Inverted roof

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Inverted roof is the name for a form of the flat roof designed as a warm roof (non-ventilated roof) . The name comes from the fact that the layer structure, through which the thermal insulation lies in the wet, is the reverse of classic warm roof constructions. It was developed in Canada in the 1950s.


Comparison of the two variants of the inverted roof
In the variant with a water-draining separating layer (right), most of the rainwater already flows off above the insulation layer.
1 - concrete ceiling
2 - sealing
3 - thermal insulation
4 - (a) fleece, (b) water-draining separating layer
5 - gravel fill or green roof substrate
6 - downpipe with sand trap

The inverted roof is characterized by an inverted arrangement of the insulation layer compared to the warm roof. With inverted roofs, the sealing layer is applied directly to the load-bearing structure (e.g. reinforced concrete ceiling ), the insulation layer is on top of the sealing. In the following, the individual component layers are explained using a schematic structure (from outside to inside):


The gravel fulfills two functions. On the one hand, it protects the insulation layer from UV radiation, as extruder foam ( extruded polystyrene / XPS) is not UV-stable. On the other hand, it secures the usually loosely laid roof structure from wind suction. There are also variants of the inverted roof, which are provided with a substrate (green roof) or a walk-on covering (e.g. concrete slabs) instead of gravel. The rule of thumb for the area load is around 120 kg / m².

Protective / filter layer

The protective / filter layer originally had the task of protecting the insulation layer from mechanical loads (application of the gravel) and preventing sand and parts of the earth from flowing into the insulation layer. With modern inverted roofs, a large part of the accumulating rainwater is drained away through this layer without seeping into the insulation layer (see drawing). Various plastic fleeces (e.g. polypropylene ) are used. It is important that the roof fleeces diffusion are thus the inverted roof works permanently. When using non-vapor-permeable nonwovens, moisture can accumulate in the insulation material over the years and thus the insulation quality may be reduced.

Insulation level

Due to the weather-exposed location, only hydrophobic insulation materials are used for the inverted roof . Typically, extruded foam (extruded polystyrene / XPS) is used. Special insulation boards with a stepped seam are to be used. A two-layer, cross-wise installation is not permitted according to the current state of the art, since experience has shown that a film of water forms between the layers, which can prevent the diffusion of vapor and thus lead to moisture penetration of the lower insulation layer.

In the past, the insulation of inverted roofs was only laid in one layer. Since 2011, some manufacturers have also been allowed to use two-layer insulation (permitted by the German Institute for Building Technology ). Larger insulation thicknesses (up to 400 mm), important for z. B. special requirements for thermal insulation can be achieved. However, this is only possible if the insulation is secured with gravel. A green roof is insufficient in this case.

Sealing level

The waterproofing is applied either directly to the load-bearing roof structure or to sloping concrete (as a drainage level). Plastic or rubber-based roofing membranes are used, as are bitumen seals that are flamed on, as well as coatings as a liquid sealing compound based on acrylic or epoxy resin, embedded with fiberglass fabric. Alternatively, the inverted roof is designed as a waterproof concrete roof construction, here the supporting structure = sealing level, according to the system of a white tank. This variant has been built for 50 years and is growing rapidly.

Load-bearing roof structure

Depending on the type of design, this can be wooden formwork, reinforced concrete ceiling, screed, trapezoidal sheet metal ceiling, etc. According to the professional rules of the roofing trade, a roof without slope can also be constructed.

Due to the reversed arrangement of the layers, the insulation (XPS) can sometimes be surrounded by water in the event of heavy rainfall. This also limits the insulation effect for a short time. A certain surcharge (up to 0.05 W / m² * K) must be included in the U-value calculation. However, if a water-draining roof fleece is attached to the polystyrene panels, this surcharge can be omitted.


The sealing membrane lies in a temperature zone that is more or less constant over the year. The thermal stress on the roofing membrane over the annual cycle is significantly lower than with conventional flat roofs, and the service life is correspondingly longer. The material fatigue due to temperature-related expansion and contraction is therefore lower. This structure does not require a vapor barrier. After the roof waterproofing has been applied, further work is no longer dependent on the weather, as the insulation is weatherproof. This allows more flexible scheduling. Due to the simple structure, the risk of structural damage due to execution errors is significantly reduced. In addition, loose laying offers the advantage of good accessibility to all component layers for maintenance and repair. The time-consuming opening and resealing of the warm roof is no longer necessary, and all the building materials that have been removed can be reused. A full-surface connection between the seal and the substrate prevents the seal from getting behind. This means that defects can be easily located and repaired, which is often not possible with a warm roof construction.


When it rains, (cold) water flows under the thermal insulation and thus leads to heat loss. According to DIN 4108-2, there is a U-value addition of 0.05 W / (m² · K) and a weight per unit area of ​​the substructure of at least 250 kg / m² as a thermally inert mass to prevent the formation of condensation because the sealing surface is below the dew point. These disadvantages are significantly reduced in the modern form of the inverted roof with a water-draining separating layer above the insulation layer.

If necessary, resistance to flying sparks and radiant heat must be specially established.

A high degree of condensation water penetration can occur due to vapor-tight layers above the insulation. For this reason, no structures that prevent re-drying may be set up or layers may be laid on the roof insulation.


  • Hans Peter Eiserloh: Handbook Roof Sealing: Structure - Materials - Processing - Details. Rudolf Müller Verlag, Cologne 2002, ISBN 3-481-01887-8
  • Beat Hanselmann, Andreas Kuster, Peter Stoller: Seals on building construction - planning and execution. Grafitext-Verlag, Treiten 2007, ISBN 3-9522490-5-X

Web links

  • Comprehensive portal around the inverted roof with very clear animation on the function and structural physics of the inverted roof.

Individual evidence

  1. a b Gravelled inverted roofs with a water-draining separating layer . Investigation of the TU Berlin.
  2. A very clear illustration of this fact can be found here: Animation inverted roof (Flash plug-in required; 27 kB)
  3. What is an inverted roof and how does it work? ›Insulation› building material knowledge. Accessed April 26, 2018 (German).