Steel sheet for ventilation ducts -
standard requirements and material selection
The material standard in the manufacture of ventilation ducts remains galvanised sheet in grade DX51D with a Z275 coating to EN 10346, in thicknesses from 0.5 to about 1.25 mm. This choice is not a matter of industry habit but of three independent requirements: the Polish regulation on technical conditions for buildings requires a non-combustible material, EN 1507 and EN 12237 link wall thickness to the strength and air tightness class of the duct, and duct production technology requires material of repeatable thickness and flatness, fed from the coil to the line in an automatic cycle.
Each of these requirements narrows the choice on its own. Together they reduce it to a narrow range of material in which differences between deliveries become visible on the production line.
Standards that describe a ventilation duct
The set of product standards is clearly divided and easy to organise.
| Standard | Scope |
|---|---|
| EN 1505 | rectangular sheet metal ducts and fittings, dimensions |
| EN 1506 | circular sheet metal ducts and fittings, dimensions |
| EN 1507 | rectangular ducts, strength and leakage requirements |
| EN 12237 | circular ducts, strength and leakage of ductwork |
| EN 15727 | ductwork components, leakage classification and testing |
| EN 12599 | test procedures and measurement methods for handing over installations |
The division therefore runs along several lines: dimensions, strength and air tightness, ductwork components and acceptance of the finished installation are each covered separately. In Poland, PN-EN 1507 replaced the previously withdrawn national standard PN-B-76001.
Air tightness classes and their effect on the material
Duct air tightness is described by the permissible air leakage factor, expressed by a formula containing the test pressure. The classes are designated by letters A to D, and each subsequent class means a factor about three times lower than the previous one.
| Class | Air leakage factor | Practical use |
|---|---|---|
| A | 0.027 × pt0.65 × 10-3 | lowest requirements, usually insufficient for ducts |
| B | 0.009 × pt0.65 × 10-3 | most commonly used in Poland, treated as the minimum level for ducts |
| C | 0.003 × pt0.65 × 10-3 | installations with increased system pressure |
| D | 0.001 × pt0.65 × 10-3 | special systems and increased hygiene requirements |
The air tightness class does not follow directly from sheet thickness but from the design of joints, the type of seams, the quality of sealing and wall stiffness. Thickness enters the calculation indirectly: a wall that is too thin deforms under operating pressure, the seam no longer seals evenly, and the air tightness test result deteriorates despite correctly applied sealing.
That is why duct manufacturers assign thicknesses not only to the duct size but also to the pressure class for which the duct is intended.
Typical sheet thicknesses
EN 1507 and EN 12237 set out strength and air tightness requirements, while the specific thicknesses result from calculations made for a given duct design. For this reason manufacturers' catalogues differ in detail, although they fall into similar ranges.
| Duct size | Typical sheet thickness in manufacturers' catalogues |
|---|---|
| rectangular duct, side up to 500 mm | 0.6 mm |
| rectangular duct, side 501-1,000 mm | 0.8 mm |
| rectangular duct, side over 1,000 mm | 1.1 mm |
| circular duct, diameter up to 500 mm | 0.5 mm |
| circular duct, diameter 560-800 mm | 0.7 mm |
| circular duct, diameter 900-1,800 mm | 0.9 mm |
The overview is indicative and comes from manufacturers' catalogues referring to the standards listed. For a specific project, the thickness resulting from calculations for the intended pressure and air tightness class applies. For a sheet supplier, what matters is that the whole range falls within 0.5-1.25 mm, i.e. in the thin material segment, where thickness tolerance and flatness have a greater effect on the process than with thicker sheet.
Building and fire safety requirements
The Polish regulation on the technical conditions to be met by buildings and their location governs this matter in two paragraphs.
Paragraph 267 requires ventilation ducts to be made of non-combustible materials. Combustible thermal and acoustic insulation and other combustible linings may be used only on the outer surface of the duct, provided they do not facilitate the spread of fire. The provision also sets the distance of uninsulated ducts from combustible materials at no less than 0.5 m, as well as requirements for flexible connectors, which must be of low flammability, not longer than 4 m, and 0.25 m when connected to fans. Separate rules apply to single-family and multi-family residential buildings.
Paragraph 268 governs duct penetrations through partitions and the use of fire dampers. At penetrations through fire separation elements, dampers with a fire resistance class matching that of the element are required, and in zones covered by a fire alarm system they must be triggered automatically by that system. Penetrations must allow for thermal expansion, and anchorages are made of non-combustible materials.
The requirement for a non-combustible duct material determines the use of steel and rules out alternative solutions in most public and industrial buildings.
