The roof is the element that most characterizes a building, in relation to its relationship with the context and the users.
In some areas, subject to constraints on historical or environmental protection, the roofs and details that compose them are elements that must be subject to precise rules of architectural integration with the surrounding context. In this case, the inclusion of new materials and innovative technologies does not contribute to an architectural progress but at a irreversible degradation.
Since the most remote times, large sheets suggested using them for cover. Initially this was done mainly in the surrounding areas to the quarries of extraction of stone material.
The roofs were built, using the superposition of slabs that could be more or less work needed, on horizontal planes offset each other by about one-third of the width.
In some areas, where there were specific constraints to reduce the use of wood for construction, further developed the culture of stone used as cover. Vice versa in other areas was more used the wood and protect the stone material.
The stones used for the covers vary from area to area; They range from thin slate of Liguria to limestone of the trulli of Puglia, to large piode della valle d'Aosta (Figure 1), to the thick and heavy gneiss of Ossola.
The stone is used primarily for two reasons:
- the restoration of old buildings; because it is an irreplaceable heritage of Mediterranean culture and a strong cultural testimony of the various methods and construction techniques.
- for adapting old buildings with additions that will integrate with the building and with the urban context of continuously reference Mimetic or non-Mimetic.
Moreover, the stone is desirable because the durable building material par excellence; It has properties that are weathering resistance only in the construction sector that make it more competitive with other materials of synthetic origin.
For the construction of sheds or arcades, the stones may have different thickness and very often the timber boarding is unnecessary because they can be placed directly in strips with more or less dense distance in relation to the total weight of the roof. It should be remembered that the weight increases as the slope of the roof and the thickness of the slabs.
Shells with simple support stones possess two very interesting aspects:
- A relative mobility when slight seismic phenomena because it is able to absorb vibrations caused by telluric phenomenon;
- A sliding resistance working with wooden bearing structure by reducing lateral thrust.
The use of stones with content width technical and structural solutions very different from those with thicker stones. In this situation the increased stress imposed on the roof structure is not given much weight contribution of stone cover but, above all, from the snow load.
The guidelines provide, in fact, gradient limit values which must not exceed 26 degrees, corresponding to 49% inclination. You need the formation of so-called "cool roof" which, through the principle of ventilation of the underside of the roof to maintain a temperature of almost identical upper and lower part of the cover plate prevents sudden melting of the snowpack. (Figure 2)
In some areas it uses the so-called dalles slabs that are placed directly on flat wooden planks so that the bottom edge of the cover is almost completely straight. The overlap of the slabs is, especially in this condition, particularly edited so as to bring down the water in the middle of the underlying slab.
With this technique the roof design is highly irregular. (Figure 3)
Another type of stone, the plaintiff in the construction of shells, is the beola. It is a type of metamorphic rock composed of feldspathic quartz and MICAS.
The rock, composed of several layers formed during the process of dynamic metamorphism, has parallel planes along which tends to fall apart more easily. This phenomenon, known as spalling, can occur naturally or artificially with a wedge or a chisel acting along the floor.
Shells in beola possess a high thickness (from 5 to 7 cm with a corresponding weight of 300 kg per square meter) and cutlery with slopes exceeding the 80%, resting on structures that support the roof trusses consisting of larch with two Struts and a tie. Such coverage is able to withstand high overloads of substantial snow or thermal stresses, being without acting tangentially over the brickwork.
The architectural language of the covers in beola integrates seamlessly with the surrounding context, because it can take on different colours according to the process of metamorphism that has formed.
The irregularity of this type of coverage, such as the one described above concerning the dalles, is essential to create sufficiently large spaces to avoid the capillary rise of water.
Beola roofs have a rough surface that slows down quite well the snow; but in areas where snowfall is abundant snow guard should place consisting of gneiss longer protruding from the rounded outline of coverage. (Figure 4)
In Puglia, in the Murge, there are very special stone constructions called Trulli.
The cover of the Trulli consists of a double casing (internal masonry blocks and slabs of limestone cover chiancarelle calls). Between the two cover casings there is a free space that serves as an inner tube, very useful to guarantee cooling of long warm summers of Puglia. (Figure 5)
PV panels or for the heating of domestic hot water are not easily integrated architectural solutions in the stone shell. The integration is greater in covers made from large slabs or in buildings with a simple volume. (Figure 6)