In 20 years of flooring installs across Columbus and Central Ohio, I've seen one problem come up more than any other: floors that fail not because of the material, but because of what was underneath it. Subfloor preparation isn't glamorous, but it's the difference between a floor that lasts 20 years and one that starts popping, squeaking, or buckling within the first year.
This guide covers the four core prep requirements — flatness, cleanliness, dryness, and structural integrity — and what we check on every single job before any flooring goes down.
1. Flatness — The Standard Most Installers Skip
Every flooring manufacturer publishes flatness tolerances in their installation specs. The standard for most LVT and hardwood products is 3/16 inch over a 10-foot span — roughly the thickness of a nickel laid flat. That sounds tight, and it is.
Why does it matter? A click-lock LVT plank is rigid. Lay it over a high spot and it rocks. Over time, foot traffic concentrates stress on the joint, and the locking mechanism cracks. Low spots cause the same problem in reverse — the plank bridges the gap, and the unsupported center flexes until it breaks.
- High spots: Grinding with a floor grinder or belt sander. Skipping this and feathering with compound doesn't work — compound on a high spot just peels off under load.
- Low spots: Self-leveling compound, applied in layers. It flows into the low area, levels itself, and cures hard. Good compound bonds to both concrete and plywood.
- Wood subfloor humps: Usually a joist or fastener issue. We either plane the hump or sister the joist below before putting anything down on top.
2. Moisture — The Invisible Problem
Ohio's climate means moisture is a constant concern, especially in basements and first-floor slabs over grade. Moisture from below moves through concrete — it's not a question of if, it's a question of how much.
For wood and hardwood flooring, moisture is the enemy. Wood expands and contracts with humidity changes. Install it over a wet slab and you're setting a timer. For LVT, moisture matters less for the floor itself (LVT is 100% waterproof) but adhesive failure and subfloor damage are still real risks if the slab is wet enough.
If a slab tests high, we have options: moisture barrier membranes, vapor-retarding adhesives, or in severe cases, a topical moisture mitigation system. None of these are cheap — but they're far cheaper than replacing a floor that buckled.
3. Cleanliness — Dirt, Adhesive, and Old Residue
A subfloor needs to be clean, not just swept. Old adhesive residue, paint, wax, or cutback adhesive (common in homes built before 1985) prevents new adhesive from bonding and causes floating floors to creak and shift.
Cutback adhesive deserves special attention. The black or dark brown adhesive used with sheet vinyl in older homes often contains asbestos. If you're in a home built before 1980 and there's a dark adhesive under existing vinyl, do not grind it. It needs to be tested before any work begins. We run into this occasionally in Columbus-area homes and always handle it properly before proceeding.
- Scrape up any remaining adhesive or compound high spots
- Vacuum and sweep — not just for cleanliness but to find loose material
- Check for wax or sealers that would prevent adhesive bond (shiny concrete = potential problem)
- Remove any existing flooring that is loose, bubbled, or multi-layered
4. Structural Integrity — What's Under the Surface
A floor is only as solid as the structure supporting it. For wood subfloors, this means checking for soft spots, rot, squeaks, and inadequate fastening. For concrete, it means looking for cracks, spalling, and movement.
Walk the room. Feel for bounce or flex underfoot. Bounce in a wood subfloor usually means inadequate joist support or delaminated plywood. Flex in a concrete slab can indicate structural settlement — which is beyond flooring and needs a structural engineer.
Plywood subfloor standards
The flooring industry standard for a direct-glue or nail-down installation is a minimum 3/4 inch (19mm) plywood subfloor, properly fastened to joists on 16-inch centers. Thinner subfloors, OSB in poor condition, or subfloors over long joist spans may need reinforcement before certain products can be installed.
Concrete vs. Wood: What Changes
- Moisture test required (always)
- Flatness: grind high spots, level low spots
- Check for structural cracks (hairline vs. moving)
- Remove existing adhesive/residue
- Prime before adhesive if slab is dusty or porous
- Moisture meter reading required
- Check all fasteners — re-screw loose spots
- Fill gaps between plywood sheets
- Sand or plane any humps at sheet joints
- Replace any soft, rotted, or delaminated sections
Can You Install Over Existing Flooring?
Sometimes. The rule is: existing flooring can stay if it is firmly bonded, flat within tolerance, and adds no more than one layer. A single layer of glue-down vinyl in good condition over concrete is often fine to float over. Peel-and-stick tile, cushion-backed vinyl, or anything loose comes up — no exceptions.
Adding height is also a practical concern. Every layer of flooring raises your finished floor height. This affects door clearance, transitions to adjacent rooms, and in some cases, stair riser dimensions. We measure and flag this during the in-home quote so there are no surprises.
Frequently Asked Questions
How flat does a subfloor need to be before installing LVT?
LVT manufacturers typically require no more than 3/16 inch variation over a 10-foot span. High spots need grinding, low spots need self-leveling compound. It's achievable in most Columbus homes — even older ones — it just takes the right prep work.
Can I install new flooring over my old flooring?
Sometimes — but only if the existing floor is firmly bonded, flat, and in good condition. One layer of vinyl over concrete is often fine. Multiple layers, loose material, or cushion-backed vinyl should come up. When in doubt, remove it.
How do I test for moisture in my subfloor?
For concrete, a calcium chloride test or in-situ RH probe is the most accurate method. For wood subfloors, a pin-type moisture meter is standard. We test every job — it's part of the prep process, not an add-on.
What happens if you skip subfloor prep?
Planks crack at the joints, floors squeak, adhesive fails, and in severe cases, moisture damage destroys the floor from underneath. Subfloor prep failures are the number one cause of flooring callbacks in the industry.
Does Tee's Flooring handle subfloor repairs?
Yes. Tony assesses the subfloor during the free in-home quote and includes any required repairs in the project scope. You'll know the full number before any work begins.