Replacing Subfloor: A Step-by-Step Guide for Bathrooms, Kitchens, and Living Areas

A spongy spot near the toilet, a soft patch under the dishwasher, or visible sag along a hallway — these are all signs the layer beneath your finished floor has rotted. Replacing subfloor is one of the most rewarding DIY projects a confident homeowner can take on, and one of the most expensive when handed off to a contractor ($1,500 to $4,000 per room is typical). The work itself is not complicated, but the sequencing matters and a few mistakes can turn a $300 weekend into a structural problem.

This guide walks through the process the way a remodeling carpenter actually does it: assess, expose, cut, sister, install, finish.

Step 1: Assess the Damage

Before tearing anything up, walk the floor in socks. Note every soft spot, listen for crackling under weight, and check the ceiling below for water staining. Use an awl or screwdriver to probe suspect areas — healthy plywood resists the tip; rotted subfloor punctures with light pressure.

Common damage zones:

• Around toilets (wax-ring leaks, the most frequent culprit)
• Under dishwashers and refrigerators with ice makers
• Below windows where condensation drips behind trim
• Along exterior walls where ice dams have driven water inward
• At HVAC closet drain pans

Map the failed area, then add 12 inches of margin in every direction. Rot extends past what you can see, and you want to land your cuts on solid wood and the centers of joists.

Step 2: Remove the Finished Flooring

You cannot replace subfloor with the finished surface still on top. Method depends on what is up there:

• Tile: hammer and cold chisel, or a small electric demo hammer rented for $40/day from Home Depot
• Vinyl plank or laminate: pry up edge pieces and disassemble in reverse of installation
• Sheet vinyl: score in 12-inch strips with a utility knife, pull each strip with pliers
• Hardwood: pry boards individually if you want to save them; otherwise cut into manageable sections with a circular saw set to 3/4-inch depth
• Carpet and pad: roll up, cut into 4-foot strips, bag for disposal

Wear an N95 respirator. Old flooring adhesives and underlayments may contain asbestos in homes built before 1985 — if in doubt, get a $30 mail-in test before disturbing material.

Step 3: Cut Out the Bad Subfloor

Set a circular saw blade depth to match the subfloor thickness exactly — typically 5/8 inch or 3/4 inch. Cutting deeper risks slicing through wiring, plumbing, or HVAC duct running between joists.

Plan your cuts to land on the center of joists so the new patch has solid bearing on both sides. Joists are usually 16 inches on center; find them with a stud finder or by tapping for solid spots. Mark cut lines with a chalk snap line.

Plunge-cut along the chalk lines, pry out the bad plywood with a flat bar, and pull all the old fasteners. Vacuum the cavity thoroughly — debris under the new sheet causes high spots that telegraph through finished flooring.

Step 4: Inspect and Repair the Joists

This step gets skipped by impatient DIYers and creates problems within 6 months. Probe each exposed joist with a screwdriver. Healthy joists are firm; rotted joists deflect or crumble.

For minor surface rot or splits, sister a 2×10 or 2×8 alongside the existing joist with construction adhesive (Loctite PL Premium) and 3-inch construction screws every 8 inches. The sister should extend at least 24 inches past the damaged section in both directions and bear on the same supports.

For severely rotted joists, the full member needs replacement, which often means temporarily supporting the floor above with adjustable steel posts. If that scope intimidates you, this is the point to call a contractor.

Step 5: Add Blocking

Where your new subfloor patch ends mid-bay (between joists), you need 2×4 or 2×6 blocking nailed perpendicular between the joists to support the seam. Without blocking, the sheet edge flexes and squeaks within months. Predrill and use 16d nails or 3-inch construction screws.

Step 6: Cut and Install the New Subfloor

Match the existing subfloor thickness exactly. Most homes built after 1980 use 23/32 (3/4) tongue-and-groove plywood or OSB; older homes may have 5/8 plywood or even diagonal solid-board sheathing. Mix-matching thicknesses creates a height transition that telegraphs through any finished flooring.

Material recommendations:

• Bathrooms and kitchens: 3/4-inch AdvanTech subfloor — moisture-resistant OSB, runs $50 to $65 per 4×8 sheet
• Living areas: 23/32 tongue-and-groove plywood, $40 to $55 per sheet
• Avoid: standard OSB in wet areas, particle board anywhere

Apply a continuous bead of subfloor adhesive (PL 400 or Liquid Nails Subfloor) along every joist top before laying the new sheet. Fasten with 2-1/2 inch deck screws every 6 inches along edges and 8 inches in the field. Stagger seams away from existing seams.

Step 7: Address the Underlying Cause

If you do not fix what caused the rot, you will be replacing the new subfloor in 5 years. Common fixes:

• Install a new wax ring or upgrade to a Danco Perfect Seal under the toilet
• Caulk along the dishwasher kickplate and check the supply line
• Add a $20 drip pan under the refrigerator water line
• Re-flash the window where water entered behind the siding
• Improve gutter and grade drainage outside the affected wall

Step 8: Reinstall Finished Flooring

Let the new subfloor acclimate to the room for 48 hours before installing tile, hardwood, or laminate. Sand any high spots flush with adjacent original subfloor using a belt sander with 60-grit. Check with a 6-foot level — variations greater than 3/16 inch over 10 feet need a self-leveling underlayment before tile or LVP.

Cost Breakdown for a 50 sq ft Repair

• 3/4 plywood, 2 sheets: $90 to $120
• Subfloor adhesive: $10
• Construction screws (5 lb): $25
• Sister joist 2×8: $20
• Blocking lumber: $15
• Disposal/dump fee: $30 to $50
• Total DIY: $190 to $240, plus 1 to 2 days of labor

Same job hired out runs $1,200 to $2,000 in most US markets, more if joists need replacement.

FAQ

Can I replace just part of a subfloor or do I need to do the whole room?

You can patch a section as long as cuts land on joist centers. There is no structural reason to redo the entire room if only one zone is rotted.

What thickness subfloor should I use?

Match what is already there. Modern code calls for 23/32 (3/4) over 16-inch joists and 7/8 over 19.2-inch joists. Tongue-and-groove edges add stiffness and reduce squeaks.

How long does subfloor take to replace in one room?

A confident DIYer can do a typical 8×10 bathroom in one weekend, including flooring removal. Add a day if joist sistering is required.

Do I need a permit to replace subfloor?

Most jurisdictions do not require a permit for like-for-like subfloor repair. If you are altering joists, replacing structural members, or moving plumbing, a permit is usually required. Check with your local building department.