It's usually possible to carry a roof box and a kayak at the same time. This does depend on how wide your kayak is and how long your roof bars are. You'll almost certainly need to be using longer 'through' style bars, shorter flush ended bars won't be long enough for a roof box and a kayak. You may need to carry the kayak on it's side to make more efficient use of the space on the bars. We have a large range of kayak carriers that will allow you to do this, covering all budgets.
You'll also need to consider the maximum load of your roof bars or car roof (be guided by the lower of the two).
If you do need to carry a kayak and a roof box we would recommend looking at our long thin roof box range (please click here for details).