The subfloor decides whether laminate or vinyl plank stays flat, quiet, and locked together. A straightedge check, a moisture test over concrete, and the right underlayment take an hour or two and prevent the failures that force a full relay.
What must the subfloor be like before laminate or vinyl plank goes down?
Clean, dry, sound, and flat. Sweep and vacuum the bare subfloor, drive down protruding fasteners, and screw loose panels to the joists. Carpet and padding always come out first.
Laminate and vinyl plank can go over concrete, plywood, OSB, or an existing hard surface that is flat, fully bonded, and dry. Pergo's installation instructions require removing any wood flooring installed over concrete before laminate goes down. The click-lock system floats on the surface, so the subfloor underneath carries the whole job.
How flat does the subfloor need to be?
Lay a 10 ft straightedge on the subfloor in several directions, along the room, across it, and diagonally, and look for gaps under the tool. The NWFA installation guideline accepts a gap of 3/16 in or less over 10 ft, or 1/8 in or less over 6 ft, and most laminate and vinyl plank product instructions state the same or a tighter value. The product instructions always win over the guideline.
A 1/4 in gap under a 10 ft straightedge fails the check. That size of dip lets planks flex underfoot, and flexed click joints separate and wear early. Note that flat is not the same as level: the subfloor can slope gently and still be flat enough, as long as the local variation stays inside the stated tolerance.
How do you fix the high and low spots?
Grind or sand high spots down to the surrounding surface. Fill low spots with a portland cement-based leveling or patching compound approved for use under the flooring, mixed and cured exactly per the compound's instructions. A bag of compound and a trowel cost far less than lifting a finished floor to fix one soft spot.
Write down the straightedge length, the largest gap found, and where it was. That record is what proves the subfloor met the product's tolerance if a joint problem appears later.
What moisture checks does a concrete slab need?
Concrete holds and moves moisture even when it looks dry. Run the checks in this order: the cheap screen first, the paid test second, and the barrier decision last.
- Confirm the slab's cure. The product instructions state the required age; one widely cited installation guide puts the common figure at 60 days. Test before that and the reading is meaningless.
- Screen with a taped plastic sheet. Tape a 2 ft square of plastic sheeting tightly to the slab and leave it for 72 hours. Condensation underneath or a dark patch in the concrete means moisture is moving through.
- Run the quantitative test the product instructions require. Many products set the calcium chloride test (ASTM F1869) at 3 lb of vapor per 1,000 sq ft per 24 hours or the in-situ relative humidity test (ASTM F2170) at about 75%. Those are common product limits, not universal rules. Pergo Outlast+ instructions, for example, allow up to 5 lb and under 80% RH. The instructions packed with the chosen floor govern.
For wood subfloors, meter the moisture and keep it at or under 12%, within 4% of the flooring's moisture content per the NWFA guideline. If a slab tests wet, find and fix the source first. A vapor barrier does not cure a real water problem.
When does the job need a vapor barrier?
Over any subfloor containing concrete, laminate needs a 6 mil (0.15 mm) virgin polyethylene vapor barrier with seams overlapped at least 8 in and taped, per Pergo's instructions. The barrier runs up to the wall base to close the envelope. Wood subfloors do not get a vapor barrier laid on their surface, but below-grade rooms still need the concrete-slab protection.
Which underlayment does each product need?
Underlayment and vapor barrier are separate decisions. The underlayment cushions and quiets the floor. The vapor barrier blocks slab moisture. Match both to the product.
| Product and subfloor | What goes under it |
|---|---|
| Laminate without attached pad, over plywood or OSB | Laminate-rated underlayment per the product instructions |
| Laminate without attached pad, over concrete | 6 mil polyethylene vapor barrier first, then the laminate-rated underlayment |
| Laminate with attached pad, over concrete | Vapor barrier only; no second underlayment |
| Vinyl plank with attached pad | Nothing extra; a second layer can void the warranty |
| Vinyl plank without attached pad, over plywood or OSB | Thin dense foam rated for vinyl, about 1 to 1.5 mm |
| Vinyl plank without attached pad, over concrete | 6 mil polyethylene vapor barrier or a moisture-rated underlayment |
| Glue-down vinyl plank | No underlayment; the adhesive bonds straight to the subfloor |
Never use thick carpet padding or standard soft laminate foam under vinyl plank. Vinyl needs a firm base. A soft pad lets the planks flex and the click joints fail early.
