ᐅ Is waterproof concrete suitable for flat garage roofs?

Hello everyone,

The shell of our double garage (8 x 6 m (26 x 20 ft)) is currently being built. Soon, the formwork for the concrete ceiling (flat roof) will begin.
Later, a bitumen coating and gravel are planned to be applied on the roof. I have often heard that the coating can be prone to leaks. Would it help in this case to use waterproof concrete (tightly waterproof concrete) despite the 18-20 cm (7-8 inches) thickness? Would this provide additional protection, for example, against standing water?
The extra cost for waterproof concrete would be about 200 euros. (It might also require using more reinforcement due to the waterproof concrete.)

Best regards

If someone tells you that for 200€ more they can turn a regular ceiling into a waterproof concrete (WU) ceiling, they are just taking the 200€ and labeling it as WU. There’s really nothing more to say about that.

No, because even waterproof concrete (WU concrete) is not completely watertight, but water-resistant. This means the amount of water allowed to pass through per hour is simply lower compared to other types of concrete, but it is still not 100% sealed. In areas in contact with the ground, this can be sufficient as moisture protection; if less moisture reaches the concrete than it allows through, the interior stays dry. However, as a roof exposed to standing water, it definitely does not work. So it’s a waste of money—better invest the 200€ in higher-quality roofing felt, a bitumen layer, metal flashing, or whatever the roofer plans to install up there.

Lumpi_LE schrieb:

If someone tells you they will turn a regular slab into a waterproof (WU) slab for 200€ more, they just take the 200€ and label it as WU. There’s not much more to say about that.

The 200 euros would only be the additional cost for about 10m³ (13 cubic yards) of waterproof concrete.

However, if this does not provide any benefit against standing water, then it might be something you could skip.

Mottenhausen schrieb:

In the below-grade area, this can be sufficient as moisture protection if less moisture penetrates than the concrete allows, keeping the interior dry. As a roof with, at worst, standing water, as mentioned, this does not work.

What about the model basements that are permanently submerged in a pool?

External waterproofing "tank" system, as commonly used for basements with standing water, I assume?