There's a straightforward reason why professional tile installers spend considerable time preparing a substrate before any tile gets set: the finished floor is only as good as what's underneath it. The industry has codified exactly how flat a substrate needs to be — and for good reason. Understanding these standards helps homeowners ask better questions and evaluate the quality of any tile installation quote they receive.
ANSI A108.02: The Industry Standard for Tile Substrate Flatness
The governing document for tile installation in North America is the ANSI A108 series, maintained by the Tile Council of North America (TCNA). ANSI A108.02 specifies that tile substrates must not vary more than 1/8" in 10 feet from the required plane, with no more than 1/16" variation in 12 inches when the grout joint is less than 1/2" wide.
In plain language: if you lay a 10-foot straightedge on the floor, the gap between the straightedge and the floor surface should not exceed 1/8" at any point. That's a tight tolerance — tighter than the requirement for vinyl plank flooring, and tighter than general construction framing tolerances. Most concrete slabs and many wood subfloors don't meet it straight from the builder without additional prep work.
For grout joints wider than 1/2", the tolerance is slightly more forgiving — 1/4" in 10 feet — because the mortar bed can be feathered out more beneath the tile edges. But wide-joint installations are less common in contemporary residential tile work.
Large Format Tile: Tighter in Practice
ANSI defines large format tile as any tile with at least one edge longer than 15". Common large format sizes in residential projects are 24x24", 24x48", and 18x36". The written standard doesn't change the 1/8" in 10-foot flatness requirement for large format tile, but the practical tolerance becomes tighter because large tiles can't flex to accommodate surface variation the way smaller tiles can.
A 4x4" mosaic sheet set in a mortar bed can bridge over a 1/4" high spot without much trouble. A 24x48" porcelain slab tile cannot — any high spot under the tile creates a fulcrum point. The tile rocks, the mortar bond breaks under the edges, and eventually the tile cracks. For this reason, many experienced tile installers — including on projects at Orleans Flooring Co. — hold to 1/8" in 10 feet as a minimum standard even where the written spec would allow more variation in larger-joint applications.
Lippage: What It Is and Why It Matters
Lippage is the vertical displacement between the edges of two adjacent tiles — the height difference where one tile sits higher or lower than the tile next to it. ANSI A108.02 limits maximum allowable lippage to 1/32" plus the allowable variation of the tile itself (since tiles from the factory are not perfectly uniform). In practice, lippage in excess of 1/16" is noticeable to the eye and noticeable underfoot.
Lippage is not just an aesthetic issue. Tile edges that project above adjacent tiles are a trip hazard, particularly for older residents. On polished large format tile, even 1/32" of lippage is visible under light. This is why lippage standards exist — and why experienced tile setters take them seriously.
Lippage clip systems — proprietary systems from companies like Raimondi, Levolution, and others — are the modern solution to lippage control. Small plastic clips are inserted under tile edges during setting, and wedges are driven in to level adjacent tile faces before the mortar cures. The clips break off after the mortar sets, leaving clean grout joints with consistent tile height across the field.
How to Measure Subfloor Flatness
The basic tool is a 10-foot straightedge. Lay it flat on the floor, move it in multiple directions across the area to be tiled, and note any gap larger than 1/8" between the straightedge and the floor surface. On larger jobs or where accuracy is critical, a long digital level or a rotating laser level can map the entire floor surface more systematically.
On every job, Joseph uses a 10-foot straightedge across the full installation area, working in a grid pattern to identify both high and low spots. Those measurements determine exactly how much prep work is needed before a single tile goes down.
Fixing an Out-of-Flat Floor
High spots on concrete are ground flat. A Hilti floor grinder or angle grinder with a diamond cup wheel removes material precisely without damaging the surrounding slab. The grinding process generates a significant amount of fine dust — proper dust collection (a HEPA-rated vacuum connected directly to the grinder) is essential both for the health of the installer and to prevent fine silica dust from settling throughout the building.
Low spots are filled with self-leveling compound or floor patching compound, depending on the size and depth of the low area. Self-leveling compound is poured as a liquid, seeks its own level, and cures to a hard surface that can be tiled over within 24 hours in most cases. It must be properly primed to bond to the substrate, and any perimeter gaps need to be dammed to keep the liquid from flowing under baseboards or into adjacent rooms.
On wood subfloors, low spots can sometimes be shimmed from below or filled with floor leveling compound compatible with wood substrates. High spots are planed or sanded. High fastener heads get countersunk below the surface.
Why Substrate Prep Matters More Than Tile Quality
A $10/sqft porcelain tile set over a properly prepped, perfectly flat substrate will look better and last longer than $50/sqft stone tile set over an uneven, under-prepared floor. The material does not determine the quality of the outcome — the prep does. This is the consistent finding across failed tile installations: the tile itself is usually fine. The substrate under it was not ready.
For more detail on large format tile specifically, see the guide on large format tile floor preparation. For tile installation services in Orléans and Ottawa, visit the tile contractor page for Orléans or the tile contractor page for Ottawa.