# An Overview of the Historical Retrofitting Interventions on Churches in Central Italy
## 3 The Main Interventions Observed in Central Italy
### Interventions on the Roof  
As mentioned in Section 2, the prevailing roofing system adopted in many masonry churches in Central Italy consisted of wooden trusses. Apart from being very light, thanks to the presence of a horizontal tie beam, trusses take on all the outward thrust transferred by the rafters, allowing the transmitting of only vertical loads to the underlying masonry walls. To avoid unthreading, tie beams were restrained to the supporting wall through a wooden stake (Figure 20a). This specific truss system, very common in Abruzzo, is known as _\"capriata impalettata\"_ (stacked truss).  
In the area of Umbria and Lazio it is common to find so-called diaphragm arches. This is a wooden roofing system in which the function of struts is replaced by arches [81] (Figure 20b).  
In the 20 th century, with the advent of new materials and building techniques, traditional anti-seismic presidia were progressively replaced by more \"sophisticated\" retrofitting systems, with the aim of upgrading the box-like behavior of unreinforced masonry constructions and boosting their structural integrity. In fact, during the post-earthquake reconstructions carried out over the last century, many timber trusses and light roofing systems were dismantled and substituted with heavy reinforced concrete (RC) roofs (Figure 21a) [14]. Moreover, to improve the wall-to-roof connections and the transmission of horizontal fores, RC ring-beams were frequently inserted all along the perimeter of the churches (Figure 21b), under the roof structure, accommodating them into longitudinal cuts excavated inside the masonry walls for more than half of their thickness and leaving only a small portion of the cladding stones for aesthetics purposes. It was a practice widely used for floor slabs in multi-story buildings to make them resistant to seismic actions.  
Figure 20: Historical presidia in coverage: (**a**) stacked truss in Santa Maria Extra Moenia church, Petogna (AQ); (**b**) diaphragm arches in San Giovanni church, Gubbio (PG) (photo from [81]).  
Though, in most cases, such additions were harmful for CH buildings and churches, not only because they caused a reduction of the cross-sectional area of the underlying masonry walls, adversely affecting the overall seismic behavior and increasing the vulnerability to out-of-plane failures when badly connected, but also because they produced a significant increase of the seismic masses at the top of the structure, contributing to higher earthquake-induced inertia forces. Whatever the mode of construction, it is known that RC elements feature a remarkable bearing capacity; however, they are too heavy and stiff to be combined with URM construction systems. Indeed, the different properties of concrete and masonry can lead to diverse vibration responses and result in negative effects when the building is subjected to seismic loading, generating high local stresses in the masonry elements. Moreover, RC-based reinforcement techniques are irreversible interventions, and for this reason, they are nowadays not recommended as a seismic improvement technique in built cultural heritage. A deeper insight into these aspects will be given in the Section 4.  
Today the new approach to conservation dictates that seismic improvements have to be achieved without invasive interventions and respecting the \"authenticity\" of the construction, both in terms of materials and geometries [82]. Accordingly, smart retrofitting strategies based on the use of dissipative and deformable roof structures (for example, with steel connections, light and reversible interventions) have been fruitfully proposed in the last few years [43].