The newly-developed method of graphic stability is applied to Robert Maillart’s iconic roof design at Chiasso. The truss has an unusual form that emerged from Maillart’s graphic statics as the shape dual to the symmetrical equilibrium forces. Although graphic statics can lead to highly efficient structural forms, there can be a tendency for diagonal bracing to be minimised or even eliminated, as at Chiasso. Whilst equilibrium is satisfied, there may be a decrease in stability, and it has not previously been possible to quantify this within the traditional graphical approach. Using Chiasso as an example, this paper demonstrates how the prestress stability of this and similar truss structures may now be assessed within the same graphical setting.