Current design rules for cold-formed trapezoidal sheeting, which predict sheeting failure for an interior support, do not provide sufficient insight into the sheeting behaviour, and can differ up to 40% in their predictions. To develop a new design rule, this article presents new experiments in which first-generation sheeting behaviour is studied for practical situations. The experiments show that after ultimate load, three different post-failure modes arise. Finite element models were used to simulate the experiments. Studying stress distributions with finite element simulations, it can be seen that there are only two ultimate failure modes at ultimate load. One of these ultimate failure modes is probably not relevant for practice. A mechanical model has been developed for the other ultimate failure mode. This model performs as well as the current design rules, and it provides insight into the sheeting behaviour.