Flat roof - vapour barrier and vapour barrier

Flat roof structures, just like pitched roof structures, must also be protected against the penetration of moisture from the inside .

As a general rule, the diffusion resistance in a component (e.g. wall or roof) must decrease from the inside to the outside, because in the critical case of cold and dry weather on the outside, vapour diffusion takes place from the inside to the outside. Since the external flat roof waterproofing is relatively vapour-tight, a vapour barrier or barrier is applied to the inside of the insulation . It is available as plastic film, aluminium composite film or bitumen film.

More about vapour barrier and vapour barrier for flat roofs

When selecting the vapour barrier or barrier, the requirements for the barrier value (= sd value) and the type of substructure are particularly decisive. The requirement for the sd value of a system-compatible membrane can be created or checked with the help of a diffusion calculation according to DIN 4108, Part 3, Section A6. Such a calculation can ensure that the moisture balance of the roof structure is positive with the selected materials. This is the case when less condensation accumulates in the winter months than can diffuse out in the summer period. The diffusion inspection of the selected structure is also carried out by the commissioned energy consultant in the course of the thermal insulation verification. Online calculators such as www.u-wert.net can also help here.

Notes on the planning

vapour barriers/barriers are not required: In special cases, the supporting structure under the insulation layer is already sufficiently diffusion-inhibiting, so that no vapour barrier or barrier needs to be installed. According to DIN 18195-5, this layer can also be omitted without proof in the following cases:

• inverted roofs (non-ventilated roofs with thermal insulation above the waterproofing) and vapour-permeable load. This applies, for example, to gravel, while green roofs are vapour-tight.
• Diffusion-tight insulation materials such as foam glass are used on a rigid supporting structure and under the waterproofing.
• Non-ventilated roof structure and at the same time diffusion-inhibiting component layer (usually supporting structure) under the thermal insulation with sd ≥ 100 m. The thermal resistance below the diffusion-inhibiting layer may not exceed 20% of the total thermal resistance.
• Ventilated flat roof structure and at the same time diffusion-inhibiting component layer (usually supporting structure) under the thermal insulation with sd ≥ 100 m. The thermal resistance below the diffusion-inhibiting layer may not exceed 20% of the total thermal resistance.

Explanation: In practice, the exceptions mentioned are rarely used, as concrete roofs, for example, usually have an sd value of only approx. 10 to 30 m. Substructures made of trapezoidal sheet metal are generally vapour-tight, but water vapour diffusion still occurs in the area of impacts and penetrations, so that vapour barriers or barriers are usually also used on trapezoidal sheets. In air-conditioned rooms, a diffusion-tight layer (vapour barrier) must always be installed due to the special climatic conditions.

Influence of the substructure: The suitability also depends essentially on the type of substructure and the connection areas. Cold self-adhesive membranes are particularly suitable where welding is not possible or only possible to a limited extent because damage could occur to fire-prone components. Here, attention must be paid to both the substructure (e.g. wooden substructure) and the connection areas (window connections, etc.).

Roof systems: The compatibility and compatibility of the various component layers with each other must be checked. However, most manufacturers also offer tested systems for entire roof structures for the different requirements, so that there is certainty of layer compatibility for their planning.

Separating and levelling layer: A separating and levelling layer in the form of a plastic fleece must be installed under the diffusion-inhibiting layer. This separates the roof structure from the subsoil in order to avoid the transfer of stresses from the supporting structure and to protect the vapour barrier from roughness and chemical influences. Depending on the approval, some vapour barrier membranes can take over this function at the same time, so that no additional separating and levelling layer is necessary if:

• the roof layers are laid loosely or
• the vapour barrier or vapour barrier is only glued in points or strips

prerequisite for both assumptions is a substrate without gaping cracks, burrs and sharp edges.

Connections: The connection area of the vapour barrier or brake must be raised at least to the upper edge of the thermal insulation, and it must be tightly connected to roof penetrations so that no water vapour can penetrate the thermal insulation level.

Notes on the construction of

the installation: The seam and joint area should cover each other by 8 -10 cm. Cross joints are to be avoided. Seams and joints are to be staggered.

Fastening: Each roof package must be secured against the wind suction forces that occur. This can be secured by mechanical fixation (dowels) in the substructure. In such constructions, vapour barriers do not necessarily have to be glued to the substrate, the position is secured by screwing the thermal insulation and sealing. Mechanical fastening of the roof layer structure through the vapour barrier does not impair its effectiveness.

Temporary cover (emergency sealing): Some vapour barriers can be used as temporary cover for a transitional period. However, this should only be a short, limited period of time, as the vapour barriers are usually not designed for use in the open weather and cracks must not occur under any circumstances. Information on the maximum duration of free weathering depends on the information provided by the manufacturers.

Wind suction protection: If a flat roof is designed with an additional load and without mechanical fixation or gluing, care must be taken to ensure that the construction progress is carried out quickly and taking into account the weather conditions and that the load is applied immediately after completion. Bonding or fixing the panels is safer.

Care: Flat roofs are usually designed with minimal slope (e.g. 2%) so that water can flow to the drains. During installation, care must be taken to ensure that this slight gradient is maintained and not reduced by inaccuracies in the substrate, residues or soiling.

Storage: The strips must be stored away from UV rays, moisture and heat.

Standards and literature

Note: DIN 4108-1 (Thermal insulation in building construction; Sizes and units) has been withdrawn and replaced by DIN EN ISO 7345

DIN 4108 Supplement 2, Thermal insulation and energy saving in buildings - Thermal bridges - Planning and execution examples

DIN 4108-2, Thermal insulation and energy saving in buildings - Part 2: Minimum requirements for thermal insulation

DIN 18195, Parts 3, 5, 8-10, Building waterproofing

DIN 18234, Structural Fire Protection of Large Roofs, Fire Exposure from Below

DIN 18338, VOB Procurement and Contract Regulations for Construction Services - Part C: General Technical Contract Conditions for Construction Services (ATV) - Roofing and Roof Waterproofing Work

DIN 18531, Part 1-4, Roof Waterproofing

DIN EN 13970/A1, Waterproofing Membranes - Bitumen Vapor Barrier Membranes - Definitions and Properties

DIN EN 13984, Waterproofing membranes – Plastic and elastomer vapour barrier membranes

DIN EN ISO 7345, Thermal insulation - Physical quantities and definitions

EnEV Energy Saving Ordinance for Buildings, Ordinance on Energy-Saving Thermal Insulation and Energy-Saving System Technology for Buildings

Flat Roof Directive (Technical rule for waterproofing), rule for waterproofing of unused roofs, rule for waterproofing of used roofs and surfaces

Source: bauwion