Roof seal as additional seal on pitched roofs

A pitched roof is not only because of the roofing tight. Underneath the pitched roof is a watertight or rain-proof layer, which can be made in different ways. How this "under-roof" is constructed, which materials are used, and what must be taken into account, read here.

Necessity of a sub roof

Covering the roof with roof tiles or roof tiles is not completely watertight. They have joints and overlaps through which rainwater can penetrate. The wind pressure also ensures that water does not run off, but is blown under the roofing.

It can also form a so-called ice block in frost. This ice block ensures that meltwater accumulates on the roof and can no longer run off. Also in this case, water can penetrate under the roof skin.

The third possible risk of moisture consists of the formation of condensation (condensation) on the inside of the roof skin. It occurs when warm, moist air penetrates through the insulation, and condenses on the cold cover.

The three main factors that make the construction of a sub-roof necessary are:

  • Wind pressure and open overlaps of roof tiles
  • Backwaters through ice barriers
  • condensation

Protection against penetrating dirt

So that the insulation and the interior is not contaminated by dust, soot and dirt particles, the under-roof provides for a corresponding protection. The roof construction is also protected from penetrating dirt.

Designs of the sub-roof

Rain-proof and waterproof under-roof

For the type of sub-roof, especially the roof pitch is crucial.

For very steep roofs, there is usually a very good drainage in the direction of the eaves. A laying below the counter battens is thus easily possible. The sealing ability of the counter battens is sufficient to prevent ingress of the water through the nail holes of the counter battens. One speaks in this case also of a "rainproof sub-roof".

However, if the roof is not so inclined, other measures must be taken. The so-called "water-bearing layer" must then be laid over the counter battens, so that no water can penetrate. The water-bearing layer is additionally either glued or welded. One speaks then of a "watertight under-roof".

The counter battens must then be designed accordingly to allow installation of the sub-roof on the battens. As a rule, either trapezoidal counter battens or lateral triangular profiles are required.

However, a watertight under-roof must be constructed only in cases where the so-called regular roof pitch is greatly undercut. In practice, such low roof pitches are rarely the case, so waterproof roofs are rare.


If a roof structure does not have a ventilation layer between the insulation and the roof, the roof underlay must have at least six times less water vapor resistance than the internal vapor barrier. This ensures that moisture and water vapor can be safely channeled out of the insulation.

used material

If the water vapor diffusion resistance is irrelevant, you can easily use roofing felt. This used to be the common material. However, the requirement for a low water vapor diffusion resistance makes other substances necessary, today mostly nonwovens made of polyethylene.

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