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Sealing a flat roof: you need to know that
Sealing a flat roof: you need to know that

Video: Sealing a flat roof: you need to know that

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Video: DIY Flat Roof Repair - Easy Paint on Fix 2023, February

Proper sealing of a flat roof is crucial because the surface is exposed to numerous weather influences and heavy loads. The sealing material must be able to withstand all possible stresses. Read below which requirements apply and which types of seals are best.

Seals on flat roofs not only have to withstand thermal loads such as large temperature fluctuations. In addition, there are usually mechanical, chemical and biological factors, so that the roof surface must meet the highest requirements. To ensure that the sealing materials are installed correctly, the best thing to do is to hire a specialist who is also familiar with the relevant rules and manufacturer guidelines. You should also check the flat roof once a year for damage.

If you want to familiarize yourself with the requirements for roof seals and waterproofing materials, you can consult DIN 18531 "Roof seals - seals for unused roofs".

Roof sealing: the requirements according to DIN 18531

According to DIN 18531-1, the quality of the flat roof waterproofing is divided into two different classes or application categories K1 and K2. Category K1 applies to flat roofs to which the usual requirements are placed. Application category K2 applies to roof seals that have to meet increased requirements, such as high-rise buildings or roofs with difficult access.

In addition, the respective requirements for sealing materials are regulated in DIN 18531-2. Accordingly, these must be watertight, sufficiently stable, stretch and tear-resistant so that they can withstand the expected temperatures, deformations and wind loads and have sufficient perforation resistance.

Mechanical stresses

A flat roof is not only exposed to thermal stress. The roof waterproofing must also withstand mechanical loads. According to DIN 18531, there are two levels:

Level I deals with high mechanical stress, which results from the following factors:

  • Base course movements
  • Support structure made of steel profiles
  • Formwork made of wood or wooden materials as a base for the waterproofing
  • Use of soft thermal insulation materials
  • Extensive greening
  • Stresses caused by the way the roof waterproofing is secured

Level II means moderate mechanical stress and is always present when level I is not present.

Sudden loads

Flat roofs must have increased perforation security so that they do not show any damage under impact loads. The test according to DIN EN 12691 serves as the basis for determining the resistance to an impact load on bitumen, plastic and elastomer sheets.

Thermal loads

Flat roofs are exposed to the different weather conditions around the clock. Sunshine, temperature fluctuations, precipitation and atmospheric oxygen cause degradation processes and thus also aging in sheets made of plastic and bitumen.

The aging behavior of a sealant is mainly determined by the quality of the material. The raw materials, the manufacturing and processing methods are decisive.

DIN 18531 also differentiates between two different levels with regard to thermal stress. Level A is always present when the material is directly exposed to the weather or has only a slight surface protection. Level B denotes moderate thermal stress and is present when the roof seal is protected from high heating and fluctuating temperatures.

Loose installed roofing membranes are particularly susceptible to damage in the cold season. Because the temperature fluctuations create tensile forces that strain the fixation points and seams.

Hailstorms also have an impact on flat roofs. For example, the EMPA (Swiss Federal Materials Testing and Research Institute) has determined that the waterproofing material according to the requirements profile (AfP) of the European Association should have a permanently tight roof (dd roof) with a hail resistance of at least 25 m / s.

Root strength of the membranes in green roofs

If you want to green your flat roof, you should consult the green roof guidelines of the "Research Association for Landscape Development and Landscaping" (FLL). There you can read about the "Procedure for the examination of the root strength of membranes and coatings for green roofs". If the lanes pass the two-year long-term test in an air-conditioned greenhouse, they receive the test certificate "Root Festival according to FLL".

Good to know: Even if you do not want to green your roof, it can be an advantage to choose root-resistant membranes, because plants can also settle through seed flight.

Flachdach begrünen
Flachdach begrünen

If you want to green your flat roof, you should definitely buy root-resistant waterproofing membranes.

Photo: MSG / Roland Krieg

Ecological aspect

Geomembranes should also meet certain ecological criteria. In addition to minimizing the use of materials and energy consumption in production and transport, it is also important to ensure environmental protection.

If you can choose the sealing membrane yourself, you should find a manufacturer who produces environmentally friendly, ecologically harmless and hydrolysis-resistant membranes.

It is also important to ensure that the building materials used are durable, low in pollutants and recyclable. Some manufacturers can prove the ecological safety of their products through test reports from neutral institutes.

Different types of seals

Anyone who reseals their flat roof as a result of renovation should make sure that the roof structure and, above all, the thermal insulation are undamaged and dry. Also make sure that the roof structure and thermal insulation are adapted to the currently valid regulations of the Energy Saving Ordinance (EnEV).

Sealing with bitumen membranes

The most common and easiest is to seal the flat roof with a special form of roofing felt, the so-called bitumen membranes. As a rule, however, these do not only consist of bitumen. Rather, a carrier insert made of polyester, glass or jute is surrounded by two layers of bitumen in order to ensure stability.

As a material, bitumen has the advantage that it is particularly weatherproof, waterproof and easy to process. As a by-product of petroleum production, the distillation bitumen is supplied to the bitumen membrane manufacturer in liquid form at 200 degrees Celsius. There, polymers are added in various processes, which, for example, ensure UV resistance. As a further protective layer, there is an aggregate on the top cover layer, which is also available in different colors. Lay roofing felt by rolling out the bitumen sheets with a certain overlap. Then they are glued with liquid bitumen and welded to the seams with a gas burner.