Why DX51D with a Z275 coating
DX51D is a low-carbon steel grade for cold forming, covered by EN 10346, with an A80 elongation of at least 22 percent. This value translates directly into the process: the sheet must withstand seam folding, forming of stiffening beads and shaping of fittings without cracking on the outer bend radius.
The Z275 coating means 275 g/m2 of zinc counted in total for both sides, i.e. about 20 µm per side. Indoors, in corrosivity category C1 or C2, this gives durability exceeding the typical service life of the installation. In buildings with a higher corrosivity category, with high humidity or aggressive substances present, heavier coating masses are used.
Installations operating in chemically aggressive environments form a separate group: swimming pools, coastal buildings and fume extraction systems. Manufacturers use stainless steel there, typically grade 1.4301, and where chlorides are present grade 1.4404 with added molybdenum, which markedly improves resistance in such conditions.
Surface finish also matters. Sheet intended for duct production should have a surface free of storage stain and excess protective oil. Staining spoils the appearance of the finished product, and excess oil makes it difficult to apply sealants in the seams, on which the achieved air tightness class depends.
Material and duct production technology
A rectangular duct line performs in one sequence uncoiling, straightening, forming of stiffening beads, cross-cutting and folding into the final shape. Typical lines handle material 0.5-1.2 mm thick, with a cutting accuracy of about ±1 mm per metre and cross-sections from about 140 by 140 mm to 2,500 by 2,500 mm.
Two feedstock parameters are particularly important here. The first is thickness tolerance: the straightener and the forming section are set for a specific thickness, so variations between batches force adjustments and lengthen changeovers. The second is flatness and absence of camber in the strip, because material deviating from a straight line leads to skewing of the sheet and, as a result, to uneven seam closure along the whole length.
Circular spiral ducts are made by helically winding narrow strip while closing the seam at the same time. Here, in turn, the key factors are strip width accuracy and edge quality after slitting, as both determine the geometry and tightness of the seam. Slitting coils into strip of the required width is usually done by a steel service centre rather than by the duct manufacturer itself.
What this means for purchasing
From the point of view of a duct manufacturing plant, three things matter: repeatable thickness across successive deliveries, stable surface quality and strip width accuracy for material intended for spiral lines. Any deviation translates into changeover time, the air tightness test result or the appearance of the product, and therefore directly into cost.
With a regular demand, it pays to define the material parameters once and keep them in successive deliveries, rather than adapting the line settings to each batch separately.
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 galvanised sheet in the grades and coating masses used in ventilation, as coils, slit strip and sheets, with the width matched to the specific product. Send us the specification and the required widths, and we will check availability and propose the most favourable option.
Frequently asked questions
What sheet is used for ventilation ducts?
The standard is galvanised sheet in grade DX51D with a Z275 coating to EN 10346, in thicknesses from 0.5 to about 1.25 mm. In chemically aggressive environments stainless steel is used, most often grade 1.4301, and 1.4404 where chlorides are present.
Which standards set the requirements for ventilation ducts?
Dimensions are specified in EN 1505 for rectangular ducts and EN 1506 for circular ducts. Strength and air tightness are covered by EN 1507 and EN 12237. Acceptance of the installation is carried out to EN 12599.
What does air tightness class B mean?
It is the level of permissible leakage described by the factor 0.009 × pt^0.65 × 10^-3, in practice treated as the minimum for ventilation ducts and the most commonly used in Poland. Each subsequent class means a factor about three times lower.
Do ventilation ducts have to be non-combustible?
Under paragraph 267 of the Polish regulation on technical conditions for buildings, ventilation ducts must be made of non-combustible materials, with combustible insulation permitted only on the outer surface of the duct, provided it does not facilitate the spread of fire.
What does the sheet thickness of a duct depend on?
On the cross-section size and on the pressure and air tightness class for which the duct is intended. Thickness results from strength calculations to EN 1507 or EN 12237, and manufacturers' catalogues give typical values for their designs.
Why does sheet surface quality matter in duct production?
Storage stain spoils the appearance of the product, and excess protective oil makes it difficult to apply sealants in the seams, on which the air tightness class of the finished duct depends.
Sources
- EN 1505 and EN 1506, Ventilation for buildings - sheet metal ducts and fittings, dimensions
- EN 1507, Ventilation for buildings - rectangular sheet metal ducts, requirements for strength and leakage
- EN 12237, Ventilation for buildings - strength and leakage of circular sheet metal ducts
- EN 12599, Test procedures and measurement methods to hand over ventilation and air conditioning systems
- Regulation of the Polish Minister of Infrastructure on the technical conditions to be met by buildings and their location, paragraphs 267 and 268
- EN 10346, Continuously hot-dip coated steel flat products for cold forming - technical delivery conditions
- Stowarzyszenie Polska Wentylacja, list of ventilation and air conditioning standards