How do you choose between laminate and vinyl plank?
Choose by the room's moisture exposure first and by wear specs second. The two products solve different problems.
| Decision factor | Laminate | Vinyl plank |
|---|---|---|
| Water exposure | Water-resistant, not waterproof; standing water swells the fiberboard core | Waterproof core; the choice for kitchens, baths, laundry, and basements |
| Scratch resistance | Hard melamine surface; strong against scratches and dents | Softer surface; a thicker wear layer improves it |
| Wear rating | AC1 to AC5 abrasion classes (NALFA); higher is tougher | Wear layer in mil, where 1 mil = 0.001 in; buying guides commonly cite 12 mil for standard residential and 20 mil for busy homes with pets |
| Comfort and sound | Floats with an air gap; can sound hollow without good underlayment | Softer and quieter underfoot, especially with an attached pad |
| Best rooms | Dry living areas, bedrooms, hallways | Whole home, including wet and below-grade rooms |
| Refinishing | None; replace when worn | None; replace when worn |
Some laminate lines now market waterproof cores, but their seams stay vulnerable and the warranty terms set the real limit. A 20 mil wear layer is 0.020 in thick, which is 0.008 in more than a 12 mil layer, and that thickness is what stands between the printed surface and daily grit. The wear-layer ranges in buying guides are planning aids, not standards. The chosen product's spec sheet governs.
Which subfloor and material mistakes force a relay?
- Laying over carpet or a loose existing floor. The planks need a firm, bonded surface. Carpet compresses under load and the joints open.
- Skipping the straightedge check. A 1/4 in dip fails a 3/16 in tolerance and shows up as bouncing planks and separating joints.
- Testing moisture after the floor is down. The slab screen and quantitative test happen on the bare, cured slab, before any material is ordered.
- Doubling underlayment under an attached pad. The extra layer softens the base and can void the warranty.
- Using laminate foam under vinyl plank. Soft foam lets vinyl flex; flexed joints break apart over time.
- Putting standard laminate in a wet room. Water-resistant marketing does not survive a dishwasher leak or a basement seep.
- Reading a retailer's wear-layer chart as the product spec. The mil and AC numbers that matter are the ones printed for the chosen product.
Sources and scope
NWFA Installation Guidelines 2019 (National Wood Flooring Association technical publication): supports the 3/16 in per 10 ft and 1/8 in per 6 ft flatness acceptance, high-spot sanding and low-spot filling, and the wood-subfloor moisture rule of at most 12% and within 4% of the flooring. The guideline states that manufacturer instructions take precedence. Pergo installation instructions (Pergo Outlast+ user manual and Pergo laminate installation guide, via manuals.plus and Home Depot product PDFs, accessed 2026-10-21): support removing carpet and padding, removing wood flooring over concrete, the 3/16 in per 10 ft flatness limit, the 6 mil virgin polyethylene vapor barrier with 8 in seam overlap over concrete, the calcium chloride limit of 5 lb per 1,000 sq ft per 24 h and RH under 80% for that product, and 48-hour acclimation at 60 to 85 F and 35 to 65% RH when delivery conditions differ from room conditions. wallcurtains.com installation-mistakes guide (accessed 2026-10-21): supports the plastic-sheet screening test, the commonly cited 3 lb calcium chloride and 75% RH limits for many products, the 60-day new-concrete figure, and the attached-pad underlayment rules. Retailer buying guides (westcoastfloorco.com, allamericandiscountcarpet.com, thedesignconfidential.com, dkflooring.ca, accessed 2026-10-21): support the wear-layer mil ranges, laminate AC ratings, and the waterproof-vs-water-resistant comparison as published buying guidance, not standards.
Scope limits: Flatness tolerances, moisture limits, vapor-barrier rules, underlayment choices, and acclimation for a specific floor come from that product's installation instructions. This page records the checks and the decision process. It does not select a product, certify a subfloor, or replace the instructions packed with the flooring.
Review note: Saleem Sial owns the research and editorial record. Source checks, fixtures, build validation and rendered QA are completed before publication. Waseem Sial is listed as External Reviewer and Engineer for ongoing external review; no completed review date is claimed.
Read How to Measure Rooms for Laminate Vinyl Plank for the room-measurement record behind the order, or use the Flooring Calculator to turn a checked net area into boxes. The Flooring Quantity Planning hub collects the published flooring resources, the calculation methodology explains units and rounding, and the corrections route accepts source or arithmetic issues.
