Galvanised steel in large-span construction -
applications and durability
In industrial halls, warehouses and agricultural buildings, sheet hot-dip coated to EN 10346 performs two functions at once: it is a structural material for cold-formed members and a cladding material. Structural grades from S220GD to S550GD provide a yield strength from 220 to 550 MPa, and coatings from Z100 to Z600 correspond to a zinc thickness of 7 to 42 µm per side. The choice of grade and coating mass decides whether the building will reach its intended service life without maintenance work.
The durability calculation is also simpler than commonly assumed. Two figures are enough: the coating thickness and its rate of loss in the given environment.
Where coated sheet works in a building
Applications fall into several groups with different requirements.
Cold-formed structural members, i.e. purlins and side rails with C, Z or sigma sections, are made from strip in structural grades, most often S280GD, S320GD and S350GD. Yield strength and consistency of properties within a batch are decisive here.
Cladding, i.e. trapezoidal sheet, wall liner trays and facings, is made from grades with lower strength requirements but higher surface requirements, because the element remains visible.
Sandwich panels use coated sheet as the outer and inner facing of the insulating core.
Flashings, drainage systems and support structures for building services form the fourth group, in which corrosion resistance and formability are the main selection criteria.
| Grade to EN 10346 | Re min [MPa] | Rm min [MPa] | A80 min [%] |
|---|---|---|---|
| S220GD | 220 | 300 | 20 |
| S250GD | 250 | 330 | 19 |
| S280GD | 280 | 360 | 18 |
| S320GD | 320 | 390 | 17 |
| S350GD | 350 | 420 | 16 |
| S390GD | 390 | 460 | 16 |
| S420GD | 420 | 480 | 15 |
| S450GD | 450 | 510 | 10-14 |
The relationship is visible at first glance: the increase in yield strength comes at the expense of elongation, and therefore of formability. For profiles with complex sections and small bending radii, the strongest grade is not always the right one.
Design and execution standards
| Standard | Scope |
|---|---|
| EN 1993-1-3 | design of cold-formed members and sheeting |
| EN 1090-2 | execution of steel structures, execution classes EXC1-EXC4 |
| EN 1090-4 | execution of cold-formed thin-walled elements and of roof, ceiling, floor and wall structures |
| EN 508-1 | self-supporting roofing and cladding products of steel sheet |
| EN 14782 | self-supporting metal sheet for roofing, external cladding and internal lining |
| EN 10346 | technical delivery conditions for continuously hot-dip coated products |
The boundary between EN 1090-2 and EN 1090-4 is a frequent source of misunderstanding. Part 2 covers steel structures in general, while Part 4 refers directly to cold-formed thin-walled elements and to roof, ceiling, floor and wall structures, i.e. what the envelope of a hall and its substructure is actually built from. EN 14782, in turn, limits its scope to non-load-bearing applications, with the permanent load limited to the self-weight of the sheet.
Coating durability: a calculation based on two figures
Coating mass translates into thickness according to a fixed relationship: 275 g/m2, counted for both sides together, corresponds to about 20 µm per side.
| Designation | Coating mass [g/m2] | Thickness per side [µm] |
|---|---|---|
| Z100 | 100 | 7 |
| Z140 | 140 | 10 |
| Z200 | 200 | 14 |
| Z275 | 275 | 20 |
| Z350 | 350 | 25 |
| Z450 | 450 | 31 |
| Z600 | 600 | 42 |
The second figure is the rate of zinc loss in the given environment, specified in ISO 9223 for the first year of exposure.
| Corrosivity category | Zinc loss [µm/year] | Typical environment |
|---|---|---|
| C1 | up to 0.1 | heated, clean interiors |
| C2 | 0.1-0.7 | rural areas, low pollution |
| C3 | 0.7-2.1 | urban and industrial areas with moderate pollution |
| C4 | 2.1-4.2 | industrial areas, moderate salinity |
| C5 | 4.2-8.4 | industrial zones with high humidity, coastal areas |
| CX | 8.4-25 | extreme environments, offshore |
Dividing the coating thickness by the rate of loss gives an indicative service life. For Z275, i.e. 20 µm per side, this gives several decades in category C2, from about ten to almost thirty years in C3, five to ten years in C4 and two to five in C5. The method is conservative, because the actual corrosion rate of zinc decreases over time as the carbonate patina forms, so the real service life is often longer.
