ROOFING
Timber is an essential part of a house. A roof is the upper part of a building, protecting against almost all vertical influences: water (rain, snow), wind and heat, and of course also physical invasions. A roof has a construction and covering materials. The construction supports itself – some bearing constructions are extremely heavy because of the spans. A roof construction can be simple or double. For longer spans and for heavier material, the construction is doubled: the primary rafters carry a horizontal top beam – and secondary rafters stretch from this beam to the overhang beams. Rafters can be tied with a tie-beam, which connects them and composes a triangle – a very stable construction. The most interesting are constructions of thatched roofs in windy regions, where the saddle constructions are flexible. In extreme winds (the bora for instance, in Karst regions) it bends itself and returns to the original position after the wind weakens. The roof can be single-sided (such as a lean-to roof), the most usual roof is a pitched roof, but it can also have hips or the gables can be open. The inclination of roofs depends on the covering material and climatic conditions. An inclination over 45 degrees is suitable for countries with a lot of rain; in general a slope of one to one (45 degrees) is used – with thatching or wooden boards. Gentle slopes can be made with materials that allow fixed joints – sheet metal mostly, which is possible to bend.
Stone can be used as roofing material too, not only its construction. The simplest self-supporting system in stone is corbelling, but it is less used because of the short spans – except for historical tombs and herdsmen’s shelters. A stone roof is composed of thin flat stone plates, slates, joined to one another with a third element, covering both. The most effective is one to one, or 45 degrees. Stone roofs are today expensive and, given the technical difficulties, are less in use. In the past, it was the only possibility in regions with a lot of stone.
Thatched roof: straw and reed are the most natural materials used for roofs. Straw consists of stems of cereals and is a softer material, with a smaller profile, while reeds have thicker blades. Straw has a longer lifespan because of slower growth in dry locations; reeds need a lot of water and ripen faster, but their lifespan on the roof is shorter than that of straw. Sheaves are tied on horizontal laths and levelled with wooden tools. The cereal ears are turned down into the roof and help slower drying of the straw. Willow branches are used for tying onto the laths. Cereal straw is used for the whole roof, except for the top, for which rye straw must be used, because of its length.
Wooden boards and shingles can be used where wood is available. Wood is not a perfect material only for the construction, but also for filling and the covering.
A problem with boards is warping during drying. This can also be the main problem in roofing.
Profiles are joined into constructions. Joints need provisional strengthening – until the wood bends longitudinally. Constructions are much stronger after this bending.
A shingle is split wood and has several types. There are two types in Slovenia: in the Gorenjska region it is split as a 1.2 to 1.5-metre long and a good centimetre thick element. These shingles are laid in two layers, overlapping by half (Juvanec, Zupančič 2014). The Carinthian shingle is smaller, up to 90 cm long, 10 wide and up to a centimetre thick, laid on the roof in two, sometimes three layers in a herringbone pattern. In Slovak mountains can be found hewn shingles with grooves.
Shingles and boards are two different types of wooden material – with very similar use. Shingles are an older and more primitive system: the trunk is split with an axe. A board is sawn, with a more advanced tool: a saw. The difference between the two systems is the surface: while shingles have whole, undamaged ribs (veins) and a rough surface, a board's surface is smooth, but the ribs are damaged. Damaged ribs decay, whole ones have a longer life.
Other systems of shingles exist as a roofing system, depending on the climate, available materials and types of wood, the experience of workers and their tools. Religious buildings have complicated ground plans (with apses, towers and projecting roofs). Smaller elements are more suitable for such objects, allowing circular and spiral shapes. Shingles were historically used on smaller religious buildings, especially in romanesque architecture, with a lot of rotundas, and ossuaries with circular ground plans.
The biggest problem is the ridge, because wooden shingles can't be bent. The only way of avoiding leakage is for the slopes to overhang, turned in the direction of the prevailing wind or a trunk, hewn and upturned to the ridge.
Pan and barrel tiles are made of fired clay: simple flat tiles, tiles with notches and barrel tiles.
Tiles with notches are linked one to another with notches. They are always rectangular, because the vertical edge has a notch throughout the length: one on each side, but upturned cyclically – above on the first side and below on the other. A projection sits in the notch. Because of the slope of the roof and the notches, this system is watertight.
The main difference between the use of simple flat tiles and tiles with notches is the depth of the roof: plain tiles have to be overlapped horizontally and at the vertical joints - the system is covered by four tiles with at least some centimetres. This means more tiles have to be used and, because of the large number of elements, the roof is much heavier. Tiles with notches are doubled only some cm in a vertical section. The construction is made of one layer only, fewer tiles are used and the roof is lighter.
All flat tiles have 'a nose' for placing them on the laths.
The third system comes from Roman times: barrel tiles. This system was originally composed of two differently shaped elements – one flat with wraps and the other, smaller, as a covering. Today’s barrel tiles are simplified into a single tile. It is non-symmetrical, with two different wraps, and is bigger and heavier at one end than at the other. Barrel tiles allow gentler slopes and are well known in all Mediterranean areas as 'Latin tiles'. The ridge can be covered with the same elements.
All metals can be used as roofing materials, all metals are waterproof. The choice is in material, its thickness and possibilities of work (handwork or industrially prepared elements).
Lead is heavy, but simple to work because of its softness. It can be built into any corner, and can be shaped with a hammer only. Its weight allows only extremely massive constructions: wide walls and massive domes, such as in oriental baths. Lead doesn't corrode.
Copper is also a soft material and can be worked by hand. It does not corrode but in time becomes covered by a noble thin, green patina, but it is not cheap.
Flat metal sheets are mostly used in modern architecture (mainly steel: galvanised with zinc, painted or coated with a plastic covering). Sheets can be connected with wraps.
Self-supporting elements such as corrugated sheets are more usable, with the undulations oriented in the direction of the slope. Connections are simply made with overhangs.
Gold is the most valued metal, mostly for decoration, but as gilding can also be used as a roofing material, displaying richness with glitter and glamour.
The gilding is made of extremely thin sheets of gold, but a lot of gold is needed for larger roof surfaces.
Dome des Invalides in Paris has a golden roof, covered by 550,000 thin sheets, but this means twelve kilograms of gold..
Some other firm materials can be used for roofing, but mostly in combinations: wood or steel or cast iron for the construction and flat elements for covering.
Glass is a very usable material, it is transparent or only translucent, but is brittle and heavy. Transparent roofs are sometimes desirable, especially for big spans of sport objects and public spaces (lobbies, halls), where natural light is needed. Glass is also often used in small parts of the roof, for creating a special atmosphere: for instance in oriental baths, where the ceiling is a dome filled with stars. This is also the case in some religious objects.
Corrugated panels of reinforced concrete (asbestos and mineral wool) are very popular as chip material. Building such roofs is simple and very quick to erect because of the large elements, and suitable connections with overhangs.
The Swiss firm Eternit was extremely successful with its corrugated panels after the Second World War, when this material saved thousands of lives – until it was discovered that asbestos is cancerous. This reinforcement was later replaced by mineral wool.