten areas. This ensures you are attaching the new subfloor to solid edges.

Does the drywall on the wall need to be removed?

In most cases, no. You can cut and pry up the subfloor without taking down drywall. Be very careful not to damage drywall edges with tools when prying up subfloor pieces.

Can I walk on the floor during subfloor replacement?

It’s best not to walk on the flooring around the exposed subfloor area to prevent breaking through weakened sections. Use boards or plywood to spread your weight if needing to access the work area.

What is the easiest way to cut the subfloor?

Using a circular saw to cut perimeter outline lines about 1/8″ deep makes cutting through the subfloor with a chisel/hammer easier and cleaner. Reciprocating saws also work well.

How do I match new subfloor height to existing floor?

Use a sander or grinder to bring the level of the new subfloor pieces flush with surrounding subfloor before re-installing flooring. Feather edges smooth.

Can I use both screws and nails?

Yes, you can use both. Typically screws are used to anchor the subfloor to wall studs, while nails are used along the joists. Stagger them in a grid pattern.

Should I use construction adhesive?

Yes, adhesive is recommended to “glue” the new subfloor pieces to the edges of wall studs for maximum stability and to reduce any creaking.

How soon can I walk on the replaced subfloor?

It’s best to wait 24 hours for construction adhesive to fully cure before walking over the replaced section. Once secured properly, it should support normal foot traffic.

Replacing just a section of subfloor while leaving walls intact is very doable. Following proper precautions and process can lead to success!

Step-by-Step Guide to Replacing Subfloor Under a Wall

Replacing rotten, water damaged, or uneven subfloor under a wall is an involved but doable DIY project. Follow these step-by-step instructions for smooth process:

Step 1: Prep the Work Area

Clear space of furniture and floor items near the wall
Detach baseboards with pry bar
Use pry bar to lift flooring edges from wall
Vacuum area thoroughly to remove debris
Mark the subfloor section to be replaced with chalk lines
Turn off water lines in area

Step 2: Demo Damaged Subfloor

Using pry bar and hammer, remove flooring pieces from subfloor
Make initial cuts in subfloor with circular saw inside chalk lines
Tap chisel along cuts to break subfloor pieces loose
Use reciprocating saw to cut subfloor flush with wall studs
Pry and pull up subfloor section; discard debris

Step 3: Install New Subfloor

Measure opened area and cut replacement subfloor panels to fit
Dry fit subfloor pieces to ensure proper fit
Apply wood construction adhesive to edges and joists
Place new subfloor pieces into opening
Drill and screw panels into wall studs and joists
Fill any gaps with wood putty; let dry completely

Step 4: Replace Flooring

Use sander or grinder to make new subfloor flush with existing
Sweep and mop floor thoroughly when subfloor is level
Starting at wall, attach original or new flooring with nailer/stapler
Tap flooring into place gently with trim block
Reinstall baseboards or quarter round with finish nails

Step 5: Complete Finishes

Roll flooring seam edges with seam roller
Use silicone caulk to seal flooring edges against trim
Install any remaining trim pieces like quarter round
Fill all nail holes with matching wood putty
Clean up work area and tools
Allow 24 hours for adhesive to fully cure before heavy use

Following these step-by-step instructions carefully will result in a successful subfloor replacement under walls. Take it slowly and ensure each piece is securely fastened. Thorough preparation is key to smooth installation.

Benefits of Replacing Subfloor Under Walls

Replacing a damaged, uneven, or rotten subfloor under existing walls provides many benefits that improve the structure and appearance of a home:

Supports new flooring properly – Rotting or warped subfloors cannot properly support flooring. Replacing the subfloor provides a flat, level surface for long-lasting flooring.

Eliminates squeaks and pops – Detached or loose subfloors rub against joists causing noises underfoot. New subfloor eliminates annoying floor squeaks.

Stops squeaking and prevents future moisture damage – Water spilled near walls or leaking pipes can degrade subfloors over time. New subfloor prevents this damage.

Strengthens weak areas – Deteriorated or compromised subfloor areas only get weaker. Replaced subfloor creates a stronger base under walls.

Makes floors feel more solid – Old subfloors can feel bouncy or hollow. New subfloor installation makes floors feel firm.

Increases home value – Proper subflooring is key to a high-quality flooring installation, which boosts resale value.

Extends the life of flooring – Smooth, level, and properly fastened subflooring maximizes the lifespan of flooring in that area.

Can match unique floor shapes – Subfloor under oddly angled walls can be tricky. Custom cutting replacement pieces is easier.

Provides peace of mind – Knowing the structural base of a home is solid gives homeowners assurance and eliminates worries.

Replacing only the affected subfloor area saves time and money compared to replacing flooring and subflooring in an entire room.

Common Mistakes to Avoid When Replacing Subfloor Under a Wall

Replacing a section of subflooring while leaving walls intact takes precision. Avoid these common mistakes for best results:

Not supporting edges – Not properly supporting subfloor edges on nearby joists or walls while cutting can lead to collapsing and injury.

Rushing demolition – Taking the time do careful pry up and cut out the old subfloor prevents damage to nearby walls and floors.

Not containing dust – Using plastic sheeting containment and proper ventilation reduces dust and particles from spreading through the home during demolition.

Improper subfloor panel spacing – Leaving too much space between panels or at edges allows movement and causes squeaking.

Inadequate adhesive – Skimping on quality wood adhesive at seams and on joists diminishes the strength of subfloor attachment.

Not anchoring to studs – Failing to screw subfloor to wall studs properly prevents stability along edges.

Ignoring unevenness – Not smoothing the transition between new and existing subfloor will telegraph through flooring.

Not waiting for adhesive cure time – Full adhesive strength takes up to 24 hours. Early walking on subfloor can compromise the connection.

Rushing flooring installation – Taking time to ensure subfloor is level and clean makes re-installation of flooring easier.

Improper flooring spacing – Not leaving proper expansion space between flooring and walls leads to buckling and moisture issues.

Avoiding these common mistakes helps ensure subfloor replacement under walls is seamless, stable, and successful for the long term.

Conclusion

Replacing a damaged or uneven section of subfloor under an existing wall is an intermediate DIY project that requires proper preparation and tools. But it can be accomplished without the major work of taking down walls or removing entire floors. With careful demolition of the old subfloor, precise installation of the new replacement subfloor cut to fit the space, and re-installation of flooring, the repair can be almost unnoticeable. Just be sure to use care around wall studs, watch for wiring or plumbing, and take time to make new subfloor pieces level. Addressing problem subfloor areas protects your investment and provides a solid base for beautiful floors.

How to Replace a Subfloor Under a Wall