T18 or T35: comparing trapezoidal profiles for your roof

When replacing a section of old covering, do not buy new steel sheet solely on the basis of a wave height measurement. A matching number does not guarantee a fit. A full cross-section and a check of the joint are needed. If the profile does not match, a separate transition detail or replacement of a larger area must be allowed for. Pulling the sheets together with screws does not replace matching geometry.

Compare load-bearing capacity for the same steel thickness, material properties and support arrangement. A higher rib usually changes the stiffness of the cross-section, but the result depends on the entire geometry. The support width is important, as is whether the sheet acts in one span or in several spans. Tables for different conditions do not give a simple ranking.

The designer takes wind and snow loads into account as well as the relevant deflection criteria. The spacing of purlins or battens should not be changed after reading the T35 designation alone. If the existing structure is to remain, start by measuring and assessing it. Only then is the covering selected to suit that data. The sheet length from eaves to top edge is not its permissible span.

The number of pieces is determined by the effective width, not by the profile height. Consider two actual dimensions found in the compared families: 1100 mm coverage for T18 and 1035 mm for T35. These are not universal widths for every product with such names. Before ordering, the cross-section of the selected variant must be checked.

For a model roof slope 12.6 m wide, 12 sheets with 1.10 m coverage or 13 sheets with 1.035 m coverage are needed. Both results are rounded up to whole pieces. The difference affects the quantity of material and the last strip at the edge. At a length of 4.2 m, the nominal total coverage area of the purchased sheets is 55.44 and 56.511 m² respectively. The method of price settlement remains a separate matter.

A profile with a height of 35 mm produces a stronger shadow line than a shallow rib, under similar lighting and colour. On a façade this can clearly change the perception of the surface. On a low roof, visibility depends on the viewing angle. Compare larger samples, because a small cut piece does not show the rhythm of whole sheets.

Also check the orientation of the outward-facing side and the intended use. The wall and roof variants may have different solutions. Do not choose the laying direction based on surface appearance alone. On a roof, water drainage and tightness take priority. Select the coating for the environment, not solely for the expected shade of the façade or joinery.

Count full pieces, laps and the fit of the last strip. Add the appropriate flashings and fixings. A lower unit price does not necessarily give a lower order value if you need an extra sheet. Pay attention to the steel thickness. A comparison of T18 in thicker steel and T35 in thinner steel describes two different sets of parameters.

Include the cost of transport and lifting, particularly for long elements. If changing the profile requires a change to the structure, that cost also belongs in the comparison. Any reduction in the number of supports must be confirmed by calculations. Do not subtract it from the budget just because the profile is higher. A good quote is based on an approved layout, not on the assumed superiority of one name.

Gather in one place the roof slope geometry, support spacing, load conditions and coating requirements. Prepare the sheet layout for both profiles. Then compare the structural data for the specific variants. Finally, assess the appearance and cost of the complete covering. This order helps avoid changing the order after installation has begun.

In the specification, record the profile with its full cross-section or an unambiguous description of the variant, the steel thickness and the cover width. Include lengths, colour and the required accessories. On delivery, check that the batch matches the order. If the supplier proposes a substitute, return to the same criteria. A similar appearance does not mean identical fit to the prepared structure.