Walk into any portal-frame warehouse, workshop or logistics hall and you will notice the steel columns and rafters - but the parts that actually carry the roof and walls are the purlins and girts. Most of these slender members are cold-formed from S220GD galvanized steel coil, an EN 10346 structural grade that has become the default raw material for the light steel building industry. This article explains what purlins and girts do, why S220GD suits them so well, and what to specify when you order the coil.
What Are Purlins And Girts?
In a steel portal-frame building the main frames (columns and rafters) are spaced several meters apart. Purlins and girts are the secondary members that span between those frames and directly support the building envelope:
- Purlins are horizontal members laid across the roof rafters to support roof sheeting or sandwich panels. They also brace the top of the main frame against buckling.
- Girts are horizontal members fixed to the outside of the columns to support wall cladding and to carry wind loads on the walls.
Both are almost always cold-formed sections - typically C, Z, Sigma (Σ) or U profiles rolled from galvanized strip. Because they are spaced at regular intervals (usually 1.0–1.5 m), a single industrial building can consume hundreds of meters of section, which is why purlins and girts are one of the biggest volume applications for galvanized structural coil in the world.
| Member | Position | Main Function | Typical Section |
|---|---|---|---|
| Purlin | On the roof slope, across rafters | Supports roof sheeting / panels; transfers roof loads to the main frame | C, Z, Sigma section |
| Girt | On the walls, across columns | Supports wall cladding; resists wind load on the facade | C, Z, U section |
Why S220GD Is The Standard Grade For Purlins And Girts
Purlins and girts are load-bearing members, but they are also mass-produced by roll forming. S220GD sits at exactly the right point of the strength–formability–cost balance:
- Guaranteed structural strength. Unlike forming grades such as DX51D, S220GD guarantees a minimum yield strength of 220 MPa. Engineers can calculate section capacity with a certified value, which is essential for load-bearing roof and wall members.
- Good cold-forming behavior. The "GD" designation means the steel is designed for cold forming - it can be roll-formed into tight C and Z profiles, punched and notched without cracking or losing its mechanical properties.
- Inherent corrosion protection. The hot-dip zinc coating protects the finished section for decades, including the cut edges where sacrificial galvanic action continues to shield exposed steel.
- Economic efficiency. For the majority of secondary members 220 MPa of guaranteed yield is sufficient; there is no need to pay for a higher S-grade unless the section is heavily loaded or the spans are very long.
Key Properties Of S220GD
S220GD is delivered to EN 10346 (material number 1.0241). Its chemical composition is tightly controlled to keep carbon and sulfur low so the strip welds and cold-forms reliably:
| Grade | C (max. %) | Si (max. %) | Mn (max. %) | P (max. %) | S (max. %) |
|---|---|---|---|---|---|
| S220GD (1.0241) | ≤ 0.20 | ≤ 0.60 | ≤ 1.70 | ≤ 0.10 | ≤ 0.045 |
| Property | Guaranteed Value | Meaning For Purlin Design |
|---|---|---|
| Min. yield strength | ≥ 220 MPa | Certified design strength for section capacity calculations |
| Min. tensile strength | ≥ 300 MPa (typical 300 – 440) | Safety margin before failure of the section |
| Elongation A80 | ≥ 20% | Allows tight bending and roll forming without cracking |
Thickness correction for elongation (EN 10346): reduce by 2 units for 0.50 mm < t < 0.70 mm, by 4 units for 0.35 mm < t < 0.50 mm and by 7 units for t < 0.35 mm. Values are for longitudinal test pieces.
What To Specify When Ordering Coil For Purlin Production
If you are a roll former or a trader buying S220GD coil to make purlins and girts, a typical specification looks like this:
| Parameter | Typical Requirement For Purlins / Girts |
|---|---|
| Steel grade | S220GD+Z to EN 10346 (material no. 1.0241) |
| Thickness | 1.5 – 3.0 mm (section thickness of common C/Z profiles) |
| Width | 600 – 1500 mm; slit coil from 30 mm for the required section width |
| Coating | Z275 (standard for outdoor buildings); Z350 for coastal / industrial areas |
| Surface | Minimized spangle or regular spangle; chromated and lightly oiled recommended for storage |
| Coil weight | 3 – 8 tons per coil |
| Certificate | Mill test certificate (MTC) per EN 10204; ISO 9001 / CE; third-party SGS, BV or TUV inspection available |
Design And Fabrication Considerations
- Bending. S220GD forms well, but keep corner radii as generous as the profile allows. Very sharp 90° bends increase the risk of micro-cracking of the zinc layer at the outer radius.
- Cut edges and punched holes. Exposed steel at sheared edges and punched holes is normal - the surrounding zinc protects them by sacrificial action. Avoid letting these areas sit permanently in standing water.
- Dissimilar metal contact. Do not fix aluminum or copper flashings directly onto galvanized sections without a separating gasket, otherwise bimetallic corrosion can shorten coating life.
- Handling and storage. Bundles of finished sections must be stored dry and tilted for drainage; condensation between nested profiles is a common cause of white rust before the building is erected.
- Design standard. Section sizes, spacing and spans must be verified by your structural engineer against the applicable design code (e.g. EN 1993-1-3 for cold-formed members) - S220GD guarantees the material properties that make that calculation valid.
FAQ
1. Why not use DX51D for purlins?
DX51D is a forming grade without a guaranteed yield strength, so it cannot be used as a certified load-bearing material. S220GD guarantees a 220 MPa yield point and is the lowest structural grade suitable for purlins and girts.
2. Which coating weight should purlin coil have?
Z275 (about 275 g/m² total both sides) is the standard for outdoor metal buildings. If the project is near the coast or in an industrial atmosphere, upgrade to Z350 or consider the +ZM (zinc-aluminium-magnesium) coating. For more detail see our S250GD+ZM channel article.
3. C section or Z section - which is better?
Both are cold-formed from the same S220GD strip. C sections are simple and economical for short spans and end bays; Z sections can be nested and lapped at the joints, which makes them the common choice for continuous multi-span roofs.
4. Can S220GD purlins support solar panels on the roof?
Yes - this is increasingly common. The purlin depth, spacing and the added snow/wind load of the PV array must be re-checked by an engineer, and the coating choice should be reviewed for the longer design life of solar installations.
5. What thickness of S220GD do you supply for roll forming?
Standard supply is 0.30 – 3.00 mm with widths of 600 – 1500 mm; slit coils are available from 30 mm. The 1.5 – 3.0 mm range covers most C/Z purlin programs. S220GD+Z color-coated coil can be supplied on the same base if your sections are painted after forming.