Truss

The following paragraphs describe systems of historical timber roof trusses. Due to significant differences in trusses depending on the shape and range of roof and different ways how to create a rigid spatial system, there are described only the basic types of truss systems (purlin system and spanner system).

Types of truss

The truss is a load-bearing roof structure with its main function to carry the roofing and protect the building against external influences (rain, snow, wind). From historical point of view there were not significant requirements on attic space, so the thermal insulation, acoustic and fire requirements were transferred to the ceiling of the top floor.

The truss is a rigid spatial structure consisting of rafters, purlins, wall beams, studs, tie beam, braces, ribbons and ribbands. The individual beams are differing by the number of purlins, by range, position and the way how purlins are supported, the position and number of braces and ribbands.

Basic classification of historical trusses is according to the method of supporting rafters to purlin system and spanner system.

Purlin system

Purlin system consists of full frames  on which purlins and yeast frames lie.

Fig.: Purlin system in the transverse and longitudinal cut.

The basic types of purlin systems are the King post truss and Queen post truss.

Fig.: Purlin system - King post truss.

Fig.: Purlin system - Queen post truss.

Spanner system

Spanner system consists only of full frames. All the rafters must be anchored to the ceiling to prevent their displacement. Care should be taken to ensure stability in the longitudinal direction, at all stages of construction and reconstruction.

Spanner system cannot be designed to larger spans and on hip roofs. The advantage of this is the possibility of using the attic space because the disposition does not interfere with the studs.

Fig.: Spanner system in longitudinal and cross-section.