Surface Accuracy & Datums: Understanding Floor Levels vs Flatness
Two terms that frequently cause confusion in screeding are "surface accuracy" (also called surface regularity or flatness) and "datums" (levels). They measure fundamentally different things, and understanding the distinction is essential for specifying, laying, and assessing a floor screed correctly.
What Are Datums?
Datums are the designed finished floor levels — the target heights that the screed surface should reach at specific points. They're established by the architect or structural engineer and transferred to site as level marks on walls, columns, or dedicated datum pegs. When we talk about a screed being "to level" or "at the correct datum," we mean the screed surface is at the intended height relative to the building's reference datum (usually the finished floor level or structural slab level).
Getting the datum right ensures that the screed meets door thresholds, connects properly to adjacent floor areas, achieves the designed thickness over insulation and services, and finishes flush with other floor zones. Errors in datum — laying the screed too high or too low — create problems that are expensive to fix after the event.
What Is Surface Regularity?
Surface regularity — or flatness — is a completely different measurement. It describes the smoothness of the screed surface regardless of its absolute height. A screed can be perfectly level (at exactly the right datum) but have an unacceptably rough or wavy surface. Conversely, a screed can be beautifully flat but at completely the wrong level.
Surface regularity is measured by placing a straightedge (typically 2 metres long) on the screed surface and measuring the maximum gap beneath it. BS 8204-1 defines three classifications: SR1 (high standard — maximum 3mm deviation under a 2m straightedge), SR2 (normal standard — maximum 5mm), and SR3 (utility standard — maximum 10mm). For a deeper dive into these classifications, see our guide to surface regularity standards.
Why the Distinction Matters
We frequently encounter situations where these concepts are confused on site. A screed can pass a surface regularity check (nice and flat) but be 10mm above the intended datum — meaning the door won't close. Or the screed can be at exactly the right level but with 8mm dips and ridges across the surface — meaning thin vinyl will show every imperfection.
Both measurements matter, and both need to be checked independently. The laying team should be working to datum marks that define the target height, while also using straightedges to ensure the surface between those datum points is flat to the specified SR classification.
Achieving Good Surface Accuracy on Different Floor Bases
The flatness of the substrate beneath the screed directly affects the ability to achieve consistent thickness and surface regularity above. For unbonded and floating screeds, where the screed sits on a membrane or insulation rather than bonding to the concrete, the base flatness is particularly important. High points in the substrate create thin spots in the screed above — and thin spots are weak spots that are prone to cracking and poor surface finish.
As a practical guide, the concrete substrate for an unbonded screed should not deviate more than approximately 10mm under a 3-metre straightedge. Where deviations exceed this, either level the substrate first with a repair compound or increase the screed thickness to compensate. For bonded screeds, the substrate flatness is less critical to screed thickness but still affects the quality of the mechanical bond.
Normal Accuracy vs High Accuracy Flooring
The surface regularity required depends on the final floor finish. Ceramic and stone tiles are relatively tolerant — SR2 or even SR3 is often acceptable, as the tile adhesive bed can accommodate minor surface variations. Thin vinyl sheet and LVT require SR1, as any surface imperfections will telegraph through the thin material and show on the finished floor. Resin floor coatings are the most demanding, typically requiring SR1 with additional local flatness requirements.
Flowing screeds — including our Ardex and Mapei self-levelling ranges — deliver SR1 surface regularity as standard, without the skilled hand-finishing that traditional screeds require. For refurbishment projects where the existing screed surface doesn't meet the required standard, a thin self-levelling compound overlay is the fastest and most reliable correction method.
Need Help?
If you're unsure what surface regularity standard your project requires, or need products to correct an existing surface, call us on 0118 370 2060. We stock levelling compounds and self-levelling screeds from Ardex, Mapei, and Weber that can achieve SR1 finishes quickly and reliably. Free delivery on orders over £600 ex-VAT.
