In Poland, the service life of trapezoidal sheet depends not on a single parameter but on matching the material to the operating conditions. Check the steel thickness, the type of protection and the product's intended use in the specification, not just the profile symbol and colour. Beside a busy road, in an industrial environment or near the sea, the coating is selected for exposure, as it is for moisture and aggressive compounds inside a building. Installation quality, storage and later maintenance also matter. A warranty period alone does not predict service time, so compare the scope of protection and its limitations.

T18 and T35 differ in profile height, but the comparison does not end there. Cover widths, steel thickness and the arrangement of supports matter. In a small building, fit to openings and edges is also relevant. A taller profile should be assessed as a complete solution, together with the structure, flashings and intended use. In the examples considered, T18 has a profile height of 18 mm and T35 — 35 mm. A deeper rib affects the geometry and stiffness of the section. However, it does not automatically mean that every T35 variant outperforms every T18 in all applications.

Fleece on the underside of the sheet metal can reduce dripping water from condensation. It acts as a temporary buffer. It absorbs some moisture and then must release it. Without conditions for drying, its capacity comes to an end. Therefore, the suitability of sheet metal with fleece is determined by the entire roof assembly and ventilation, not by the presence of the soft layer under the sheet alone. Moist air can come into contact with a surface colder than the dew point temperature. Then vapour condenses on the underside of the covering. The phenomenon can also occur without precipitation. Not every trace of water under the roof therefore means a hole or a defective lap.