How best to mend old flat concrete roof?

Our garage is 50 years old and has a flat concrete roof. The concrete is crumbling away, so we want to mend it this year before it freezes again (this is in central Europe). We have thought about glueing thick roof felt on it, the kind with gravel on it. But I don't think we are upto doing it with the burner and so on. Is it easier to do than it looks? Is there a better way?
We already jetwashed it to get the holes clear.