Use this guide as a planning framework for room conditions, subfloor preparation, movement and moisture. Confirm the exact installation tolerances and warranty terms in the documentation for the product you choose.
Confirm the substrate is structurally sound
Walk the room slowly and listen for squeaks, movement or hollow areas. Wood subfloors should be firmly attached and free of damaged sections. Concrete should not have loose material or active deterioration. Floating flooring can bridge minor cosmetic imperfections, but it should not be used to hide structural movement. If you suspect rot, significant cracking or ongoing moisture intrusion, resolve that first.
Check flatness with a long straightedge
Flatness is different from level. A room can slope gradually and still be flat enough for a floating floor, while a level-looking room can have a sharp hump that stresses plank joints. Use the tolerance stated by the flooring manufacturer. Mark high and low areas, then use an appropriate method for the substrate—such as sanding a wood high spot or using a compatible patching/leveling product where allowed.
Remove debris and incompatible surface material
Dried joint compound, paint ridges, staples and small stones can create pressure points. Scrape or remove protrusions and vacuum thoroughly, especially along walls where installers kneel and where expansion gaps are set. If an old resilient floor, adhesive or coating is present, identify it before aggressive scraping. Older materials can require special handling, and uncertain materials should be assessed rather than disturbed casually.
Handle moisture and underlay as a system
Do not assume every underlay is a vapor barrier or that more layers are better. Check the flooring instructions for the subfloor type, moisture testing method and approved underlay. Concrete installations may have specific vapor-control requirements. Products with attached pads may prohibit an additional cushion layer because excess softness can strain locking joints.
Practical planning notes
Preparation is part of the flooring system
Treat the subfloor as a component of the finished floor, not something that disappears once the planks are installed. Record moisture checks when the product requires them, use a long straightedge to find local high and low spots, and vacuum immediately before underlay or flooring goes down. Small debris can create movement or noise later. Also confirm door clearances and transition heights before leveling work changes the finished floor elevation.
Project checklist
- Check for movement, damage and squeaks
- Measure flatness to the flooring specification
- Remove staples, ridges and loose debris
- Follow the moisture-testing requirement
- Use only an approved underlay or vapor-control layer
Common mistakes to avoid
- Treating “flat” and “level” as the same thing
- Stacking underlay layers to make the floor softer
- Installing over an active moisture problem
- Assuming a floating floor will hide every subfloor defect
Frequently asked questions
Does a subfloor have to be perfectly level?
Usually the critical requirement is flatness within the flooring manufacturer’s tolerance. A gradual slope may be acceptable even when a localized hump is not.
Can I put underlay over existing carpet?
Generally no for standard laminate or click-lock vinyl installations. Follow the specific flooring instructions.
Should I fill every concrete crack?
Not every hairline crack has the same significance. Active movement, moisture or wide cracks need evaluation; compatible patching may be appropriate for stable surface defects.
Can underlay fix an uneven subfloor?
Underlay is not a substitute for correcting significant high or low spots unless the flooring manufacturer explicitly allows a specific system.
Why does subfloor moisture matter?
Moisture can affect flooring, underlay, adhesives, trim and the building assembly even when the decorative surface itself is water resistant.