Steel Sigma Profiles and Sigma Purlins — Roll-Formed to Drawing

A sigma profile is a lipped steel open section cold-formed from coil on a roll-forming line, with two inward steps formed into its web; seen end-on it resembles the letter Σ, which gives it its name. The steps act as stiffeners running the length of the web, so at the same thickness a sigma section has more bending capacity and local-buckling resistance than a flat-web C-profile. That is why it is used for longer-span roof purlins and side rails in steel buildings, as load-bearing members in racking and storage systems, and as heavily loaded piles in solar plants.

The sigma profiles we make are structural steel sections, roll-formed from galvanized, Zn-Al-Mg (ZM) coated or black steel coil in 1–6 mm, each on its own roll set and to the customer's drawing. The solar use is covered separately on our Y Solar sigma pile page.

In a thin-walled profile the first part to give way under load is the wide, flat web plate: the wider it is relative to its thickness, the earlier local buckling sets in and the less of the section carries load. The two inward steps of a sigma section split the web into shorter, stiffer elements, so more of the section works effectively and bending capacity rises at the same thickness.

The stepped web also brings the shear centre closer to the plane of the web, so a vertical load applied through the flange twists the section less than it would a C-section of the same size. Sigma purlins are therefore chosen for roofs with longer spans and higher loads. The steps are formed on the roll-forming line in the same pass as the rest of the section; they need no separate operation.

C, Z and sigma profiles are all lipped, cold-formed open sections made on our roll-forming lines; the difference is in the web and flange geometry. A C-profile is simple and common, and the most economical choice wherever the load allows. A Z-profile, with flanges pointing in opposite directions, laps over the supports so that long purlin runs act as a continuous beam. A sigma profile, with its stepped web, gives more bending capacity at the same gauge.

At the same strip width and thickness the three sections weigh the same per metre; what differs is how the steel is distributed in the section. Span, load and connection detail decide which section carries the load with the least tonnage. We support section selection with engineering input; the project's structural calculation sets the final section.

A sigma section is defined mainly by its web depth (h), flange width (b), lip (c), the depth and position of the web steps, and its thickness (t). Unlike C and Z sections, sigma geometry varies from one design to the next, so there is no single standard size series. We roll each section to the drawing on a roll set we design and make in-house.

Steel grade is a design lever too: with S420GD or S450GD instead of S350GD the same load is carried at a thinner gauge, lowering tonnage. Thin-walled cold-formed profiles are designed to EN 1993-1-3, using effective rather than gross section properties; the project's structural calculation sets the final section. We support projects from the start with section optimisation and material and coating choice.

Because sigma geometry varies by design, weight is found most accurately from the developed (strip) width: strip width (mm) × thickness (mm) × 7.85 ÷ 1000 = kg/m. The developed width is the sum of the centre-line lengths of the web segments, steps, flanges and lips; the bend radii shorten it slightly. We work out the exact developed width of your section from your drawing.

For example a sigma profile rolled from a 300 mm strip in 2.5 mm weighs about 5.89 kg/m. Every value in the table below is calculated with the same formula for example strip widths, without the coating. For your own section, use the "Sigma / Custom (strip)" option in our steel weight calculator.

We form sigma profiles from pre-coated coil — Zn-Al-Mg (ZM120–ZM430) or galvanized (Z275–Z600) — so the profile leaves the line coated and needs no post-galvanizing bath. Zn-Al-Mg self-heals at sheared and punched edges and lasts longer than galvanizing at the same coating mass, which matters most on heavily punched purlins and piles. See our Zn-Al-Mg vs galvanized guide.

Structural purlins, side rails and piles are rolled in S350GD, S420GD and S450GD (EN 10346); general-purpose profiles are rolled from DX51D+Z galvanized steel. Sigma profiles that will be painted or welded into a fabrication are also rolled from black steel. Where the specification calls for hot-dip galvanizing, we supply that too.

Yes. Decoiling and levelling, roll forming, in-line punching and flying cut-off to length all happen on one line. Purlin connection holes, splice holes and adjustment slots are punched in the same pass as the forming, so hole positions are identical across thousands of parts and there is no drilling or cutting on site.

Profiles are cut to the lengths on your order list, then bundled and labelled by section and length. Because we design and make the tooling and rolls in-house, a sigma geometry for your drawing or platform moves quickly to first article and series production. Our article roll forming vs press braking explains when roll forming beats press braking.

In solar plants the sigma section works as the driven pile that carries the table. The sigma piles we make under our Y Solar brand have a 120–240 mm web depth and 2–6 mm thickness, in S350GD–S450GD, rolled from Zn-Al-Mg or galvanized pre-coated coil. High wind and snow load, large 2V tables, and soft ground that calls for deeper driving are where the sigma section pays off.

Some single-axis tracker platforms also use sigma or similar stiffened sections as posts; the platform design is the tracker supplier's, and we manufacture to the specified section, tolerance and coating. See the sigma pile and tracker steel structures pages for details. Under the Y Solar brand we have supplied more than 1 GW of solar steel to date.

Profiles are packed by section and length, strapped with steel banding and labelled; we prepare the loading plan around the profile length. We ship across Türkiye and to Europe, the Middle East and North America by truck or container.

EN 10204 3.1 mill certificates and certificates of origin are provided on request; delivery terms (EXW, FOB or CIF) and the lead time are agreed at quotation. Production runs under an ISO 9001 quality management system.