T18, T35 and structural steel profiles: how to read the designations

In common designations, the number indicates the nominal height of the corrugation, expressed in millimetres. T18 is therefore a family of profiles around 18 mm, and T35 around 35 mm. The exact shape and dimensions are confirmed by the technical drawing. The abbreviation should not be treated as a complete product description.

Manufacturers may use additional letters, digits and their own family names. Their meaning must be read from the relevant catalogue. A symbol used by one supplier does not have to have the same interpretation with another. This applies in particular to roof, façade and high structural profile variants.

The same nominal height does not define the width of the crests, valleys or rib spacing. The number of corrugations per sheet and the geometry of the lap may also differ. This affects the cover width, appearance and technical parameters. Random sheets should not be combined simply because both are described as T18.

Likewise, T35 is not one universal product. When extending an existing covering, an exact drawing and identification of the profile are needed. Measuring the height alone is not enough. Well-preserved purchase documents often make it possible to avoid ordering sheet that cannot be correctly joined to the old sheets.

Overall width covers the full sheet, including the part used for the lap. Cover width defines the usable dimension after adjacent elements have been correctly joined. The correct value must be used when calculating quantities for a roof. Swapping these figures can lead to a shortage of material at the end of the roof slope.

The distinction also affects the price. Suppose a sheet is 1.10 m wide overall and covers 1.05 m once laid. At the same sheet length, €9.72 per square metre of overall area works out at about €10.18 per square metre of cover. This is a calculation example, not a quotation for a particular sheet. Always check which area the price refers to.

Thickness is a separate parameter, usually given in millimetres. A profile of the same height may be available in several variants. The thickness of the steel core must not be confused with the thickness of the coating, often expressed in micrometres. Both pieces of information describe different parts of the product and have different significance.

The steel grade affects the mechanical properties. In structural trapezoidal profiles it matters when using load tables. Greater thickness alone does not remove the need to check that the material matches the design. The data should be linked to the declaration and documentation for exactly the variant that will arrive on site.

The colour designation describes the appearance, but not the complete surface protection. The same shade may occur in different organic coatings, textures and gloss levels. The type of protection and its application must be checked. A matching colour number does not prove that two materials have identical resistance.

The metallic coating beneath the organic layer and the protection of the reverse side are also important. Not every commercial code can be explained from a general list of abbreviations. When ordering, keep the full material description. This will later make it possible to select flashings and clarify any differences between deliveries.

A roof variant may differ from a façade variant in profile orientation, the side carrying the coating and the lap detail. The installation drawing must be checked, not just a photograph. A high structural trapezoidal profile may form a load-bearing element beneath further roof layers. It is not always at the same time the external covering that drains water.

For structural profiles, the support scheme and the way the tables are used are important. Values for a single span do not have to correspond to a multi-span arrangement. Verification of load-bearing capacity must be distinguished from deflection checks. The product designation identifies the material, but does not replace calculations for a specific building.

Record the full profile, intended use, thickness, coating, colour and lengths. Add the number of sheets and the basis for calculating area. Establish how the delivery is to be protected and which accessories are required. An order described in this way reduces the risk that the seller and the installer understand the same abbreviation differently.

On delivery, compare the markings on the packs with the documents. If data on the variant is missing, clarify it before cutting and installation. It is advisable to keep the label and the profile drawing together with the invoice. They are far more useful during a later repair than general information about graphite-coloured sheet on the roof.

Photograph the label of each material variant before unpacking. Keep it with the cross-section drawing and the coating description. When ordering a few additional sheets, the full designation will help avoid mismatched laps.

T18 or T35 is a useful starting point for identification, but not enough for a reliable order. Suitability is determined by the full geometry, thickness, coating and conditions of use. Read the designation together with the drawing and documentation, and compare prices for the same cover area.