Bathroom pipes and waterproofing: how pipe installation damages the seal

By: Ron Arad, civil engineer

A bathroom has to be waterproofed over the concrete floor, with cove fillets (raised edges) running up the walls, so that moisture cannot reach the apartment below or pass through the bathroom walls.
Applying the waterproofing correctly is the obvious part.

It also has to be coordinated with everything else in the room, above all the hot and cold water pipes and the drain pipes that run through it.
The water pipes run across the concrete floor and "climb" into the walls, and from there reach the fixtures: sink, toilet, shower or bathtub.

Some of the drain piping stays entirely on the floor, as under a shower stall or a bathtub, and some of it also "climbs" the wall.

Which should come first, the waterproofing or the pipes?
Unfortunately no standard settles this, and every site manager sequences the work according to his own experience and understanding.
What happens when the pipes are laid before the waterproofing

If the pipes go in first and care is not taken to bury all of them beneath the waterproofing, they are forced to pass out of the waterproofing layer and back into it.
We documented exactly this in a luxury apartment in north Tel Aviv, during a routine home inspection (bedek bayit) while the building was under construction.

In the master shower room, the pipes passing into and out of the waterproofing cause it to fail. A crack has already formed between the waterproofing and the pipe.

The failure is at the point where the pipe comes out of the waterproofing. That point cannot be sealed hermetically, and over time (sometimes immediately) a crack, a gap, opens between the waterproofing and the pipe.
Water that collects under the floor tiles from showers, leaks and so on passes through the crack to the underside of the waterproofing, and from there into the apartment or into the apartment below.

How do you prevent it?
In a case like this, where the waterproofing was applied only after the pipes, all of the piping should have been buried beneath it, and the pipes should have emerged inside the walls only.

Note that even when the water pipes are entirely buried beneath the waterproofing, a weak point still forms around the drain piping, part of which has to stick up above the waterproofing, as the photos above show.

That is why it is always better to install the pipes after the waterproofing and then apply another layer of waterproofing over them. The result is waterproofing both beneath the pipes and above them.