Subfloor preparation is invisible the day a job finishes, it is the simplest thing to trim to win a job on price, and it is the biggest single reason one floor lasts twenty years while an identical-looking one fails in three.
Flat is not the same as level
A floor can slope gently across an entire room and be perfectly fine to install over. What causes problems is local unevenness — dips and humps over short distances.
Manufacturers publish flatness tolerances, and installing outside them is what voids warranties. It is measured with a straight edge rather than judged by eye, because a floor that feels solid underfoot can easily be out of tolerance.
What happens when it is skipped
A click-together floor over an uneven substrate flexes at its locking joints every time somebody walks across, and those joints are not designed for repeated flexing. Eventually they separate, which shows up as an unexplained gap in the middle of a floor or a hollow spot you can hear.
Under glue-down material, a dip means the adhesive never made full contact, leaving a void that becomes a soft spot. Under tile, it means a tile bridging the dip rather than sitting on full support, which is how tile cracks without anything being dropped on it.
Moisture, especially in lower levels
Concrete passes moisture vapour upward, and how much varies through the year. Putting a moisture-sensitive floor or the wrong adhesive over a slab that is giving off vapour produces a slow failure — adhesive breakdown, cupping, sometimes a smell.
Lower levels across the Coulee Region make this a live question rather than a theoretical one, particularly on properties down in the valley where the ground stays damper. The answer is to measure rather than assume, and to use the appropriate barrier where the reading calls for it.
What proper preparation includes
- Complete removal of the old floor and all residue — adhesive, staples, tack strip.
- Flatness checked against the manufacturer tolerance rather than by eye.
- Moisture measured on any slab or below-grade floor, not assumed.
- Loose or squeaking wood subfloor refastened before anything covers it.
That last one is worth insisting on. A squeaky subfloor does not stop squeaking because a new floor went over it, and once the new floor is down the opportunity has gone.
Why quotes differ
When two quotes for the same material differ substantially, subfloor preparation is usually most of the gap. It is hard to compare unless it is written down, because "install flooring" can mean anything from full preparation to laying planks over whatever is there.
Ask what the quote assumes about the subfloor and what happens if it needs work; it is one of several items covered in how to compare flooring quotes, and subfloor build-up is also how height differences at transitions between rooms get solved properly. We write it as its own line so you can see it — see what to expect during installation for how the rest of the job runs.