They don't come cheap but if you're a regular camper or like to head off on weekends away, a car roof box (also known as a luggage box or roof pod) can definitely be worth it.
A roof box fits onto the roof bars of your car and can add an extra 300 to 600 litres of storage space. But more importantly, they make a long drive safer. They improve your rear visibility (if you tend to pile things up in the boot or back seat) and reduce the risk of objects flying around the cabin in the event of an accident.
Installation
Roof boxes fix to your roof racks either with a basic U-bolt system or a 'claw'-style attachment. The claw attachments make it easier and quicker to mount and remove the box.
Compatibility
The roof box must be compatible with your roof bars. Check the shape, width and distance between your bars.
Size and shape
A roof box that's too big or the wrong shape can interfere with rear hatch or wagon opening. Narrow boxes can be difficult to reach, particularly on wider or taller vehicles. There are a range of shapes and sizes on the market, some with more specialised designs if you need to carry skis or other sporting equipment.
Opening
Boxes that open from both sides offer the most flexibility and practicality. Boxes that open from the driver's side only present a hazard if you can't pull off the road.
Load rating
Roof boxes have load ratings. Make sure you choose one that will safely hold the weight of your luggage.
Loading
Put heavier items in the boot of your car and use the roof box for lighter items. This will have less impact on your car's centre of gravity and make items easier to stow and retrieve.