Hi Karl
We have a roof box, I am sure it is okay.
We have taken into consideration the max weight that you can load your car up to. We only put the garden table and chairs in the roof box, along with the Cadac so nothing heavy just the awkward items.
I know our roof box does have a weight limit, so if you purchase one then check how much it allows you to carry. We bought ours from Halfords and I think it is 75kg.
I have really done my homework with regards to the weight that we carry. We always make sure that the caravan isn't loaded up to its max and the items in the boot of the car are only the cool box, the kids bags/toys and a holdall/suitcase with our clothes in.
As long as you keep within all your weights, then it shouldn't be a problem.