FLAT ROOF AS A TIMBER CONSTRUCTION: THE IMPORTANCE OF VENTILATION AND VAPOR BARRIER
Unventilated flat roofs are no longer a modern construction method, as this has led to a great deal of damage in the past. The correct arrangement of timber components, ventilation level and vapor barrier is crucial to prevent damage caused by moisture and air leaks.
In order to prevent moisture from the room air (e.g. from breathing air, showers, cooking) from penetrating the wooden structure of the flat roof, a vapor barrier is usually installed.
Unfortunately, practice has shown that it is almost impossible to install the vapor barrier so airtight that it can withstand the function of sealing against humidity from the living space in the long term. On the one hand, this is due to material fatigue, unclean work during the construction process or subsequent damage.
Once moisture has penetrated the roof structure, it can no longer escape from the roof, as a (bituminous) sealant is applied as the top layer of the roof structure to seal the roof against precipitation. As this seal is not only watertight but also airtight, moisture collects as condensation in the roof boarding and begins to decompose the wood.
The moisture present in the timber used during installation can also collect in the coldest areas of the roof structure and also trigger the growth of wood-destroying fungi. The roof literally composts itself.
Over a period of several years, wood-destroying fungi can develop unnoticed in the wooden components of the roof and spread unnoticed. As a rule, the damage is only noticed when it reaches the living space. This is when moisture damage or unpleasant odors become noticeable. In most cases, however, a large part of the roof structure has already been damaged and needs to be replaced.
An air layer underneath the waterproofing layer can dissipate the moisture before it can penetrate the wooden structure and cause damage. For at least a decade, flat wooden roofs have therefore been planned with a ventilation layer. Older flat roofs, which in earlier times were built without an air layer, are now often in need of renovation.
Recommendations for the arrangement of an air layer in the flat roof:
- Air layer for ventilation: An air layer in the flat roof should be arranged in such a way that it allows sufficient ventilation. This reduces the risk of moisture and damage to the wood.
- Positioning: The air layer is often positioned between the insulation and the roof cladding. This positioning helps to prevent condensation caused by temperature differences between the inside and outside.
- Removal of moisture: The air layer should be designed to ensure that any moisture or vapor diffusion that may have penetrated from the inside is conducted to the outside.
- Ventilation openings: Sufficient ventilation openings should be planned in to ensure air exchange. These can be located at the lowest point of the air layer to promote effective ventilation.
- Manufacture and materials: The standards can provide specific guidance on the materials used, such as what type of insulation material is best suited to a flat roof construction to prevent mold growth.
