Under roof
The sub-roof is a second water-draining layer under the actual roof covering to protect against drift, capillary, backwater, condensation or dirt. The underroof usually consists of roofing felt or vapor diffusion-open film (underlay sheet) on roof boards or of a free-spanning underlay sheet .
tasks
Water protection
Roof coverings laid like roofs, such as roof tiles or concrete roof tiles , drain rainwater along the roof slope . Joints are either folded or, as with beaver tail , covered, but not completely watertight . By wind pressure rainwater or snow may be driven under cover and melt water can accumulate in forming an ice barrier. If individual roof tiles or roof stones break, rainwater can also penetrate under the cover.
When the air temperature rises in the morning, condensation can form and drip off on the underside of the roof covering, which is still cold . Water vapor can also get into the insulation from heated rooms and condense on the cold underside of the cover.
Depending on the roof pitch or particular stress, a sub-roof must be designed differently in terms of watertightness :
- Rainproof sub-roof
- if the roof is steep enough so that water on the sub-roof drains quickly towards the eaves, the water-bearing layer can be laid under the counter batten. The nails of the counter batten damage the lower sheet, but the sheet is sealed there by the contact pressure of the counter batten. The counter batten can also be provided with sealing cords or sealing compound.
- Waterproof sub-roof
- If the roof pitch is below the standard roof pitch , the water-bearing layer is guided over the counter batten. In addition, seams and joints are welded or glued. In this way, the nail holes in the roof battens are at the highest point. In order to enable the roof membrane to be laid in a form-fitting manner, the counter batten should have a trapezoidal profile or be provided with lateral triangular strips. This design is more complex.
dirt
Dust , soot , fallen leaves and insects can penetrate the joints in the roofing, connections to openings (e.g. skylights , chimneys ), ventilation elements or damaged roof tiles . The sub-roof keeps this dirt away from the insulation, roof structure and interior.
Wind barrier
Open-pored thermal insulation must be protected against cooling by wind and convection. The sub-roof also acts as a wind barrier.
Openness to water vapor diffusion
In the case of warm roofs , the water vapor that diffuses into the insulation despite the internal vapor barrier must be discharged to the outside. This is only possible if the sub-roof has the lowest possible water vapor diffusion resistance. The S d value of the sub-roof should be six times lower than that of the internal vapor barrier.
In the case of cold roofs , the water vapor diffusion resistance does not initially play a role, as water vapor should be conducted away to the ridge through a sufficiently dimensioned ventilation layer under the sub-roof. For this purpose, the air inlet at the eaves as well as the air outlet at the ridge must be ensured. If the air-conducting layer is interrupted by roof windows, dormers or the like, the air flow above and below must be enabled via ventilation roof tiles or in some other way.
material
The material of a sub-roof must be watertight and airtight, as well as possibly also permeable to water vapor (ie water vapor permeable).
The classic material for the sub-roof is roofing felt . However, in order to achieve a lower water vapor diffusion resistance, nonwovens made of polyethylene are also used as the underlay membrane . Common products are Tyvek or products from doerken , isover or Klöber .
Bituminized wood fiber insulation boards are water-repellent, open to water vapor diffusion and form an additional insulation layer on warm roofs .
construction
The roofing membrane is usually on a roof boarding on unless there is a cantilevered roofing felt . The counter battens are attached to the sub- roof in order to create a gap between the roof battens and the rear ventilation.
The roofing membranes are laid overlapping in the direction in which the water flows. They are fastened with roofing paper nails or welded when using welding strips. Boards, such as wood fiber insulation boards, are laid in tongue and groove and glued to the joints. The spring should point upwards.
The water-draining layer is either fed into the gutter or ends in front of it, so that any water that runs off drips under the gutter. Drainage under the gutter is used, for example, on flat roofs with a roof pitch of less than 25 °, since otherwise the membrane to the gutter would have to be led upwards, which would create a water pocket. Usually only very small amounts of water drip off the eaves. If larger quantities are involved, this is an indicator of damage to the roof.
Openings for skylights, chimneys or ventilation pipes are passed through the sub-roof. A drainage channel made from the underlay or sheet metal used is installed over the opening. The penetrations are cut into the roofing membrane and must be laid twice on both flanks and overlapping on the ridge side in order to achieve watertightness. In order to achieve windproofness, this must be done on all four sides.
Even if the sub-roof fully fulfills the function of the roof covering, it is not designed for long-term weathering. During the construction phase, however, the sub-roof can temporarily take over the function of the main roof.
literature
- Walter Holzapfel: Pitched roofs: requirements, planning, execution. Fraunhofer IRB Verlag, Berlin 2010, ISBN 978-3-8167-8214-8 .
Individual evidence
- ^ A b Eberhard Schunck , Thomas Finke, Richard Jenisch, Hans J. Oster: Dach Atlas: Inclined roofs. Birkhäuser, Berlin 1996, ISBN 3-7643-6479-3 , p. 200.
- ↑ Central Association of the German Roofing Trade - Professional Association for Roof, Wall and Sealing Technology eV (Ed.): Roof, Wall and Sealing Technology . Verlag Rudolf Müller, 2014, ISBN 978-3-481-03146-6 , p. 37.
- ↑ Susanne Rexroth, Friedrich May, Ulrich Zink: Thermal insulation of buildings: contemporary and versatile. VDE-Verlag, Berlin 2014, ISBN 978-3-8007-3570-9 , p. 175.
- ^ A b Walter Holzapfel: pitched roofs: requirements, planning, execution. Fraunhofer IRB Verlag, Berlin 2010, ISBN 978-3-8167-8214-8 , p. 51.