Flatness for porcelain slabs is measured with the method in ISO 10545-2. The allowed limits come from the product standard your market uses, such as EN 14411 or ISO 13006 in Europe and much of the world, and ANSI A137.1 or ANSI A137.3 in the US.
Those limits are written as a percentage of the slab's dimensions, with a cap in millimetres. On a large slab, the percentage alone would allow a big bow, so the millimetre cap is the figure that matters. Many buyers agree a stricter figure with the factory than the standard requires.
What flatness means
ISO 10545-2 measures three kinds of deviation from flat:
- Centre curvature: how far the centre of the slab rises or dips compared with its corners.
- Edge curvature: how far the middle of each edge rises or dips compared with its ends.
- Warpage: how far one corner sits out of the plane set by the other three, a twist across the slab.
Why slabs are harder
The same ideas apply to floor tiles, covered in how flat a floor tile should be. Slabs make them harder to achieve, because a long, thin panel is more likely to bow in the kiln.
Why the standard limit is not enough
A percentage limit grows with size. On a 1200x2400 mm slab, even a small percentage of the diagonal is several millimetres, so the standard's millimetre cap usually applies. The standards were also written mainly for tiles, not panels used as worktops.
For your customers, what counts is lippage: the height step between two neighbouring slabs on the floor or wall. A slab that passes the standard can still give visible lippage if it bows and is laid with narrow joints, or in a long offset pattern.
Check the current edition of the standard your market uses for the exact limits. Then decide whether you need a tighter figure for your product and installation method.
Agreeing a figure with the factory
- Ask the factory for its own flatness limits for each size, in millimetres, and for its ISO 10545-2 test report.
- Write the agreed maximum centre curvature, edge curvature and warpage into your purchase order, with the method of measurement.
- Make it clear that slabs outside the limit are rejects, not a lower grade shipped in your order.
- For worktops, state the limit for the full slab, because fabricators cut from the whole panel.
How flatness is checked in practice
Lab tests use a calibrated rig with dial gauges. At the factory or warehouse, an inspector can do a practical check:
- Lay the slab face up on a flat, rigid table, or check it in its frame against a long straightedge.
- Place a straightedge along each edge and both diagonals, and measure the largest gap with feeler gauges.
- Check several slabs from different frames, not just the top one.
- Record each reading against the agreed limit.
Check again on arrival
Slabs can also bend in storage if they are not supported evenly. Check flatness before loading and again on arrival. Our guide on inspecting slabs before loading covers the full check.
Next step
We agree measurable flatness limits with the factory and check them on production slabs before loading. To set flatness terms for your next order, book a free sourcing review.