Roofbox - do I have to get bars if my car has roof rails?

[bumpertobumper]

I need a roof box,and we have an estate with roof rails. I can't find a clear answer on eg halfords site so hoping the wisdom of mumsnet can help.
A friend has offered to lend me their box and I'm trying to work out if I need the special bars if I already have roof rails on the car.
TIA

I'm afraid you do need bars even with rails.

The bars go horizontally across the car (at right angles to your rails) and it's on them that you put the box.

I disagree - I have a Renault with roof rails and I do not need bars too. I've got a massive roof box on my car, fixed straight to the rails. In Halfords, their system automatically recommends the bars you need, so if it's recommending none, you don't need them.

It's clearly dependent on the car then. I have bars and roof rails. To get a roof box on my car without bars would need a really wide box and it would have to go on sideways!

Sounds like it depends on the box and the car. Thanks for your input. I'll go to halfords tomorrow.