Two suppliers can quote the same section at the same thickness and the same coating. What separates them is how close to the drawing the steel actually arrives, and whether it arrives that way on the tenth truckload as well as the first.
That is what tolerance standards are for. IS 811 and EN 10162 set the permitted deviations for cold-rolled steel sections: how far the web can be from its nominal depth, how straight the member has to be, how much twist is acceptable, and where the holes are allowed to sit. This guide walks through what they cover and why the numbers matter on site.
What the standards actually cover
IS 811 is the Indian standard for dimensional tolerances on cold-formed light gauge structural steel sections. EN 10162 is the European specification for cold-rolled steel sections, covering technical delivery conditions and dimensional and cross-sectional tolerances. They serve the same purpose and a supplier quoting both is telling you the section is held to a published, checkable limit rather than to whatever the line happens to produce.
Tolerance is not the same as quality control. A tolerance is the permitted deviation; quality control is whether anyone measured. Both matter, and the second one is the question buyers forget to ask.
Section dimensions: web, flange, lip, thickness
The permitted deviation on cross-section dimensions scales with the dimension itself, which is why a single number never covers a whole range.
| Feature | Range | Tolerance |
|---|---|---|
| Web | 40 to 300 mm | ≤ 100 mm: ±0.75 · ≤ 200 mm: ±1 · over 200 mm: ±1.5 |
| Flange | 40 to 90 mm | ≤ 100 mm: ±0.75 |
| Lip | 10 to 25 mm | ≤ 40 mm: ±0.75 |
| Thickness | 0.7 to 3.0 mm | ±8% |
| Angle | 90° | Shorter flange ≤ 10 mm: ±3° · 10–40: ±1°45′ · 40–80: ±1°15′ · 80–110: ±1° · over 110: ±0°45′ |
The thickness figure deserves a second look. A tolerance of ±8% on a 2.0 mm section means anything from 1.84 to 2.16 mm is compliant. At the bottom of that band you have roughly 8% less material than nominal, and section capacity follows material. This is not a defect — it is the standard — but it is why a design should not sit right on the edge of capacity at nominal thickness.
Length, and why the band widens
Length tolerance is banded, and the band widens with length:
| Cut length | Tolerance |
|---|---|
| ≤ 2 000 | ± 1 |
| 2 000 to 6 000 | ± 2 |
| 6 000 to 10 000 | ± 3 |
| over 10 000 | ± 4 |
Maximum length on our profiles is 12 metres, and on slotted strut 6 metres.
Straightness, twist and concavity
These are the tolerances buyers skip and installers notice.
Straightness is limited to 2 mm per metre of length. On a 12 metre rafter that permits 24 mm of bow — enough to see, and enough to matter when the member has to line up with a pile cap.
Twist is limited to 1° per metre. Twist is the one that makes a member fight you during erection: the holes at one end line up and the holes at the other do not, because the section has rotated along its length. A twisted purlin also means the module sits on a surface that is not quite flat.
Concavity and convexity of the web is limited to 0.8% of the web height, with a minimum of 0.5 mm; on slotted strut it is 0.5 mm. A dished web changes how a bolt seats and how load transfers into the connection.
Straightness and twist are the tolerances that decide how a crew's day goes. They are rarely on a purchase order and almost always on the complaint.
Holes, slots and cumulative error
Hole diameters of 12 and 14 mm carry ±0.5 mm. Slots of 12 × 24 and 14 × 28 mm carry ±1 mm. Hole and slot centres carry ±1 mm, and on slotted strut the centres are at 50 mm pitch.
The number to think about is not the individual tolerance but what happens across a row. If centres are set from a single datum along the member, error does not accumulate. If each hole is measured from the previous one, it does — and by the fiftieth hole the drift is meaningful. This is a punching-setup question, and it is a fair thing to ask a supplier about directly.
The same logic applies across bays. Twenty tables in a row, each with a few millimetres of length tolerance in the same direction, and the last table is not where the layout said it would be. Slotted holes at splices absorb this deliberately, which is why jointer plates are usually slotted rather than round-holed.
Five questions to ask a supplier
- Which tolerance standard do you work to? If the answer is not a published standard, there is no agreed limit to hold anyone to.
- Do you measure, and can I see the report? Ask for a dimensional report against the standard for the batch, not a generic certificate.
- What is your straightness and twist limit? Named separately, because these are the two most likely to be quietly outside spec.
- How are hole centres set out? From a single datum, or hole to hole. This decides whether error accumulates.
- Will a repeat order match? If the profile is recorded, a batch six months later rolls to the same dimensions. If it is set up fresh each time, it may not.
None of these questions are unreasonable, and a manufacturer who takes tolerance seriously will have quick answers. Full tolerance tables for our profiles and strut are published on the sizes and tolerances section, and we can supply a dimensional report against any batch — see Engineering Services.
Want the tolerance data for a specific section?
Tell us the profile, thickness and coating and we will send the physical properties and the dimensional limits we hold it to, before you order rather than after.
Request section data