Sounds of rainwater drainage on flat roof

Hello everyone,

I have a quick question about a problem with our flat roof on the dormer.

When it rains, an unpleasant noise occurs – but not directly from the rain itself, rather only when a certain amount of water has accumulated and starts to drain.

Then there is a loud "gurgling" that lasts for several seconds up to a few minutes. It is so distinct that you get the feeling in the room that someone is drinking a nearly empty glass with a straw.

The tinsmith who installed the gutter and the drain is at a loss. According to him, even the manufacturer of the drainage system cannot provide an explanation.

He said it might, but only might, help to replace the drain with a drain box – however, at my expense.

Does anyone know this problem?
Is this a common defect in such installation situations?
Could it have been detected in advance?
And could this possibly be considered a defect in terms of construction performance?

Thank you in advance for your help!

Here is a photo of the installation situation:

Hi,

the described gurgling often occurs with flat roof dormers when draining rainwater flows together with air through an improperly sized or poorly ventilated drain. The cause is usually missing pressure equalization: air is sucked in, the water briefly backs up, then both escape audibly. Technically, this is not an isolated case, but it is not the norm with proper planning.

Whether this counts as a defect depends on how severe the impairment is in everyday use and what was guaranteed in the construction contract. If the drainage otherwise works flawlessly, it is often legally difficult to demand improvements – an expert can provide clarity here. A drain box can help, but it is not mandatory as long as the system is tight and reliable.

Schnackischnak