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Sealing is the crucial point of a flat roof. If its very low slope allows it to evacuate rainwater, it is insufficient to ensure that the parts below will be dry! So how do you choose the waterproofing membrane of your flat roof? And how to do it taking into account both technical and environmental requirements?

How to seal a flat roof?

A flat roof or roof terrace is composed of a single pan. It does not have a carpentry but a carrier element (concrete slab, steel sheets, wood, etc.). It has a slight slope (1-5%) for the rainwater to flow out.
Above the supporting structure, the flat roof receives a thermal insulation that protects from cold and heat and a waterproof membrane that must be foolproof!

Currently you find 2 main types of membrane:

  • Multilayer asphalt and gravel membrane: With more than 150 years of hindsight, this type of membrane has proved its worth. But he is accused of contributing to the phenomenon of urban heat islands (ICU).
  • Synthetic membranes monolayer or bilayer: Light and resistant, they are more or less flexible and easy to install. Although the oil industry, they are considered the least polluting membranes

Multilayer asphalt and gravel membrane

This membrane is composed of 4 to 5 layers of felt paper between which liquid asphalt is applied. To protect the asphalt from UV, it is covered with a layer of gravel.

  • Advantages of multilayer asphalt and gravel membrane: It is easy to set up especially on flat roofs where are located many vents, fireplaces, roof windows, etc., as much obstacle to bypass. It has good resistance over time (often beyond 25 years). Its robustness is even recognizable on flat roofs where the water tends to stagnate (too low slope or clogged drains).
    It is the least expensive of all roof waterproofing membranes.
  • Disadvantages of multilayer asphalt and gravel membrane: It is heavy and polluting. Its carbon footprint is high. Protective gravel can clog drains. An annual inspection is required.
    It contributes to the phenomenon of urban heat islands unless it is covered with white gravel.

Synthetic membranes monolayer or bilayer

The family of synthetic waterproofing membranes comprises several members including:

  • Membranes EPDM (Ethylene Propylene, Diene Monomer).
  • Membranes PVC (Polyvinyl chloride)
  • Membranes TPE (Thermoplastic polyolefin or thermoplastic elastomer)

Different in their composition, the synthetic monolayer or bilayer membranes must be chosen according to the specificities of your flat roof.

  • Advantages of synthetic membranes: They all have in common their lightness compared to the asphalt and gravel membrane.
    Their lifespan is from 30 to 50 years.
    They are rot-proof and offer good resistance to insects except perhaps rodents that sometimes appreciate their "rubbery" flavor... Therefore, it is better to put a protective coating.
    They make an excellent base for hosting a green roof.
    They exist in white in order to avoid the urban heat island effect.
    They are recyclable.
  • Disadvantages of synthetic monolayer membranes: More or less flexible, they are more or less easy to implement.
    Monolayer membranes have no room for error when installed. They must therefore be asked by a specialist and be regularly checked.
    Depending on the type, they stick to cold or hot. Hot laying (using a gas torch) of two-layer membranes can pose a safety problem.
    If they are incorrectly installed, joints may fail and be a source of leaks.

Did you know? Naturally asphalt, an oil of very viscous consistency outcrops on the surface. It has been used for a very long time. Noah would have used it to seal his ark. In antiquity, the hanging gardens of Babylon, were also sealed with asphalt.


Video Instruction: Application of the polyurethane waterproofing membrane ISOFLEX-PU 500 on flat roofs