Flat roof leaking, repair or build a roof structure over it?

I sealed my 120 sqm flat roof myself, that already adds up to a lot of money, just for materials about 10,000 euros. Including wood, bitumen primer, weldable elastomer bitumen vapor barrier membrane 4mm, ESP 035 insulation board 240mm, cold self-adhesive elastomer bitumen underlayment membrane 3.5mm that goes on the insulation board, and then a good welding membrane, etc. I will not advertise the manufacturers now. I only bought high-quality products, as I am a roofer myself ;) If you then calculate the company's labor costs... you quickly lose 20,000 euros or more. Flat roofs with bitumen membranes now last 30 years and longer... if you buy good material ;) 32 years ago I also welded my first house with a flat roof myself and it has been watertight until today :) But we sold it 2 years ago and built a new age-appropriate one ;) with a flat roof and roofing felt.

with a flat roof and roofing felt.

I will gladly ask the expert about the roof construction. Roofing felt sounds cheap.

440 euros per m2 is usury.

[cold self-adhesive elastomer bitumen underlay membrane 3.5mm that goes on the insulation board,]
Does it have to stick cold because otherwise you would burn the insulation board?

20,000 EUR would still be half of 40 ...

With this one, it has to stick cold, otherwise you burn the insulation board?

yup, that's also how it's done in new construction, for example on tapered insulation. But only the first layer.