In roof structures, rafters or rafters are the girders that run from the eaves to the ridge and support the roof cladding . When viewed from above, they are usually arranged at right angles to the ridge. The rafter comes either as an inclined single-span beam or inclined multiple - span beam , each with or without a cantilever arm . The rafters transfer the load of the roof skin in the rafter roof to the roof or ceiling beams and in the purlin roof to the purlins .
The word rafter has been used in German since the 11th century (Middle High German "sparre", Old High German "sparro"); it has a common basis with the verb "lock" meaning "straighten up beams, spread legs (like the rafters on the Roof) "assumed.
The totality of the rafters of a roof forms the planes of the roof surfaces and is called the rafter layer . The concept of the rafter position can also appeal to the graphic representation of a rafter supervision. Today the graphic representation of the supervision of the entirety of the rafters of a roof, for example in the context of an implementation planning , is also referred to as a rafter plan .
The rafter is a pair of rafters that belong together, but it can also address all rafters (or all wooden components) of a roof. Bundle refers to the pair of rafters that belong together, including any connecting elements such as the collar beam .
The lower end of a rafter is also known as the rafter foot ; if it protrudes over the edge of the outer wall and is visible from the outside, it is referred to as a rafter head .
The bird's mouth (see. To carve English. Carve, carving ) is to be here a dihedral notch (triangular cross section) from the bottom wooden rafters forth a horizontal usually bearing surface on a roof beam to create.
Rafters with a special shape and processing are:
- Rafters are located on the so-called roof ridges , for example the outer edges of a hip or pyramid roof .
- Valley rafters are located on the inner corners of a roof, the so-called throats , for example on thecourtyard side ofthe square courtyard.
- Schifter (or Schifters rafter) are shorter rafters that do not go through to the ridge, but nestle against the hip or valley rafters on an inside or outside corner of the roof. In the case of large roof overhangs, the outer corners (e.g. a hipped roof) can have such short shifts that they no longer rest on the purlin. In this case, they should not be laid parallel to the other rafters, but approached at an angle to the hip or valley rafter until they reach an appropriate length and can transfer their share of the roof load to the purlin.
- Gable rafters or verge rafters lie on or on the gable .
- Rafters are exposed in front of the gable of a building and are z. B. supported by projecting purlins. They form an open space (also known as a floating gable or open space ).
- Painting rafters lie directly against a (gable) wall. High gable walls that are not supported by interior walls can be stiffened by the roof structure by connecting them with painted rafters .
- The rafter is at the so-called point of incidence of a hipped roof.
Material and task
In the past, only solid wood rafters were used in historical buildings . At the beginning of the 20th century it was recommended for such wooden rafters that their free length without support should not exceed 4 meters. Today, rafters made of glued laminated timber can be used to bridge much larger distances. Constructions made from other materials also exist, and so do rafters made from steel profiles , aluminum or reinforced concrete .
The rafter has the task of introducing loads into an underlying structure. This can be done, for example, via purlins in the load-bearing walls, or by means of a wooden roof truss . The DIN 1055 (loads in construction) distinguishes between permanent loads (the building itself), traffic loads , snow loads and wind loads . The distance between the rafters is given as the center distance ( e ) and in timber construction is usually between 50 and 100 cm, today often between 65 and 80 cm. With these rafter spacings, the statically necessary rafter dimensions between 6/12 and 10/20 cm result in solid wood. In the case of roofs over heated rooms, thermal insulation is usually placed between the rafters today . Increasing demands on thermal insulation in recent years have led to greater insulation thicknesses in Germany. Which in turn means that when dimensioning the rafters, not only static requirements play a role, but also the need to dimension them according to the required insulation thickness. In this respect, the rafter heights in new buildings are rarely less than 20 cm today.
Window openings in the roof surface or components that penetrate the roof, such as a chimney, make so-called replacements necessary. A horizontal upper change and a lower change distribute the load in the adjacent change rafters . Due to the static system, larger changes in the rafter roof and the related collar beam roof are considered problematic. Dormers that extend over more than two rafter fields should be avoided there. With purlin roofs , larger replacements are considered relatively unproblematic, and in this respect also the construction of larger dormers.
References and footnotes
- Sentence after Theodor Böhm: Handbook of wood structures. Verlag Julius Springer, Berlin 1911. (6th reprint of the original edition from 1911, Reprint-Verlag-Leipzig), p. 298.
- Schneider: Construction tables for engineers Werner Verlag (16th edition 2004).
- Isabell Hermann: The farmhouses of both Appenzell . Appenzeller Verlag, Herisau 2004, ISBN 978-3-85882-387-8 , p. 104-105 .
- Dr.-Ing. Thomas Eißing: Roof structures , University of Bamberg; In: Baugeschichte.A.TU-Berlin.de; accessed in July 2019
- Sentence based on "Kluge Etymological Dictionary of the German Language", 24th edition, 2002.
- Theodor Böhm: Handbook of wooden structures. Verlag Julius Springer, Berlin 1911. (6th reprint edition of the original edition from 1911, Reprint-Verlag-Leipzig), pp. 288, 301 ff.
- sentence based on Günther Wasmuth (ed.): Wasmuths Lexikon der Baukunst , Berlin, 1929–1932 (4 volumes), Lemma Sparren bis Sparrenwerk .
- see Günther Wasmuth (ed.): Wasmuths Lexikon der Baukunst , Berlin, 1929–1932 (4 volumes), Lemma Gespärre .
- cf. Wood lexicon . Leinfelden-Echterdingen, 2003, DRW-Verlag
- Explanations on weird Schiftern by Martin Ahammer, teacher at the state vocational school in Wals; accessed in December 2016
- sentence based on Hagen Prehl: Wooden roof structures , 2nd edition, Düsseldorf, 2001, Werner Verlag, p. 15.
- Sentence with terms according to Tanja Brotrück: Basics Roof Construction , 2007, Birkhäuser, p. 25, Fig. 19.
- on Hagen Prehl: Wooden roof structures , 2nd edition, Düsseldorf, 2001, Werner Verlag, p. 17.
- on Hagen Prehl: Wooden roof structures , 2nd edition, Düsseldorf, 2001, Werner Verlag, p. 29.