Roof seal at the flat roof


Flat roofs pose a particular challenge in sealing. While the pitch of the pitched roof can be relatively smooth, a flat roof must be much denser. How the seal of the flat roof is constructed, and how it works, tells you this post.

Flat roof definitions

Flat roofs may still have a low roof pitch. In Germany one speaks of flat roofs, if the roof pitch is less than 10°. In Austria, for example, roofs are only considered as flat roofs if they have less than 5° roof pitch.

Structurally, inclinations between 1.1° and 2.9° are constructively recommended in the so-called flat-roof directive. In the field of building regulations, however, no fixed specifications are made, therefore, flat roofs can also have quite inclinations of 5 - 10°.

The Federal Immission Control Ordinance, which deals with chimney heights, on the other hand, considers every roof with a roof pitch less than 20° as a flat roof, each roof above it as a pitched roof.

Impact on the seal

Steep roofs - ie roofs with more than 10° angle of inclination - are covered, while the flat roof is always referred to as a seal.

This seal varies depending on the angle of inclination. DIN 18531 knows two so-called roof pitch groups. The assignment to one of the two groups is decisive for the type of sealing:

  • Roof pitch group I comprises flat roofs up to an angle of inclination of 3°
  • Roof pitch group II comprises flat roofs between 3° and 5°
  • Flat roofs between 5° and 10° slope or higher are no longer considered in the DIN

Flat roof construction and the effects

The construction of a flat roof can be different:

  • as a warm roof (without ventilation)
  • as cold roof (with ventilation)
  • and as a reversed roof (external insulation)

In addition, even so-called plus roofs are common, where on an old, already existing flat roof, a new is simply built while the old remains.

The design features of the flat roof can be disregarded in the selection of the sealing methods. It only counts the roof pitch.

Sealing methods at a glance

The waterproofing of the flat roof must be completely watertight, as well as resistant to small, burning parts and heat radiation in case of fire. As a rule, the flat roof is sealed with membranes made of different materials.

bitumen sheets

They can be made either plastic or elastic. Accordingly, their characteristics are slightly different. Their big advantage is that they can be easily penetrated even after installation. Experts also speak of "bituminous seals" regardless of the type of bitumen sheeting used.

EPDM sheets

These tracks are special plastics. EPDM stands for ethylene-propylene-diene monomer. They have been widely used in recent years, but they also have disadvantages.

Other plastic sheets

Their properties often differ fundamentally from those of the EPDM membranes. PVC webs, EVA or PIB webs as well as webs made of PE plastic are widely used.

Liquid plastics

Liquid plastics that are sprayed are normally only used in the field of renovation. But they can also be used to revise and renew an existing seal.

Roll seam-welded metal roofs

This is a special construction made of different metals. The method can also be applied to renovation, creating a completely waterproof and airtight roofing.


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