The conclusion for a building designed for 25-50 years without maintenance is nevertheless clear. In categories C1 and C2 a Z275 coating is sufficient with a margin. In the upper range of C3 it approaches the limit. In C4 and C5, i.e. in industrial and coastal environments and in livestock buildings, a stronger solution is needed: a higher coating mass, an organic coating on the sheet, a zinc-magnesium coating or, for load-bearing members, hot-dip galvanising after fabrication.
For zinc-magnesium coatings it is worth remembering that a study by an independent institute found thickness loss for ZM250 about 2.2 times slower than for Z275 at non-coastal sites, and ZM310 was judged to give protection no worse than Z600. This makes it possible to achieve the same durability with almost half the amount of metal.
Continuous galvanising versus hot-dip galvanising after fabrication
Both solutions are used in the same building, but for different elements.
| Feature | Coated sheet to EN 10346 | Hot-dip galvanising to EN ISO 1461 |
|---|---|---|
| When the coating is applied | before cutting and forming, on a continuous line | after the element is fabricated |
| Typical coating thickness | 7-42 µm per side, depending on coating mass | 45-85 µm average, depending on steel thickness |
| Edges, holes and welds | no factory coating where processed | covered, because processing precedes galvanising |
| Typical application | cold-formed profiles, trapezoidal sheet, liner trays, flashings | load-bearing structure, welded elements, substructures |
The requirements of EN ISO 1461 increase with element thickness: for steel over 6 mm the minimum mean thickness is 85 µm, for the 3-6 mm range 70 µm, for 1.5-3 mm 55 µm and below 1.5 mm 45 µm. The difference compared with coated sheet is therefore twofold or more, which for load-bearing members translates directly into the maintenance-free service period.
The limitations of hot-dip galvanising are the dimensions of the galvanising bath and the need to design vent and drain holes. The standard requires a sufficient number and size of holes to allow safe venting and drainage of zinc, and unvented closed spaces must not be galvanised because of the risk of explosion. This requirement concerns the geometry of the element after welding, so it must be taken into account at the design stage, not when the galvanising is ordered.
Cut edges, holes and connections
In continuously coated sheet, every cut and drilled hole exposes the steel core. Zinc protects such a spot cathodically, but the reach of this protection depends on the material thickness.
GalvInfo research shows this relationship clearly: 1 mm thick sheet showed no rust on the cut edge after six months of exposure, whereas 2.3 mm sheet did after two weeks. At thicknesses below 1 mm, a cut edge can remain rust-free for the entire service life of the product.
The practical conclusion: in cladding made of thin sheet the edges need no special treatment, whereas in thicker elements and at points welded or drilled after galvanising the protection must be restored, usually with zinc-rich paint.
Bolted connections between galvanised elements do not create a risk of galvanic corrosion, because the potential difference is negligible. The risk arises on contact with other metals.
Special environments
Contact with copper and brass. In a humid environment it accelerates zinc corrosion, and even water run-off from these metals is harmful. Electrical isolation with non-conductive spacers is recommended.
Contact with treated timber. Modern copper-based preservatives are significantly more corrosive than older formulations. Hot-dip galvanised steel or stainless steel are indicated as acceptable solutions for fasteners and fittings in contact with such timber.
Contact with concrete. Galvanised steel in concrete withstands a chloride concentration several times higher than black steel before depassivation, and zinc corrosion products, less voluminous than iron oxides, migrate into the concrete matrix without causing spalling of the cover. Note: aluminium-zinc coatings behave the opposite way in this environment and should not come into contact with fresh concrete.
