Flat roof renovation 80 sqm old building

I have been a roofer for 40 years and we do 98% flat roofs with bitumen torch-on membrane. I have sealed all my roofs with bitumen torch-on membrane.

Thanks for the answer! The roofer believes that no slope is necessary. I just imagine that without a slope water will stand on the roof. He said that is not a problem. What do you think about that?
How long should a new flat roof last approximately?
Thanks!

Thanks for the answer! The roofer believes that no slope is necessary. I just imagine that without a slope water will stand on the roof. He said that is not a problem. What do you think about that?
How long should a new flat roof last approximately?
Thanks!

How long should a new flat roof last approximately? Thanks! So with good materials, about 30 years. There are differences in quality, for example. Bitumen membranes come in many different versions and qualities. Usually, mainly polymer bitumen membranes and elastomer bitumen membranes are offered. Sounds complicated at first, but actually it isn’t. Polymer bitumen membranes are bitumen membranes to which plastics have been added as a building material. In elastomer bitumen membranes, elastic substances have been mixed in, making them more elastic. Besides these two main building materials, there are countless other mixed and special types of bitumen membranes. These differ not only in the composition of the materials but also in the choice of carrier fabrics. Mostly glass or polyester fleece is used here. Each of these special types has its own specific technical property and thus a different application area. This makes bitumen an incredibly versatile building material. To ensure the result is absolutely waterproof, bitumen membranes are usually installed overlapping and laid in two layers. Anyone who has taken a closer look at a flat roof will have noticed that it mostly shines slightly green or gray. That is because the top membrane is usually covered with slate. Because: slate protects the bitumen membranes from UV radiation. Since bitumen membranes are temperature-dependent, they are also very sensitive to sunlight. What also helps besides a slate coating is the application of gravel. This prevents sunlight and thus UV radiation from penetrating and acts as a natural shield for the bitumen membranes. Due to the many different properties and application areas, the prices of bitumen membranes range from inexpensive to quite expensive. Just type in on the internet: Unterschiede Bitumenschweißbahn anzeigen ;) there are many reports.

Wow thank you for the answer.
Two more questions:
1. So, I believe that the house's ceiling has a slope of 2 percent. I roughly measured the room heights on both sides of the house and that should be about right. Is that enough for now or does it absolutely have to be more slope?
2. According to the offer, the bitumen will be installed in two layers by the company Bauder. Are they any good?
Thank you very much!!!

1) Slope for the flat roof.
The minimum slope for a flat roof in Germany should be 2.9 percent or approximately 1.1 degrees. However, since this minimum slope is actually extremely low, it is better to plan for five percent or 2.9 degrees.
At that time, they deliberately built “wet roofs”: A wet roof refers to a flat roof without significant slope, on which rainwater remains standing in a controlled manner due to elevated drains. The water level is intended to provide climate buffering, similar to a green roof.
So everything your roofer says is correct.

2) Bauder is a top brand, but there are quality differences even there:
Bauder KARAT
Polymer bitumen roofing membrane, 5.2 mm thick, with an extremely mechanically highly loadable 300 g/m² polyester composite carrier in combination with a premium bitumen compound. Chipped surface in the colors green-white and graphite black. Price about 12.00 euros per sqm.

BAUDER Elastomer bitumen roofing membrane PYE Photovoltaik 200 S 5 EN
Material: Elastomer bitumen, carrier: polyester fleece 250 g/m²
Price about 6.00 euros per sqm.