Change the slope of the garage roof

Hello,

we have a garage roof that slopes slightly backwards from the front/garage door side towards the pergola.

So far, the rainwater has flowed from the garage roof over the glazed pergola into a downpipe.

Now the pergola is to be reconstructed. Therefore, it is necessary that the rainwater from the garage roof, which consists entirely of concrete, no longer flows backwards over the pergola, but flows forward via a changed slope into a rainwater pipe yet to be installed.

Is such a variant possible, where the sloping surface on the roof is raised at the front with a concrete layer and then the overall slope is created towards the front?

Is it even possible to apply a concrete layer on an existing concrete layer?

What costs would we expect for this variant for a length of 15 m and a width of 5 meters?

Thank you very much for an answer!

You want to pour so much concrete in a wedge shape on a 15 m long concrete roof that just by that its lower end becomes a higher end? - but this collapse attempt is not supposed to be the ultimate wisdom, is it?

That already sounds a lot like "Technische Hochschule Schilda"

Well, maybe one should have asked first about the slope before giving that answer, then our idea might not necessarily have been a "Schilda wisdom." The garage roof has a very slight slope, is actually a "flat roof" and it would be more of a "leveling screed." Hard to imagine that there would be a collapse here. If the flat roof of the pergola were "filled up" so that it has the same height at the front (towards the garage door)... and then collapse????? And the "screed would then only have to be smoothed over by a few centimeters from back to front so that a slight slope is created again. After all, we don't want a new garage roof but just to slightly reconstruct the old one! Apparently, every craftsman has a different approach here – and certainly the costs can be set differently as well. Just as a small example: pigeon spikes are to be installed on the roof. One craftsman needs an expensive scaffold for this... the other roofer goes out, sticks the things on and that's it... two ways to one result. In other words, "one can make an elephant out of a mosquito" .....

Insulation is the magic word. Every roofer should be able to create a slope for it, price around 3-4k euros depending on the size.

Alternatively, use EPDM foil and create a tray with it. Everything below the drain then simply evaporates. Price around 1800-3000 euros.

Many things are possible. If the statics of your garage allow it, talk to a screed installer; he can make screed with a three-dimensional slope for you. Or first get opinions from a couple of roofers. Styrofoam boards with a slope can be installed and then sealed with foil or bitumen membrane.

Well, maybe one should have asked first how the slope actually is before this answer, then our idea might not necessarily be a "Schilda-wise".

I have read that

at a length of 15 m [...] it slopes down towards the pergola at the back.

I interpret this as a 15 m long section of a slope that is supposed to be converted into an incline. With less than 2° inclination, you can hardly use a slope for drainage. Then the delta between the current low point and the new high point at the same edge is a proud one meter and four. That carries weight if you want to do it with concrete.

At half a degree, it's still 26 cm. That is also far from light.