The most beautiful hardwood floor in the world will fail if the subfloor beneath it isn’t sound. At Skyline Flooring, we inspect the subfloor on every single project — and roughly 40% of the homes we work in across Sherman Oaks, Encino, Calabasas, and greater Los Angeles need some level of subfloor repair before we can install.
Here are the five warning signs that your subfloor needs attention, what causes each problem, and what the fix actually involves.
Sign #1: Squeaking or Creaking When You Walk
Squeaky floors are so common in LA homes that many homeowners assume they’re normal. They’re not — they’re a symptom.
What Causes Squeaking
In homes with raised wood subfloors (pre-1950 construction, some hillside homes):
- Plywood or OSB panels have loosened from the joists below. Nails have backed out over time, allowing the panel to flex and rub.
- Joists have dried and shrunk, creating gaps between the joist and the subfloor panel.
- The subfloor material itself has deteriorated — particularly in homes where the original subfloor was particle board, which disintegrates with any moisture exposure.
In homes on concrete slabs (most post-1950 LA homes):
- Squeaking over concrete is less common but can occur when a floating floor or glue-down floor has hollow spots where adhesive didn’t fully cover.
- If there’s a plywood underlayment over the slab (sometimes installed as a leveling layer), that plywood can develop loose spots.
The Fix
- For raised subfloors: re-screw the plywood to the joists using construction screws (not nails), add blocking between joists where needed, and replace any deteriorated sections.
- For slab-based squeaks: identify and fill hollow spots, or remove and relay the affected sections with proper adhesive coverage.
- Cost: $200-$1,500 depending on scope. This is done during demolition and prep, before new flooring goes down.
Sign #2: Soft or Spongy Spots
Walk across your floor. If any area feels soft, bouncy, or spongy compared to the surrounding floor, you have a subfloor problem that must be addressed.
What Causes Soft Spots
- Water damage: Past leaks from plumbing, appliances (dishwasher, water heater, washing machine), or rain intrusion. Even a small, slow leak can weaken plywood or OSB over months.
- Termite damage: LA has active subterranean and drywood termite populations. We’ve pulled up floors in North Hollywood, Burbank, and Glendale and found subfloor joists that looked like Swiss cheese.
- Dry rot: Fungal decay from prolonged moisture exposure. Common in bathroom subfloors, laundry rooms, and around sliding glass door thresholds where rain water pools.
- Particle board deterioration: Many homes built in the 1970s-1990s used particle board as subfloor material. It swells and crumbles with any moisture contact. If your home has particle board subfloors, we strongly recommend replacing them with plywood before any hardwood installation.
The Fix
- Cut out the damaged section and replace with new plywood of the same thickness.
- Address the source of moisture before patching — otherwise the new material will fail too.
- If joists are damaged (termites, rot), they need to be sistered (a new joist attached alongside the damaged one) or replaced. This may require a general contractor or structural specialist.
- Cost: $300-$2,500+ depending on the extent of damage and whether joists are involved.
Sign #3: Visible Moisture or Failed Moisture Tests
Moisture is the single biggest threat to hardwood flooring. It causes cupping, buckling, mold, adhesive failure, and finish problems. In Los Angeles, most moisture issues are related to concrete slab conditions.
Where Slab Moisture Comes From
- Ground moisture wicking through concrete: Older slabs (pre-1980) often lack an adequate vapor barrier beneath them. Moisture from the soil migrates up through the concrete continuously.
- Landscape irrigation: Overwatering near the foundation drives moisture under the slab. We see this constantly in Encino, Tarzana, and Woodland Hills where lush landscaping butts right up against the house.
- Plumbing leaks under or in the slab: Copper pipes embedded in older slabs can develop pinhole leaks. You might not see water, but the slab becomes saturated in one area.
- Recent concrete work: If an addition was built or a slab was poured within the last 60-90 days, it may still be curing and releasing moisture.
How We Test
Before every hardwood installation, we perform moisture testing:
- Calcium chloride test (MVER): Measures moisture vapor emission rate from the slab surface. Must be below 3-5 lbs per 1,000 sq ft per 24 hours depending on the adhesive manufacturer’s requirements.
- Relative humidity probe (ASTM F2170): A more accurate test that measures RH within the slab at 40% depth. Must be below 75-80% RH for most products.
- We test multiple locations — especially near exterior walls, bathrooms, kitchens, and areas near the water heater.
