Subfloor Assessment + Repair Reference

Why this matters

Subfloor problems show up to service techs constantly - soft spots near toilets, bouncing kitchen floors, sagging at the dishwasher, rotted boards at the bathroom door. Plumbers, cleaners, carpet techs, + pest control all walk on these surfaces + can spot trouble. The tech who recognizes subfloor compromise alerts the homeowner before catastrophic failure. This is the field card.

Subfloor types

Plywood:

• 3/4" (or 23/32") most common new
• 5/8" some older
• T+G (tongue + groove) common for floors
• More resistant to water than OSB
• Most homes 1980+ on raised foundations

OSB (Oriented Strand Board):

• Lower cost
• More vulnerable to water damage (swells, delaminates)
• Common in modern tract construction
• Identifiable: visible wood strands; not smooth ply

Particleboard / underlayment:

• Older homes
• Catastrophic failure when wet
• Often beneath sheet vinyl in 1970s-80s

Concrete slab:

• Slab-on-grade homes
• No subfloor per se; concrete IS the floor
• Different issues (cracking, moisture from below)

Plank flooring (old construction):

• Solid wood planks as subfloor
• Pre-1950s homes
• Surprisingly durable when dry
• Original layer often visible from below

Concrete + plywood combo:

• Concrete with plywood layer (for trim leveling, sound)
• Found in some basements + additions

Joists below the subfloor

The structural members the subfloor sits on:

• Solid sawn lumber: 2x8, 2x10, 2x12 (dimensional)
• Engineered I-joists: prefab; lighter; very strong; common 1990+
• LVL (laminated veneer lumber): heavy beams
• Trusses: floor trusses (open web)

Joist spacing:

• 16" OC most common
• 19.2" OC newer (with engineered joists)
• 24" OC older OR with engineered systems

Spacing matters for subfloor span ratings.

Visible subfloor symptoms

Soft spots when walking:

• Subfloor compromised at that location
• Typically water-damaged
• Joist below may also be damaged

Bouncy / springy floor:

• Excessive deflection
• Joist sag OR span exceeded
• Could be original construction OR joist failure

Squeaks:

• Subfloor moving relative to joist (loose fastener)
• Subfloor edges rubbing (loose T+G)
• Old finish flooring rubbing subfloor
• Often cosmetic but can indicate fastener failure

Sloping floor:

• Foundation movement (see foundation article)
• Joist failure
• Beam / girder sag

Visible discoloration (from below):

• In crawlspace OR basement
• Black stain = active OR past water
• White / fuzzy = mold OR efflorescence
• Crumbling wood = rot

Soft when probed:

• Use awl OR screwdriver
• Probe near plumbing, around toilets, dishwasher, refrigerator
• Sound wood resists; rot gives way

High-risk subfloor locations

These spots get water + subfloor damage:

Bathroom toilet:

• Wax seal failure = chronic seep
• Subfloor rots from below
• Common: soft spot at toilet base
• Cross-trade: plumber sees this routinely

Bathroom shower / tub:

• Edge of shower pan where leaks accumulate
• Behind tile + cement board
• Crawlspace below shower often shows damage first

Kitchen sink:

• Drain leaks slow
• Supply leaks slow
• Cabinet base + subfloor below absorb water

Dishwasher:

• Supply OR drain hose leaks
• Often discovered when subfloor sags

Refrigerator (ice maker line):

• Slow drip behind / under
• Long-term damage before detection

Washing machine:

• Hose failure (sudden)
• Drain hose backup (sudden)
• Catastrophic flooding when it happens

Hot water heater:

• Pan leak OR tank failure
• Adjacent flooring + subfloor

Window leaks (water entry through wall):

• Sill plate + subfloor
• Long-term hidden damage

Below-deck inspection

For crawlspace OR basement homes:

What to look for:

• Sagging joist
• Cracked joist (split with the grain)
• Notched OR drilled joists (DIY damage)
• Water stains on bottom of subfloor
• Rotted wood at joist ends (sill plate contact)
• Insect damage (termite, carpenter ant, carpenter bee)
• Mold (white / black / colored growth)
• Loose fasteners
• Boots / blocks installed by previous repair (often hides original problem)

Joist failure types:

• Crack along grain (split)
• Crack across grain (broken; serious)
• Rot at supports
• Insect tunnels reducing cross-section
• Compressed at bearing point (over time)

Cross-trade observations

Plumber:

• Soft floor around toilet = wax seal failure (also subfloor)
• Spongy floor at kitchen sink = leak detection
• Water heater pan staining = check subfloor below

Cleaner / Carpet tech:

• Spongy area noticed during cleaning
• Tack strip won't hold (rotted subfloor)
• Damp smell from carpet = potential subfloor moisture

Pest control:

• Termite damage visible in crawlspace
• Carpenter ant frass below joists
• Mouse nesting in insulation = potential moisture source

Roofer:

• Roof leak source = subfloor below leak point eventually
• Stain on ceiling = subfloor above the staining

HVAC:

• Duct condensation = potential subfloor damage below ducts
• HVAC return air pulling from crawlspace = moisture transport

Severity assessment

Cosmetic / minor:

• Squeaks
• Small soft spot (less than 6")
• Surface discoloration without softness
• No structural failure

Moderate:

• 1-2 sq ft soft area
• Single joist with surface damage
• Multiple squeaks indicating fastener issue
• Detectable bounce but not sloping

Severe:

• Large area (4+ sq ft) of soft subfloor
• Joist visibly broken / sagging
• Active water leak (still present)
• Floor noticeably sloping
• Mold / fungal growth visible

Severe = stop adding load (don't put heavy items on the area) + structural evaluation.

Repair approaches

Subfloor patch (small area):

• Cut out damaged section
• Add blocking under cut edges
• Install new subfloor matching thickness
• Subfloor adhesive + screws
• Cosmetic only; doesn't address joist if affected

Subfloor full replacement (room-size):

• Remove finish flooring
• Remove subfloor
• Inspect joists
• Replace subfloor (full or partial)
• New finish floor

Joist sister:

• Add new joist alongside damaged
• Bolt OR nail together
• Bear on same supports
• Strengthens but doesn't replace

Joist replacement:

• Major work
• Shore up structure above
• Remove damaged joist
• Install new
• Re-fasten subfloor

Underlayment repair (top of subfloor):

• Below tile / sheet flooring
• Adds rigidity for tile bond
• Doesn't fix structural

Materials

For new subfloor:

• 3/4" T+G plywood (best)
• 5/8" plywood (where thickness needed)
• AdvanTech / Zip-Floor (engineered subfloor; water-resistant)

For underlayment:

• 1/4" plywood under tile / sheet
• Cement board for tile (separate from subfloor)

Fasteners:

• Subfloor screws (#8 or #10, 2" - 2.5")
• Screws beat nails for new work
• Subfloor adhesive on joists (eliminates squeaks)

When to call structural engineer

References

• APA Engineered Wood Association (subfloor span ratings)
• AdvanTech / Huber technical data
• IRC Chapter 5 (Floors)
• Manuall internal: Foundation Problem Signs, Mold Remediation Basics