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

Sheathing is carpentry. The frame under it is engineering. The line is whether the load path is in question.

Call an engineer, and stop loading the area, when you see:

• A broken, split, crushed, or rotted joist, or more than one joist affected in the same bay group. Sistering a single member with surface damage is routine; a run of them is not.
• A cut, notched, or drilled joist or beam beyond allowable limits, especially the classic plumber's notch through the bottom of a joist for a drain or the oversize hole for a duct. Engineered I-joists are unforgiving here: cutting a flange, or drilling outside the manufacturer's permitted web zones, voids the member's rating entirely and there is no field repair without the maker's detail.
• Visible sag, deflection you can feel walking across the room, or a floor that has gone measurably out of level. Bounce is stiffness; slope is failure.
• Any load-bearing wall, post, column, or beam that is involved, including a wall the previous owner removed.
• Damage at a bearing point: a rotted sill or band joist, a crushed post, a girder end sitting in a wet pocket, a failed pier or column.
• The floor supports a concentrated load: a large tub or spa, a masonry hearth, a heavy safe, tile over a long span, or an interior wall stacked above.
• Foundation movement is in the picture: stair-step cracks in masonry, doors that will not latch, a wall that has rotated. Fixing the subfloor over a moving foundation is money thrown away.
• Termite or fungal decay in framing, not just sheathing, or any decay whose extent you cannot see the end of.
• You are being asked to size a repair beam or a temporary shore. Shoring a floor that carries a wall above is exactly where people get hurt.

Give the engineer photos, spans, joist size and spacing, species if known, and what you found. Their letter is what the permit office wants and what protects you from owning a structural opinion you were never paid to give.

References

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