The Fix
- Mild moisture (slightly above threshold): Moisture-mitigating adhesive or a topical moisture barrier like Bostik MVP4 or Mapei Planiseal VS.
- Moderate moisture: Epoxy moisture barrier system applied to the slab surface. This creates an impermeable layer between the slab and the flooring.
- Severe moisture: Address the source first (plumbing repair, drainage correction, landscape modification), then apply moisture mitigation.
- Cost: Moisture mitigation adds $1-$3/sq ft to the project. It’s not optional — skipping it on a wet slab will destroy your new floors within a year or two.
Sign #4: Damaged Slab After Tile Removal
This is one of the most common subfloor issues we encounter in Los Angeles. Tile removal — especially ceramic or porcelain tile set in thinset mortar — is an aggressive process that often leaves the concrete slab in rough shape.
What Tile Removal Does to a Slab
- Surface gouges and pitting: Power chisels and scrapers dig into the slab during removal. Some areas may have 1/4″ to 1/2″ gouges.
- Residual thinset: The mortar used to set tile doesn’t come off cleanly. There’s almost always a layer of hardened thinset that varies in thickness from paper-thin to 3/8″ thick.
- Uneven surface: Between the gouges and remaining thinset, the slab surface can have 1/4″ to 1/2″ of variation — far too uneven for hardwood installation.
We cover tile removal and its implications in detail in our floor demolition guide.
The Fix
- Grinding: Diamond grinders remove residual thinset and level high spots. This is standard after any tile removal.
- Self-leveling compound: For slabs with significant variation, we pour self-leveling underlayment (SLU) over the entire surface. This creates a perfectly flat, smooth substrate for the new hardwood.
- Patching: Deep gouges are filled with hydraulic cement or polymer-modified patching compound.
- Cost: Grinding runs $1-$2/sq ft. Self-leveling is $2-$8/sq ft depending on the depth needed. Most post-tile slabs need at least grinding, and about half need some leveling compound. See our floor leveling services for more information.
Sign #5: Uneven Floors from Foundation Settling
LA sits on some of the most geologically active ground in the country. Earthquakes are the obvious factor, but the bigger day-to-day issue is soil movement — especially in hillside neighborhoods and areas with expansive clay soil.
What Settlement Looks Like
- Sloping floors: A marble placed on the floor rolls consistently in one direction. Some degree of slope is normal in older homes — up to 1/4″ over 10 feet is within tolerance for flooring. More than that needs correction.
- Cracked slabs: Hairline cracks are cosmetic. Cracks wider than 1/8″ or cracks where one side is higher than the other indicate structural movement that may need engineering evaluation.
- Doors that stick or won’t close: This is often a sign of ongoing settlement rather than past, stable movement.
- Earthquake damage: Major seismic events can shift a slab enough to create noticeable unevenness. We saw this extensively after the 1994 Northridge earthquake, and many homes in the San Fernando Valley still have settlement from that event.
The Fix
- Minor leveling (up to 1/2″ variance): Self-leveling compound can correct this during flooring prep. Standard approach for most projects.
- Major leveling (1/2″ to 1″+ variance): May require multiple pours of leveling compound or installation of a plywood subfloor system over the slab to create a flat plane.
- Structural issues: If settlement is ongoing, a structural engineer should evaluate before any flooring work. Cosmetic fixes won’t last if the foundation is still moving.
- Cost: Standard floor leveling runs $2-$8/sq ft. Structural engineering evaluation is a separate cost (typically $500-$1,500 for a report).
Why Subfloor Prep Isn’t Optional
Some flooring companies skip or minimize subfloor prep to keep their bid low. We’ve seen the results countless times — floors that squeak within months, planks that pop loose, adhesive failure, finish cracks along joints, and visible undulation across the floor surface.
At Skyline Flooring, subfloor inspection and any necessary repairs are included in our project planning from day one. When we quote a job, we tell you exactly what your subfloor needs and what it costs. No surprises on installation day.
Get Your Subfloor Evaluated
If you’re planning a hardwood floor project and suspect your subfloor might need work, the best time to find out is before you order materials. Skyline Flooring provides free in-home consultations including subfloor evaluation across Sherman Oaks, Encino, Beverly Hills, Calabasas, Studio City, and all of LA and Ventura County. With 20+ years of experience and a 5.0-star Yelp rating from 109+ reviews, we diagnose and fix subfloor issues so your new floors perform perfectly for decades. Contact us or call (818) 300-2205.