What's the safest way to inspect and repair a soft or moldy bathroom subfloor?

Short answer

The safest way is to stop the leak, protect yourself with proper PPE, contain dust, and remove all soft or moldy materials back to sound wood. Clean and dry the framing, repair or sister any damaged joists, then replace the subfloor with the correct thickness plywood/OSB using construction adhesive and screws. Let everything dry to safe moisture levels before rebuilding the floor system and reinstalling fixtures.

What to do first

• Find and fix the moisture source: Common culprits are a failed toilet wax ring, leaking supply lines/shutoff valves, shower pan/liner issues, cracked grout/failed caulk, or a sink drain leak.
• Safety and containment:
  • PPE: N95 (or P100 half-face respirator for heavy mold), gloves, goggles, long sleeves.
  • Close the bathroom door, set up plastic sheeting at the doorway, and use a HEPA vacuum during cleanup.
  • If the home is pre-1990 and you have sheet vinyl/old tile or black cutback adhesive, test for asbestos before demo. If positive or suspected, call a pro.
  • If visible mold is >10 sq ft, or you smell strong musty odors in walls/adjacent rooms, consider professional remediation.

Tools and materials

• Tools: oscillating multi-tool, circular saw with depth set to subfloor thickness, pry bar, drill/driver, impact driver, jigsaw/recip saw, hammer, wood chisel, tape measure, square, chalk line, moisture meter, HEPA shop vacuum, fans and/or dehumidifier.
• Fasteners/adhesives: construction adhesive, 2–2½ in deck or structural screws, cement board screws (if tiling), thinset (for cement board), new toilet flange/bolts if needed.
• Materials: 23/32 in (3/4 in) T&G exterior-rated plywood or high-quality OSB (e.g., Advantech) to match existing thickness, 2x lumber for blocking/sistering, backer board or uncoupling membrane (for tile), EPA-registered mold cleaner or detergent, borate wood treatment (optional), new wax ring (or waxless seal), silicone caulk.

Step-by-step inspection and repair

1. Prep the area

   • Shut off water to the toilet and fixtures; drain the toilet. Remove toilet and vanity if needed. Plug the toilet flange with a rag to block sewer gas.
   • Protect nearby rooms with plastic and cover HVAC registers. Put a fan in a window to exhaust air outward if possible.

2. Diagnose the leak

   • Toilet: food-color the tank; if color shows around base after a flush, the wax ring likely failed.
   • Shower: plug the drain and fill the pan to just below the curb; look for drops below.
   • Check supply lines, shutoffs, and trap connections for seepage. Re-caulk gaps around tub/shower and penetrations.

3. Remove finished floor and assess

   • Carefully demo tile (rotary hammer + chisel), peel up sheet vinyl, or unclick LVP/laminate. Remove any underlayment/backer board.
   • Probe the subfloor with an awl/screwdriver; mark all soft/spongy or delaminated areas. Light surface mold on firm wood can often be cleaned; soft or crumbling wood must be replaced.

4. Cut out damaged subfloor

   • Set circular saw depth to subfloor thickness and cut square to the joists. Aim to land cuts over joist centers. Use an oscillating multi-tool to finish cuts near walls and the toilet flange.
   • Remove panels and vacuum with a HEPA vac.

5. Inspect and repair framing

   • Check joists and blocking. If the top edge is rotted, sister with matching 2x lumber spanning at least 3–4 feet past damage on each side, glued and screwed/bolted.
   • Add 2x blocking/ledgers around the opening so all new panel edges are fully supported.

6. Clean, treat, and dry

   • HEPA vacuum, then wash wood surfaces with detergent and water or an EPA-registered mold cleaner. Avoid bleach on raw wood; it doesn’t penetrate well and can degrade fibers.
   • Optional: apply a borate treatment to deter future fungal activity once surfaces are dry.
   • Dry with fans/dehumidifier until target readings are reached.

Moisture targets before closing:
- Subfloor/joists: ≤ 15% wood moisture content (≤ 12% is ideal)
- Dry time: often 24–72 hours, longer for heavy saturation

1. Install new subfloor
   • Match existing thickness; bathrooms over 16 in OC joists typically use 23/32 in (3/4 in) T&G plywood/OSB. For 24 in OC joists, consider thicker panels or high-performance subfloor.
   • Dry-fit pieces with a 1/8 in gap between sheets and 1/4 in at walls. Apply construction adhesive on joist tops and blocking.
   • Fasten with screws (not nails) per schedule.

Subfloor specs:
- Panel: 23/32 in T&G Exposure 1 (or Advantech); face grain perpendicular to joists
- Screws: #8–#10, 2–2½ in
- Spacing: edges 6–8 in; field 8–12 in

1. Rebuild the finished floor system

   • Tile: set 1/4 in cement backer board in thinset and screw per grid, or use an uncoupling membrane over plywood per manufacturer. Don’t rely on backer board for strength; the subfloor must be solid first.
   • Sheet vinyl/LVP: follow manufacturer’s underlayment and flatness specs. Use floor patch or self-leveling compound where needed.

2. Reinstall the toilet correctly

   • The closet flange should end up on top of the finished floor. Replace/repair the flange if corroded or too low; use spacer rings if needed.
   • Install a new wax ring (or waxless seal), new bolts, and set the toilet level. Caulk around the base, leaving a small gap at the back as a telltale for future leaks.

Tips for best results

• Use a moisture meter to verify drying and check for hidden wet spots around tub surrounds.
• Upgrade to braided stainless supply lines and high-quality shutoff valves.
• Seal grout and recaulk tub/shower joints with 100% silicone.
• A leak detector alarm near the toilet and under the vanity is cheap insurance.

Common mistakes

• Patching over wet or soft subfloor instead of removing to sound wood.
• Ignoring the root cause of moisture.
• Using bleach on framing or painting over mold without cleaning and drying.
• Setting the toilet flange below the finished floor height.
• Skipping blocking under patch edges or using nails instead of screws.

Cost and time

• DIY materials: $150–$600 for typical bathroom floor areas; more if tiling or replacing a flange.
• Time: 1–3 days of work plus drying time, depending on demo and repairs.
• Pros: $800–$3,000+ for subfloor/framing repair; more if shower pan replacement or major remediation is required.

When to call a professional

• Visible mold area >10 sq ft, persistent musty odors, or respiratory sensitivities.
• Suspected asbestos/lead, or you’re not set up for dust control.
• Structural rot in joists/plates, or bouncing floors you’re unsure how to sister.
• Failed shower pan/liner or complex tile replacement.

Handle the leak first, work methodically, and confirm dryness before you close up. A solid, properly fastened subfloor is the foundation for every durable bathroom floor.