When does T35 offer an advantage over T18 in a new building?

The example roof T18 has a cover width of 1100 mm and an overall width of 1142 mm. The selected roof T35 covers 1035 mm with an overall width of 1082 mm. These are the data for two specific variants. They should not be applied to every product with these designations. The façade version may also have different dimensions from the roof version.

The height influences the section properties, but on its own it is not enough for comparison. The thickness, steel grade and exact geometry must be retained or taken into account. The load direction and the span arrangement are also important. It is not possible to calculate from the 35 to 18 ratio how many times greater a load the finished roof will carry.

In practice, the data for the selected variant and the actual supports are compared. The limiting factor may be strength, deflection, bearing or connection. A higher profile does not exempt you from selecting fastenings for wind suction. T18 may be sufficient in one building, while T35 is more suitable in another. What decides is verification for the loads, not the roof area alone.

Consider a simple roof slope 7.6 m wide along the eaves and 6.2 m long down the slope. It has an area of 47.12 m². With a cover width of 1.10 m, seven T18 strips are needed. Their total nominal coverage is 7 × 1.10 × 6.2 = 47.74 m². With a cover width of 1.035 m, seven T35 strips give too little width, because only 7.245 m.

For this T35, eight strips are needed, i.e. 51.336 m² of nominal coverage. The result does not mean that T18 always produces less waste. It depends on the width of the specific roof slope. With other dimensions, the difference may look different. The calculation assumes strips without transverse joints and the stated end lengths. Eaves and ridge details as well as the possibility of using offcuts must be checked separately.

Deeper profiles create a more pronounced rhythm of light and shadow. On a large hall it may look different from a small building close to a house. A low profile gives a calmer surface, but the perception also depends on colour, gloss and laying direction. A sample should be viewed at the angle at which the finished façade or roof slope will be seen.

Changing the profile requires the details to be re-fitted. This applies to flashings, closures and transition elements. Do not assume that a part fitted to T18 will sit correctly against T35. Water drainage and the intended ventilation must be maintained. A higher rib alone does not allow the roof pitch to be reduced or the overlap shortened at will.

Calculate full sheets based on the actual coverage width. Then compare the same material class, thickness and coating, or explicitly describe the differences. The rate per metre alone can hide a different billing unit. It must be established whether this refers to effective area, total area or linear metre of sheet.

If you are also comparing different support arrangements, include their cost only after both solutions have been confirmed. A higher profile can be advantageous in a specific project even when it requires purchasing a larger area. In another building it will not bring the required benefit. Add the appropriate accessories, transport and installation to the calculation. A change of section makes sense when it improves the entire solution.

Prepare two specific variants instead of a general question ‘T18 or T35?’. Each should have a cross-section drawing, steel thickness, coating, sheet lengths and intended supports. Ask for the parameters to be checked for the building. Only then choose the variant that suits the cost, appearance and method of execution.

Pass the approved cutting plan and details to the contractor. If the profile or thickness changes during purchase, return to the agreements. Two sheets may look similar in a photo yet require a different roof slope closure. A well-chosen material is one whose actual properties match the building. It does not need the highest trapezoidal rib to fulfil the task.