The choice of roof structure and shape depends on many factors, to which they belong: purpose of the building (a – hence – its dimensions and roof span) and climatic conditions (mainly average wind force, average amount of precipitation and duration of snowfall). This choice is made by the designer, judging, what type of structure will be most suitable for a given roof.
The most popular material for making roof structures, that is, the roof trusses are made of wood.
Advantages of a wooden truss:
• Simplicity and ease of implementation – the simplest truss is a system of oblique beams (rafters) forming a kind of tent. All the more complicated structures are modifications of this simplest rafter roof.
• Reliability.
• A large number of qualified contractors, that is, carpenters. Knowledge on how to construct a bond is passed down from generation to generation for many years. Tools are widely available and relatively inexpensive. Half a century of domination of cube houses with flat roofs did not cause the decline of the carpentry art.
• Material availability – wooden roof structures are made of softwood of coniferous trees (pine and spruce), while pins and blocks serve as connections – made of hardwoods of deciduous species (oak and acacia) and more and more often made of galvanized steel.
The wood for the elements of the roof truss should be properly dried (preferably in a specially prepared drying room – its humidity should not exceed 20%) and protected against the action of biological factors and therefore impregnated. The most effective is vacuum-pressure impregnation performed industrially by specialized companies, while on small construction sites, surface impregnation consisting in painting is used, spraying, or short-term bathing of elements in a solution with impregnation. The roof truss structures are made by carpenters. Currently, classic carpentry joints are replaced by screws, nail or barbed plates.
The type of structure is strictly dependent on the size of the loads, which it is supposed to carry. The cross-sections of individual elements of the roof truss and its type are selected on the basis of static and strength calculations. The results of these calculations depend on the following parameters:
– distances between the outer walls,
– the angle of inclination of the roofing material,
– the type of coating used,
– wind zone in which the building is to be located,
– the snow zone in which the building is to be located.
Among the currently used wooden roof structures, these still prevail, which belong to the centuries-old practice and construction tradition - roofs with rafter trusses, collar and forceps and purlins.