Many industrial and military constructions built between the end of the IXX century and the first years of the XX century show long span roof structures constructed using slender steel truss members. This characteristic makes the structures almost invisible and the interior space appears larger than it is. After 100 years or more, structures like these are still in good conditions although they may have fallen into disuse and neglect. Usually these buildings are protected by Cultural Heritage regulations. Therefore, it would be interesting to define suitable intervention strategies to maintain, conserve and possibly reuse them. This paper compares three different steel roof trusses, covering some buildings of a military installation in Pavia (Northern Italy), built from the end of the IXX century. The residual capacity has been probabilistically evaluated in order to check the efficiency of the most vulnerable members of the structures and investigate systems’ robustness in case of sudden failure of the critical components. The method proposed can be considered as a reliable approach to recognise which structural element is most like to fail, and schedule proper selective maintenance interventions.