Livestock buildings. Elevated temperature and high humidity hinder the formation of a protective patina on the zinc coating. Common practice is additional protection of the lower zone of elements, which is most exposed to moisture and bedding.
Condensation under the roof sheeting. The mechanism is analogous to storage stain: moisture trapped without air circulation keeps the surface wet. Ventilation measures in the roof build-up are as important here as the choice of coating mass.
Cost and environmental arguments
The advantage of the galvanised solution becomes apparent in a calculation covering the entire service life, not in a comparison of material prices. The protection does not need renewing in typical corrosivity categories, and the cost of any access to the structure in an operating building can be many times higher than the cost of the coating itself.
On the environmental side, steel remains a material with a high recovery rate. The life cycle assessment methodology used by worldsteel for structural sections assumes 90 percent material recovery at end of life and accounts for 0.677 tonnes of external scrap per tonne of product as recycled input.
It pays to settle the grade and coating mass for a specific building when the material is ordered, not after the first years of service.
Available steel grades, coating types and formats of coils, slit strip and sheets are presented in the HEN-STOL steel sheet offer, with separate pages for galvanized steel sheet, cold-rolled steel sheet and hot-rolled steel sheet.
See how we can help
HEN-STOL supplies coated sheet in the grades and coating masses used in construction, as coils, slit strip and sheets, and helps match the parameters to the corrosivity category and the intended service life of the building. Send us the specification and a description of the application, and we will check availability and propose the most favourable option.
Frequently asked questions
Which sheet grade is used for purlins and side rails?
Most often structural grades to EN 10346: S280GD, S320GD and S350GD, with a yield strength of 280, 320 and 350 MPa respectively. The choice depends on the loads and on the required formability of the section.
How many years does a Z275 coating last?
Several decades in corrosivity category C2, from about ten to almost thirty years in C3, and five to ten in C4. The estimate is based on a thickness of 20 µm per side and zinc loss rates to ISO 9223, and the actual service life is often longer because the corrosion rate decreases over time.
What is the difference between galvanised sheet and a hot-dip galvanised element?
When the coating is applied and how thick it is. Coated sheet has its coating applied on a continuous line before cutting and forming, typically 7-42 µm per side. A hot-dip galvanised element receives its coating after fabrication, typically 45-85 µm on average, covering edges and welds as well.
Do cut edges need protection?
In thin sheet, zinc protects the edge cathodically, and at thicknesses below 1 mm it can remain rust-free for the entire service life. For thicker material and at points welded or drilled after galvanising, restoring the protection with zinc-rich paint is recommended.
Which coating should be chosen for a coastal or industrial building?
In categories C4 and C5 a Z275 coating does not provide a service life of several decades. Higher coating masses, zinc-magnesium coatings, organic coatings on the sheet or hot-dip galvanising of load-bearing members are used.
Can galvanised steel be in contact with concrete?
Yes, and it performs well in concrete, withstanding higher chloride concentrations than black steel. The limitation concerns aluminium-zinc coatings, which should not be used in contact with fresh, wet concrete.
Sources
- EN 10346, Continuously hot-dip coated steel flat products for cold forming - technical delivery conditions
- EN ISO 1461, Hot dip galvanized coatings on fabricated iron and steel articles
- ISO 9223, Corrosion of metals and alloys - corrosivity of atmospheres - classification, determination and estimation
- EN 1993-1-3, EN 1090-2 and EN 1090-4, design and execution of structures made of cold-formed members
- The Steel Construction Institute, report RT1999 on zinc-magnesium coatings
- GalvInfo Center, GalvInfoNote 3.6, cut edge behaviour of galvanised sheet
- American Galvanizers Association, materials on contact with other metals, concrete and treated timber
- worldsteel, LCA eco-profile, structural sections