Flatness has a number
Manufacturers specify flatness, commonly around 3/16 of an inch over a 10-foot span. That is not a suggestion; installing outside of it typically voids the warranty and produces the hollow, clicking spots homeowners notice within months.
We check with a long straightedge across the room in several directions and mark the high and low areas before anything else happens.
Moisture testing
Concrete gets a relative-humidity or calcium chloride test. Wood subfloors get a meter reading, and we compare it against the flooring's own moisture content.
If the numbers are out of range, the honest options are to wait, to add a moisture mitigation layer, or to change product. Installing anyway is how floors cup.
What comes out
Old carpet, pad and tack strip; cushioned sheet vinyl; and failing particleboard underlayment all have to be removed. Particleboard shows up constantly in 1960s and 70s Boise Bench and Nampa homes and cannot support tile or hold fasteners.
Adhesive residue from old glue-down vinyl gets scraped or encapsulated depending on what the new product's manufacturer allows.
Filling, grinding and leveling
Low spots get patch or self-leveling compound; high spots get ground down. On older homes with real settlement, full self-leveler across a room is sometimes the only path to a floor that performs.
This is the line item people are tempted to cut. It is also the one that decides whether the floor is quiet and tight in year three.
- Squeaks get screwed down before the floor covers them
- Soft or water-damaged decking gets cut out and replaced
- Transitions between subfloor heights get feathered or ramped
Why estimates differ so much
Two bids on the same house can differ by thousands because one includes prep and one assumes the subfloor is perfect. When you compare quotes, compare what each one says about demolition, leveling and moisture testing — not just the price per square foot.