Ralf Spilker, Rainer Oswald, Silke Sous, Matthias Zöller
Bibliographic info:
9th International BUILDAIR Symposium, 8-9 May 2015 Kassel, Germany

Purpose of the work

Wooden flat roofs are being increasingly constructed without the tried and tested sub- ventilation layer of the sealing sheet. However, some regulations refer to the high risk of damage to such structures if the sealing on top of the insulation is largely vapor proof. This research was to analyze whether more recent recommendations, as for example installing a variable vapor retarder, lead to a significant reduction in the risk of damage.

Method of approach

The study is mainly based on a survey of experts for building damage, as well as experts in the fields of roofing and wood construction.

Content of the presentation

The presentation shows the most important aspects and results of the study “Reliability of wooden roof structures without sub-ventilation of the sealing sheet”. The research work was sponsored by the Federal Office for Building and Regional Planning in Bonn and was finished in March 2014.

Results and assessment

The 1,657 experts interviewed named 337 cases of damage to flat roofs. In about one third of these wooden roofs, a moisture-variable vapor retarder had been installed. In 28 cases, extensive information on damage progression, assessment, and recommendations for repairs and their implementation was also given. In only four of these 28 cases would the risk have been reduced by installing moisture-variable vapor retarders.

In the German-speaking neighboring countries Austria and Switzerland, there are specific recommendations and principles for the planning of wooden flat roofs without sub-ventilation of the sealing sheet. However, these also strongly limit their scope of application and explicitly warn against the risk of moisture in unventilated flat-roof structures.

In Belgium and the Netherlands, the structures analyzed here have only recently been discussed. At the moment, they are reviewing which conditions they can be used under. It can be assumed that only a few roofs of this type have actually been built and if so, they were constructed against all recommendations by the relevant institutions.


Even the use of moisture-variable vapor retarders and the mandatory certification for the structure through transient hygrothermal calculations should not obscure the fact that the moisture in the implemented materials must be controlled. If necessary, construction time must be extended correspondingly until wooden beams and wooden framework/wooden casing have sufficiently re-dried.

The risk of damage is only slightly reduced by installing moisture-variable vapor barriers, even when considering a number of conditions for the structure to dry out without being damaged. With regard to building physics and under controlled conditions, in particular when pre- fabricating, the roof structure can certainly be implemented without flaws, but in spite of such up-to-date materials, ageing damage to the roof membrane, changes to the roof structure, or changes in use during the lifespan of the building can lead to significant damage to the wooden structure.


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