Reading T18, T35 and other trapezoidal-sheet designations

In a typical designation, T identifies a trapezoidal profile and the number its nominal height in millimetres. This is not a universal geometry code across the market. After identifying the name, consult the product sheet for the actual cross-section and dimensions.

A 35 mm profile height and 0.50 mm sheet thickness describe different features: the formed rib and the sheet material respectively. State both in an enquiry, since confusing them prevents a meaningful assessment of price and product performance.

Manufacturers may use different flange widths, rib spacing and side-lap geometry. An effective cover width of 1100 mm is not the same as 1128 mm, and overall width is another dimension. These differences matter particularly when adding new sheets to an existing covering.

Use a sample or measured cross-section for matching rather than a top-view photograph alone. Compare grooves and lap orientation too. Forcing almost matching sheets together can deform the metal and compromise the joint instead of resolving dimensional differences.

An extended designation may include height, profile pitch and effective cover width, but the order depends on the manufacturer. Do not infer its meaning from a similar listing. Match the name to a technical drawing identifying the dimension represented by each number.

For example, 18/157/1100 may describe height, spacing and coverage rather than the ordered sheet length. Length must then be specified separately. Before payment, obtain a written specification with units so the shortened designation does not replace the actual order details.

Extra letters sometimes indicate application, but their meaning is not consistent across brands. Check the decorative face, drawing and water-shedding lap detail. Simply turning over a façade sheet does not establish its suitability as a roof covering.

Distinguish lightweight cladding from high-profile structural decking supporting roof layers. Structural decking has its own load tables and support conditions. Do not assign that role to T35 merely because it feels stiffer than a lower profile when handled.

Use effective width after the side lap to calculate covered area. With 1.10 m cover width, a 3 m sheet provides 3.30 m² before any end lap. The portion of overall width covered by the neighbouring sheet does not add to that figure.

If pricing uses gross sheet area, convert the cost to effective coverage. A price per linear metre requires the effective width. Compare schedules of sheet lengths and quantities alongside covered area, rather than a single figure without a defined pricing unit.

Read thickness from the documentation for the exact variant. Steel grade concerns the substrate, while coating concerns its protection. Gloss or matt describes the finish, not a load class. A colour number alone does not define resistance in a particular exposure environment either.

Performance checks require a table matching thickness, support arrangement and load direction, including deflection assessment. T18/0.5 does not contain all those conditions and should not by itself determine batten or purlin spacing.

Combine the full profile designation with application, thickness, coating, colour and decorative face. Add lengths, quantities, effective coverage and pricing unit. If anti-condensation fleece is required, specify it as an additional feature rather than assuming every sheet includes it.

Keep the technical sheet and cross-section drawing with the delivery record. “Graphite trapezoidal sheet” is insufficient when the covering needs extending years later. A complete specification helps check the delivery and identify a genuinely compatible product in future.