Waterproofing of Upstairs Bathroom Floor (Concrete Slab)

[togaf]

Tilers are finishing up on a new home we're building and have not water proofed upper storey bathroom floor (suspended concrete slab) - showers have been fully water proofed upstairs and down.

I just noticed that BCA recommends waterproofing of upper storey bathroom floors. How much of a problem is this?

thanks,

Sam
Perth.