What are inverted roofs?

An inverted roof is typically defined as type of roof ceiling constructions, where the waterproofing layer is installed under the insulation layers on the bearing structure, which has been given the proper slope.

The inverted roof solution is mainly used for roof ceilings that are exposed to high mechanical loads – operable and traffic-laden roof ceilings, green roofs, terraces, roof ceilings of various underground structures, etc.

What are the advantages of an inverted roof?

  • The waterproofing layer is protected
  • The functional life can be considered to be significantly longer
  • More environmentally friendly
  • Thermal shocks are among the most important factors involved in ageing and fatigue for waterproofing membranes which are not the case for inverted roofs
  • Better protection in case of storms
  • Can withstand traffic loads
  • They remain unaffected by freeze/thaw cycles

Which waterproofing system to choose?

Depending on the base structure, either liquid waterproofing sheets or solid waterproofing roll membranes are chosen as the waterproofing system of the inverted roof.

Liquid membranes – one of their advantages is that they adhere to the substrate and therefore it is very difficult for rainwater to penetrate between the substructure and the waterproofing layer. At the same time, the disadvantage of liquid membranesis that their ability to bridge cracks is significantly lower compared to roll membranes. However, waterproofing membranes, which have the same or better crack-bridging ability than roll membranes, are significantly more expensive in terms of price.

Bitumen roll membranes – you can choose between roll membranes glued or torched onto the substrate. The advantage of roll membranes is very good bridging of cracks, better weather resistance during installation and immediate rain resistance after installation.

Description of the system build-up

Such solutions, where the membrane is installed loosely on the substrate or fixed mechanically (for example, PVC, TPO or other synthetic membranes), i.e. which are not adhered to the substrate, must be considered very carefully and, if possible, avoided altogether in the case of inverted roofs. In the case of loosely installed membranes, it must be possible to carry out a water test in order to be completely convinced of the waterproofness of the membrane throughout the entire installation. In the case of a loose membrane, it is almost impossible to find installation errors or damage in the membrane after backfilling, because water can penetrate between the membrane and the concrete surface and enter the building from a completely different location compared to the location of the damaged membrane.

When making a choice between liquid membranes or bitumen roll membranes, the installation time should be taken into account (if most liquid membranes require a temperature of at least +5 degrees for installation, for example, in the case of bitumen roll membranes, the installation temperature can even be below 0 degrees). The exception is liquid plastic, which is a liquid multi-component waterproofing membrane reinforced with a special super-strong woven fabric, the installation of which can be carried out at temperatures down to -10°C.

The base surface must always be inclined to ensure free drainage of water from the waterproofing layer. In addition to slopes, it is very important that a drainage layer is built directly on top of the waterproofing layer. To build a draining layer, you can use either a multi-layer drainage mat (which does not exert a point load on the waterproofing) or insulation boards with specially designed grooves (do not recommend due to unknown capacity of the water volume). Ensuring the free drainage of water from the waterproofing layer is a guarantor of the structure’s water resistance.

We recommend using the Köster KSK SY 15 cold-adhesive bitumen roll membrane, which has very good properties and has proven itself on many sites over the years:

  • Frost-resistant, pressure-water resistant, self-adhesive
  • No hot air or propane gas required
  • Waterproof immediately after installation
  • Very elastic based on rubber bitumen
  • Solvent free
  • Bridges cracks
  • Very fast installation
  • Extremely age-resistant
  • Laminated on top with highly tear-resistant foil
  • Radon resistant
  • Vapor resistance µ: <130,000
  • Can be used on all mineral substrates as well as plastic and metal substrates

The lifetime of Köster KSK SY 15 is considered to be the lifetime of the structures to be designed in the case of new buildings. Buildings are usually designed for 50 years, and thus the lifetime of the material is 3-5 times longer than, for example, SBS-based materials. The reason is that SBS bitumen roll materials lose much of their elasticity over time, as the plasticizers used in them to provide elasticity evaporate from the material. However, the elastic polymers used in Köster Bikuplan KSK SY 15 are resistant to aging and the material never loses its elastic properties. In addition, the material is also radon-proof.

Application speed

Since Köster Bikuplan KSK SY 15 has been tested under pressurized water conditions and can withstand water pressure of up to 4 bar, the material is generally installed in one layer (except for up and down turns). Therefore, it is possible to waterproof very large areas in a very short time. During one working day, it is possible to install 2000m2 or more square meters. With good planning, it is possible to cover areas of more than 10,000 m2 with roller membrane in one working week.

Environmental sustainability

If you compare cold adhesive roll membranes with a weight of approx. 1.5 kg/m2 with SBS roll membranes with a weight of approx. 4 kg/m2, the difference is noticeable. In inverted roof constructions, SBS is mostly used according to the Finnish standard V80R, which means that at least 3 layers must be installed, which makes the waterproofing weight of 1 m2 approx. 3 x 4kg = 12kg/m2. The total weight of the Köster Bikuplan KSK SY 15 system is 1.5 kg/m2. In other words, the difference is eightfold. As the main component of both materials is bitumen, it can be roughly deduced from this calculation how much more resources (production, transport, storage, installation) the production of SBS roll membranes requires compared to a cold adhesive roll membrane.

Traveter Group has over 20 years of experience in installing cold adhesive bitumen roll membranes.

We perform waterproofing of underground ceilings, i.e. inverted roofs, with mineral waterproofing mambranes, membranes based on epoxy resin, self-adhesive bitumen roll materials, and roll materials based on synthetic material. In terms of price and quality, the Köster KSK SY15 single-layer bitumen roller membrane, which we have installed on hundreds of thousands of square meters over the years, has justified itself.

Inverted roofs

System build-up