Flat and level are distinctly different measurements that fail in different ways. Level means the surface does not slope in any direction — a spirit level placed on the floor in any orientation shows it is horizontal. Flat means the surface has no high spots, low spots, or dips — a straightedge laid across the floor in any direction shows no gaps between the tool and the surface. A floor can be perfectly level but not flat, with subtle waves and undulations that a spirit level misses but that show up immediately when tile or stone is laid on top of it. Those waves cause the adhesive to be thin or non-existent where there are gaps, and the tile sits on high spots, resulting in cracking under load or pressure.
Deflection is the amount the floor moves and flexes under weight and loading. A floor that noticeably flexes when you walk on it is a deflecting floor. The amount of deflection that occurs governs which finishes are viable on top of it. Rigid finishes like ceramic or porcelain tile demand a very stiff floor with minimal deflection. Flexible finishes like vinyl and vinyl plank tolerate significant deflection. Laminate needs a fairly stiff floor but less rigidity than tile requires. The subfloor system itself — the species and thickness of plywood, the joist spacing and sizing, or an existing concrete slab — determines the baseline stiffness and deflection characteristics. Adding more support like additional joists or sistering can increase stiffness, or removing a damaged joist decreases it.
Levelling compound is used for small localized rises or dips — high spots in a concrete slab from settling, slight waves in wood subflooring, or depressions where objects have settled over time. A sheet overlay — a layer of engineered board or cement board — is better for large widespread corrections or for systematically stiffening a deflecting floor. Adding any thickness to the floor is constrained by the height budget: if thresholds and door clearances are already tight or marginal, adding thickness to the floor makes the situation worse and can make doors difficult to close or prevent them from closing.
Moisture in the subfloor is a separate critical assessment from flatness and stiffness. Wood moisture content affects how the wood behaves over time and can change with seasonal humidity variations. Concrete slab moisture has to be managed carefully and understood before any finish goes on top, especially tile. The substrate itself — what is underneath, how old it is, what condition it is in, whether it is damaged — fundamentally shapes what correction method is appropriate and whether the cost of correction is justified or whether structural work is necessary first.