The easiest and most common is the sealing with bitumen membranes. Depending on the type, either the welding process or the cold gluing process is selected.

Photo: iStock / PhanuwatNandee

Depending on the type of bitumen membrane, either the welding process or the cold gluing process is selected. In the welding process, the sheets are heated in the lower area and connected to the flat roof without additional adhesive. The sheets are then welded at the seams so that the roof is particularly tight.

The cold gluing process requires special bitumen sheets that have an adhesive surface on the underside.

Bitumen removal is problematic. Especially in the welding process, the sheets are very firmly attached to the roof, so that residue-free removal is hardly possible. You either have to laboriously remove the bitumen layer with a spatula and chisel, or you hire a specialist to do the work with dry ice or a special chisel.

You can easily dispose of non-tar roofing paper via normal building rubble.

Plastic sealing

Plastic sheets are a good alternative to bitumen sheets. In contrast to bitumen membranes, they are laid in one layer and loosely on a protective plastic fleece underlay. It is essential to ensure a careful seam and connection connection. They can be secured mechanically as well as by a load. In addition, strip or punctiform gluing is possible, surface gluing is not carried out. The plastic seal saves time and material. Further advantages are the almost weather-independent laying and the lower fire risk. Both PVC and EPDM are used as materials. A plastic seal is extremely resistant to UV light and is usually between 1.5 and three millimeters thick.

Liquid flat roof waterproofing with bitumen

With the liquid flat roof waterproofing with bitumen, the liquid material is poured out and a special fleece is pressed in for reinforcement. This process is then repeated. The advantage of this roof sealing is that there are no seams through which moisture can penetrate.

Liquid bitumen is best suited for the integration of complicated structures such as penetrations, skylight domes or roof drains, because the material fully adapts to the contours of the substrate. Mechanical attachment to the upper edge of connections is completely eliminated.

Liquid sealing with plastic

There are also 1-component liquid plastics for sealing flat roofs. They do not have to be mixed and are ready for immediate use. They are also rainproof. The liquid plastic is either processed with a non-woven film or it already contains fibers for reinforcement. This also enables use in difficult areas and guarantees an effective seal. However, make sure that the surface is dry, clean and free of dust, grease or oil.

EPDM geomembranes

The abbreviation EPDM stands for the abbreviation ethylene-propylene-diene rubber from the M group and refers to a synthetic rubber that has been used as an alternative to roofing felt since the 1960s. The material is suitable for particularly high-quality flat roof seals and is offered both in sheets and as prefabricated tarpaulins. The advantage of EPDM tarpaulins is that they are extremely elastic and can be expanded to a multiple of their original size. Due to the prefabrication of the tarpaulins, almost no seams have to be carried out on the construction site. This minimizes the risk of leaks. In addition, the EPDM seal can be so strong that it is even protected against root penetration. Other advantages are:

  • Very long service life of more than 50 years
  • Resistant to UV, ozone and many chemicals
  • Compatible with bitumen
  • Fast processing without exposure to heat on the flat roof insulation
  • Permanently elastic
  • Lighter than bitumen sheeting, suitable for lightweight halls

Slope of a flat roof

When renovating a flat roof, you should make sure that the roof has an incline of at least two to a maximum of eight percent. This ensures that water can drain off and that there are no puddles of water for a long time. To achieve the slope, a plaster spread with decreasing thickness can be applied. In addition, there is the possibility of installing appropriate slope insulation elements in the substructure.

Pfützenbildung Flachdach
Pfützenbildung Flachdach

Flat roofs must have a slope of at least two to a maximum of eight percent.

Photo: iStock / Beeldbewerking

Penetration of roof waterproofing

Penetration is understood to mean “a component that penetrates the roof waterproofing”. This includes pipelines, railing supports, chimneys and lightning protection systems. Keep in mind that any penetration of the roof surface always means a weakening of the roof and should be avoided at best. In most cases, this is not possible, but the penetrations can be kept to a minimum. DIN 18531 specifies the corresponding specifications for the design and spacing of roof penetrations.

As a rule, penetrations are carried out using adhesive flanges, sealing sleeves, clamping flanges or liquid seals. Make sure that there is a distance of at least 30 centimeters between the penetrations and other components.

Good to know: There are already solutions for solar and air conditioning systems that do not require penetration. Also find out whether the manufacturer of waterproofing membranes also offers various molded parts for penetrations as system accessories.

Sealing a flat roof: the costs

Since the sealing of a flat roof usually goes hand in hand with a renovation of the roof, the costs for the disposal of the old roofing paper are usually the first to be incurred. For 100 square meters of roofing felt with a volume of five cubic meters, you can expect around 500 to 920 euros.

For simple bituminous waterproofing membranes you can expect around two euros per square meter, high-quality bituminous roofing membranes are already between seven and nine euros per square meter. The cost of plastic sheets is between five and 12 euros. EPDM sheets are available from 10 to 25 euros and the most expensive is the liquid plastic at around 25 euros per square meter.

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