A purchase order says "6 mm aluminum checkered plate, 5052, 20 tonnes." The material arrives. A caliper over the raised bars reads 6.5 mm. A caliper on the flat between the bars reads 5.8 mm. Both readings are correct. Neither party is lying, and the argument will last three weeks.
The order was unfulfillable as written, because in tread plate "thickness" names two different measurements.
Two measurements, one word
Tread plate is not a flat sheet with something added on top. The pattern is rolled into the surface by a patterned roll, so the base metal is thinned slightly at the bars and the raised profile is formed from the same material.
That gives two distinct dimensions:
Base metal thickness — the flat, unpatterned portion. This is what the sheet would be if the pattern were absent.
Overall thickness — from the back face to the top of the raised bar.
The gap between them is the pattern height, and it is set by the roll tooling, not by the specification. Different mills, different tooling, different height. On a five-bar pattern in the 2–6 mm range, that height is on the order of half a millimeter, but "on the order of" is not something you can accept material against.
Why this is not just a wording problem
The standard that governs this material in US practice is ASTM B632/B632M, Standard Specification for Aluminum-Alloy Rolled Tread Plate. Its scope covers rolled flat tread plate, mill finish, with a raised pattern on one side, in the alloys, tempers and thicknesses given in its tables.
Two details in that scope matter to a buyer.
First, the standard is dual-unit, and it says so explicitly: the inch-pound and SI values are not exact equivalents, each system must be used independently, and combining values from the two systems results in nonconformance. That is a direct warning against the most common way this order gets written — 0.250 in (6.35 mm) — because under the standard's own rule, that is not one specification of one thickness.
Second, the standard's tables define thickness in a way that has to be matched to the drawing. If your drawing is dimensioned in inches and the mill works in millimeters, you are already in the zone the scope warns about.
For European supply the equivalent document is EN 1386, covering aluminum checkered plate. Same two-measurement situation, different tables. Do not assume the two standards are interchangeable on thickness.
What the wording actually produces
Three common phrasings, three different shipments.
"6 mm checkered plate" — the mill will produce 6 mm base metal, because that is the trade convention and it is what the standard's thickness column refers to. Your incoming inspector, measuring over the bars, will get something larger and may reject a conforming delivery.
"6 mm overall" — the mill targets 6 mm to the top of the bar, which means roughly 5.5 mm of base metal. If your design assumed 6 mm of structural section, you have less material than you calculated.
"6 mm base, pattern excluded" — unambiguous. This is the phrasing to use.
There is a fourth case worth naming, because it is where tonnage goes wrong: the order is placed on theoretical weight calculated from overall thickness. Pattern material is real material, and the pattern adds a few percent to the weight per square metre depending on the design. If the price was calculated on a thickness that includes the bars but the mill ships base thickness, the tonnage will not match the drawings, and if it was calculated the other way, you will pay for metal you did not receive. Weight calculations on tread plate should state which thickness they used.
How to write it on the order
The line that removes the argument:
Tread plate, ASTM B632/B632M
Alloy/temper: 5052-H32
Thickness: 6.0 mm base metal; raised pattern excluded from this dimension
Pattern: five-bar, pattern height to be stated by supplier
Width × length: 1500 × 3000 mm
Quantity: 20 t, weight basis to be confirmed
Four things are doing work there.
The standard is named, so there is a reference document if a dispute arises.
"Base metal" is written, and "excluded from this dimension" removes any reading in which the bars count.
The pattern height is requested as information. You are not specifying it — you cannot, it is tooling-dependent — but you are requiring the supplier to tell you what it is before you accept. That one number tells you whether the plate will clear the clearance in your design.
The weight basis is flagged. Whoever quotes will then have to state the assumption, and an assumption stated in writing is an assumption you can check.
If the plate is going into a structure, add the temper. 5052-H32 is the common choice for tread plate because it combines moderate strength with corrosion resistance; 3003-H22 is cheaper and adequate for dry internal floors; 6061-T6 is used where strength matters and bending is not. Temper changes the yield strength, and yield strength is what the floor is designed against.
The pattern is also a specification
Base versus overall is the dimension argument. The pattern itself is a separate choice, and it is usually left unstated too.
Five-bar is the most common industrial pattern: five short parallel bars in a repeating group, usually with the groups offset so that some bars run across the traffic direction. Good drainage, grip in more than one direction, and the pattern most mills tool for.
Diamond uses raised diamonds in two alternating orientations. Slightly more decorative than five-bar, similar performance, and the default for truck and trailer flooring in North America.
Lentil — an oval, lower-profile pattern — is common in European supply and reads as a flatter, less aggressive surface. It is specified for appearance as much as grip.
Two practical notes. The pattern is directional: laid one way it drains, laid the other it traps water against a joint. For anything outdoors, state the intended orientation on the drawing or accept what arrives.
And a higher pattern does not automatically mean better slip resistance. Wet grip depends on the profile, the alloy, surface contamination and footwear. If slip resistance is a safety requirement rather than a preference, it needs a stated test method and an acceptance figure — pattern appearance is not evidence, and no mill will warrant grip from a photograph of the surface.
How to inspect it
Three rules, and they matter more than the instrument.
Measure between the bars for base thickness, away from edges, at several points per sheet. Never measure over a bar and call it base thickness.
Measure over the bar for overall thickness, accepting that the reading varies with where on the profile you land. Take several and use the range.
Record which you measured. The single most common inspection failure is not a bad instrument or a dishonest supplier. It is a report that says "6.4 mm" with no statement of what was under the caliper.
For a first order from a new mill, ask for a sample or for the pattern height in writing before production. For repeat orders, keep the first certificate and the first measurement report — they are the reference that makes the second delivery quick to accept.
The tolerance question underneath
Even once you agree on base metal, the acceptance band is still open. Standard tolerances for tread plate are wider than for flat sheet of the same nominal thickness, because the rolling process is different.
If your process needs a tighter band — a floor panel that has to fit a frame, a plate that feeds an automated line — say so. It is quotable. It is not free.
And if the answer to "what tolerance" is "standard", ask for the number. A supplier who can produce the figure from the standard is a supplier who knows what they are shipping.
The short version
Tread plate has two thicknesses. Write which one you mean, name the standard and its edition, ask for the pattern height in writing, and state the weight basis.
That is four lines on a purchase order. It is cheaper than the three weeks.
If you have a tread plate requirement and want the dimension line checked before the order goes out, send us the drawing. We will tell you whether a mill can measure against it as drawn.