Is WU concrete useful for flat garage roofs?

Hello everyone,

our shell for the double garage (8 x 6m) is currently being built. Soon, the formwork for the concrete ceiling (flat roof) will begin.
On the roof, a bitumen coating and gravel will probably be applied later. I have often heard that the coating is not watertight. Would it help here to use WU concrete (despite the 18-20 cm thickness)? Would that provide additional security, e.g., with standing water?
The extra cost for WU concrete would be only about 200 euros. (It could be that more rebar is needed because of WU).

Best regards

If someone tells you that for 200€ more they make a normal blanket into a WU blanket, then they take the 200€ and write WU on it. There really isn’t much more to say about that.

No, because WU concrete is not waterproof, but water-resistant. That means the amount of water allowed to pass through per hour is simply lower than with other types of concrete, but it is still not sealed 100%. In the area in contact with the ground, this can be sufficient as moisture protection; if less moisture comes in than the concrete allows through, it stays dry inside. As a roof with potentially standing water, as mentioned, this does not work. Therefore, it's a waste of money; better invest the 200€ in higher quality roofing felt, bitumen layer, flashing, or whatever the roofer plans up there.

If someone tells you that for 200€ more they turn a normal slab into a WU slab, then they take the 200€ and just write WU on it. There is actually nothing more to say about that.

The 200 euros would only be the additional cost for about 10m³ of WU concrete. If that doesn’t provide any added value in case of standing water, then one could of course save that.

In the earth-contact area, this can be sufficient as moisture protection; if less moisture comes in than the concrete lets through, it stays dry inside. As a roof with, in the worst case, standing water, as mentioned, this does not work.

What about the sample cellars that are permanently swimming in the pool?

Outer sealing "tub," as is common with basements with standing water, I assume?