I think you would say increase petrol consumption...
I bought my car with a roof rack on it, but this year I threw it away cause of rust and bought a new one (smaller but prettier) on ebay for 30 euros... Painted it black and mounted. So I do not ruin my interiors with wooden boards or tools... And I can put on it my spare wheel during travels, as in its place I have the lpg tank...
It would be great to find ancient alu ones, there is plenty of them in France but tey always miss rubbers and fixings...