T18 or T35 roofing sheets: how the profiles differ and which to choose

When comparing, ask for exact dimensions. The nominal name and the measured height are not always identical to the millimetre. Do not match a new sheet to an old one based on the T18 symbol alone. The edges and the rhythm of the corrugations must actually fit together.

Below are two actual roof cross-section types. These are examples of geometry, not dimensions binding for entire families. When ordering, the selected variant and available thicknesses must be confirmed.

Another T18 or T35 may have different values. The roof and façade variants must also be distinguished. The width of one profile should not be combined with the load-bearing capacity table of another.

The cross-section shape affects stiffness and load transfer. Taller ribs may allow different support spacings. However, a single value cannot be given for all T35. Thickness, steel grade, span pattern and bearing width all matter.

Both load-bearing capacity and deflection must be assessed. The fixings, including the roof edge zones, require verification. When changing from T18 to T35, it is not enough to substitute the name in the order. The solution should be verified for the actual loads and building structure.

Assume a roof slope strip 12.4 m wide. With a cover width of 1.10 m, twelve full sheets are needed. With a cover width of 1.035 m, twelve are also needed, but the total cover width is 13.2 and 12.42 m respectively. The remaining allowance to be adjusted is therefore very different.

This is an example of the width distribution alone. It does not cover side details or any length restrictions. It shows why a smaller cover width does not necessarily mean more waste. The result depends on the proportions of the specific roof slope, not just on the material name.

A taller profile is not always needed. With suitable supports and loads, an appropriate T18 may meet the requirements. Conversely, wider support spacing may justify analysing a different cross-section. The decision should follow from the design, not from the assumption that a larger number is inherently better.

How the interior is used determines further requirements. A heated workshop needs proper insulation. In an unheated storage area, condensation may become an issue. Neither T18 nor T35 alone solves temperature, humidity or noise. These are matters for the whole roof and ventilation.

Pay attention to the junction between the roof and flashings. A different rib height may require matched components and seals. Do not assume that accessories from a previous covering will fit after a change of profile.

The steel thickness, coating type and scope of supply must be standardised. Check whether the price refers to a gross or effective metre. Calculate full sheets according to the cutting plan. Then add flashings, fastenings, transport and any changes to the support.

For one T18 variant with 0.5 mm steel, the mass is approximately 4.43 kg/m² effective, and for the specified T35 approximately 4.71 kg/m². These are values for specific products. They do not replace the data for the ordered batch and should not be used to determine the mass of all sheets with these symbols.

First establish the structural requirements and the function of the room. Then compare two complete variants. Check the fit of length and width to the roof slope. Look at surface samples if appearance matters. Finally, approve the full scope of the order.

If the roof has an unusual pitch, penetrations or transverse joints, assess these details separately. Do not transfer the minimum pitch from any sheet to the entire T18 or T35 type. The requirements apply to a specific product and solution. A well-chosen trapezoidal sheet is part of a coherent design.