Whether you go for a narrow, medium-width or full-width box should depend on whether you want to carry anything else on the roof next to it, such as a bike or two.

And don’t forget to think about the roof bars, the necessary support for a roof box. Aluminium bars with T-track slots will carry a wider load than steel bars.

On a typical family car, a roof box of 90cm will leave space for one bike carrier if aluminium bars are used.

On a good roof box, the materials, locks and fittings should be robust and the box should be fully waterproof.

We tested for all these things, along with the ease of mounting and unmounting and opening and closing, along with how heavy the boxes were and how much room there is inside (and whether they are easy to pack).

It is safer and easier if two people fit and remove a roof box – longer boxes tend to be caught by the wind on windy days and wider boxes are more difficult to move single handily than medium width or narrower boxes. But mostly, it comes down to a person’s strength and the height of a car.

Also remember that roof boxes increase drag and therefore fuel consumption, but modern designs include features to keep the drag to a minimum, with driving speed and style having a far bigger effect.

The best boxes are approved by TÜV, a Germany test centre that sets standards for roof boxes, among other things. Be warned that some cars have very large spoilers which prevent the tailgate from opening fully if a long box is fitted, while other cars have roof bars that are fitted too far apart for some roof boxes. But experienced resellers should know about these problems.

You can trust our independent reviews. We may earn commission from some of the retailers, but we never allow this to influence selections, which are formed from real-world testing and expert advice. This revenue helps to fund journalism across The Independent.

KAMEI 510 roof box: £389.95, Roofbox

A top seller from KAMEI, Germany’s specialist roof box manufacturer, this is a full length and full width box – a big family load lugger which can be opened from either side (which also makes fitting easier) and, like all KAMEI boxes, it has low profile fittings that take up minimal luggage space and “just pull to shut” closing (so much easier to work than the key operated type of central locking system). There are three luggage straps for keeping everything secure and the shape of this huge box (510l) means you should be able to carry one bike beside the box if you use Aero bars with the T-channelled slots. It also comes with an impressive six year guarantee.

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Hapro 3300 roof box: £164.95, Roofbox

This wallet-friendly, short wide box – which comes from the second biggest roof box manufacturer – looks very smart. It’s hinged from the front, so you can open it from either side (which makes it both far simpler to pack and to fit the roof bars) and includes features usually only found in more expensive boxes, such as claw-style box-to-bar fittings which are particularly quick and easy to fit to nearly all roof bar profiles. The 330l box makes excellent use of space and can swallow a surprising amount of luggage, yet at 79cm wide you should still be able to fit a bike or two next to it if you’re using aluminium bars.

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Thule motion XT roof box: £460, Thule

Made by the world’s biggest car-rack brand, this large box has award-winning aesthetics and stand-out durability. Fitting an impressive 450l, it’s got enough space for weeks away with a big family and dogs and the design means you can pack it full and yet close and open it easily, no matter what the weather, due to the grip-friendly, well-designed handles and supporting high quality lid-lifters. And because the “slidelock” opening and “shut-lock” closing system is independent of the actual opening and closing of the box, there’s no danger of breaking the key. It’s simple to mount and unmount so you can remove it in under a minute you arrive at your holiday destination if you need to. Available in both black glossy and titan glossy, including in other sizes.

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Exodus 470L roof box: £299, Halfords

This carbon-fibre effect black roof box is really simple to attach and detach to your car’s roof rack and has no problems coping with extreme temperatures, as well as doing well in impact tests. It looks more sophisticated than many designs and accommodates a whopping 470l. It opens both sides, which makes loading and unloading a cinch – and although it can take a little longer than usual to get used to the mechanism, it poses no problems once you have the knack. For family trips, especially camping, this is a great buy.

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KAMEI Husky L roof box: £194.95, Roofbox

This German-made, medium-width family-sized box looks good and is ultra-secure. Made from patented ABS plastic, it comes from an award-winning name in the industry and at under £200, it’s exceptional value. It includes a “just pull to shut” closing system, which means you can stuff it full to the brim, yet it can still easily be closed by one person. It’s very robust and the sliding “box to bar” fittings mean you won’t have to drill holes if your roof bars aren’t the usual distance apart. Depending on the length of your roof bars, there should be space for at least two bikes beside this optimum shaped 330l capacity box.

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Halfords 250L grey roof box: £100, Halfords

This small and budget-friendly box is good for anyone who wants a reliable way to carry a bit of extra gear. For the price tag – and compared to other roof boxes around this price – it’s surprisingly durable and scratch resistant, although definitely not on par with the more expensive roof boxes here. There’s a single rear lock and because it’s made with rear opening, you can access it from both sides, which makes it easier to both fit it to the roof boxes and to load. Be warned the lock is a bit trickier to work than with most and don’t forget to check with Halfords that it won’t be too short for your car.

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Packline NX215 roof box: £764.95, Roofbox

This low profile box – which is nice and long and full width – is the king of all roof boxes, although you’ll need deep pockets to afford it. Made in Norway from a double layer of ABS plastic, it has a uniquely stiff structure, making it exceptionally robust, and the base and lid comes together without any external overlap and no external rivets, making it beautifully streamlined. It’s a rear-opening box, but the lid swings forward from the base, rather than hinging from the front and this ‘up and forward’ opening makes access a cinch. 450l capacity.

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Atera Casar L roof box: £394.95, Roofbox

This spacious, medium width and length family-sized box has a sleek gloss black finish and dual-side opening so you can mount it in the middle or at the side of your car. Another German-made box, everything about it feels and looks top quality and it boasts some innovative features including a box opening handle which lifts the roof box lid from the base smoothly and easily. The handle – which is just like a car door handle – means closing the box is easy; simply pull the lid down and once this handle is lying flat you know the box is properly closed. 420l capacity.

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Thule Force XT M: £375, Amazon

Only available in matt black aeroskin (like a golf ball dimpled finish), this 400l box certainly stands out from the crowd visually. It’s also incredibly easy to mount with and is extra wide to fit around all types of roof bar. Thule’s newly designed “lockknob” system means that it’s more comfortable to open and close than previous models and that it’s almost impossible to damage to the key (with most roof boxes, the key can be strained if the box hasn’t been closed correctly, which can make the user force the lock and result in breaking the key in the lock). We also like the dualside opening for easier mounting and access.

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Hapro Traxer 4.6 roof box: £279.95, Roofbox

This modern looking, short- and full-width dual-side opening box includes great features such as easy to use and fast to fit MasterFit bar fittings. It has passed all the usual crash tests with flying colours and, as with many other roof boxes here, there’s peace of mind from the fact that you can’t remove the key unless it’s properly shut. At 97cm wide, there is unlikely to be enough roof-bar space for a bike carrier but capacity is generous at 370l and there are internal luggage straps to keep everything secure, such as golf clubs which, by the way, fit very nicely indeed.

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The verdict: Car roof boxes

It’s hard to fault the KAMEI 510, a full width box that will make all the difference to car journeys for a family of four or five, with space on the roof for a bike carrier as well. It’s well-priced, does everything you’d want a box to do, and does it very well.

IndyBest product reviews are unbiased, independent advice you can trust. On some occasions, we earn revenue if you click the links and buy the products, but we never allow this to bias our coverage. The reviews are compiled through a mix of expert opinion and real-world testing.