How do I fit and load a roof box safely?

Fitting bars and box securely, the two weight limits that both apply, distributing the load so the car still handles, and what changes about driving.

Difficulty: intermediate
Time: 1 hr 30 min
Read: 4 min
Safety: warning

Short answer

Fit the bars exactly to the car's fitting kit, centre the box between them, and check the tailgate still opens. Load the heaviest items low and central over the bars, keep within the lower of the bar and vehicle roof limits, and check everything again after your first stop.

A roof box makes a family journey possible, and it also raises the car's centre of gravity, adds drag and puts a large weight above the roof skin. All three of those are manageable if the load is right.

Safety

Two separate limits apply and you must respect the lower: the roof bar manufacturer's load limit, and the vehicle manufacturer's maximum roof load stated in the handbook, which is often only 50 to 75kg and includes the weight of the bars and the empty box. An overloaded or badly secured roof load changes how the car handles, and a box that comes off at speed is lethal to whoever is behind. Check the fixings before every journey and again after the first stop.

What you'll need

Roof bars with the correct fitting kit for your car
The roof box
The vehicle handbook
A torque wrench if the kit specifies torque figures (optional)

Loading a roof box so the car still handles

Step by step

Find your car's maximum roof load.It is in the handbook, and it includes the bars and the empty box, not just what you put in. Many cars allow only 50 to 75kg in total, which is far less than the box's own capacity.
Use the correct fitting kit for your exact model.Roof bars are car-specific. The wrong kit either will not clamp properly or damages the roof and the seals. Manufacturers publish fitment guides by model and year.
Fit the bars to the specified positions and torque.Many kits state a bar spacing and a clamping torque. Both matter — bars too close together let the box pivot, and over-tightened clamps deform the roof.
Position the box centrally between the bars.Not overhanging one bar. Roughly centred fore and aft between the axles keeps the added weight where it affects handling least.
Check the tailgate and the sunroof still open.Slide the box forwards or back until the boot opens fully. Finding out at a service station with everything packed is a common and avoidable annoyance.
Check it does not obstruct anything.The box must not block the driver's view, cover lights, or overhang the windscreen enough to affect airflow to the wipers.
Load the heaviest items low and in the middle.Over the bars, between the axles, lying flat. Weight high or at one end raises the centre of gravity and makes the car far more sensitive to crosswinds and sudden steering.
Put light and bulky items at the ends.Sleeping bags, coats, empty bags. Soft items also stop everything else sliding about.
Stop things moving inside the box.Straps, or simply packing it full. A load that shifts under braking changes the balance of the car at exactly the wrong moment.
Keep heavy items inside the car instead.The roof is the worst place for weight. Anything dense that fits in the boot should go in the boot, and the roof box should take the bulky light things.
Check every fixing before setting off, and again after the first stop.Bar clamps, box mountings and the lid catches. Vibration and load settling mean the first hour is when things loosen.
Adjust how you drive.Allow for extra height at car parks, barriers and drive-throughs, expect more crosswind sensitivity and longer braking, and accept the fuel consumption penalty. Remove the box when you are not using it.

Tips

Write the car's total height with the box fitted on a note taped near the driver's line of sight. Car park barriers are unforgiving.
A roof box costs a noticeable amount in fuel. Taking it off between trips pays for itself over a summer.
Lock the box and the bars. Both are attractive to thieves and both are quick to remove without a lock.

Common mistakes

Loading to the box's capacity rather than the car's roof limit — The box may hold far more than the car's roof is rated for. The vehicle handbook figure includes the bars and the box itself, and exceeding it stresses the roof and the handling.
Putting the heaviest bags on the roof to free up boot space — It raises the centre of gravity exactly where it does most harm. Dense heavy items belong low in the car; the roof is for bulky light things.

If it doesn't work

Whistling or roaring at speed

Cause: Airflow over the bars or a badly positioned box — Fix: Fit the wind deflector strip supplied with most bars, and check the box is square and fully seated.

Car wanders in crosswinds

Cause: Weight too high or too far back — Fix: Repack with the heavy items low and centred over the bars, and move dense items into the car.

Boot will not open with the box fitted

Cause: Box positioned too far back — Fix: Slide the box forward on the bars until the tailgate clears, then re-check the bar spacing is still within the kit's specification.

Box moves on the bars

Cause: Mountings not tight, or the wrong bar profile — Fix: Retighten to the specified torque and confirm the box's clamps suit the bar shape. Do not drive with a box that can shift.

Questions people ask

How much can I put on my car's roof?

Check the handbook for the vehicle's maximum roof load — often 50 to 75kg — and the bar manufacturer's limit, then use whichever is lower. Both figures include the bars and the empty box.

Does a roof box affect fuel consumption?

Noticeably, especially at motorway speeds where drag dominates. Removing the box and often the bars between trips is worth doing.

Can I use a roof box on a car with a panoramic roof?

Often yes, but the fitting kit and the roof load limit are what decide it, and glass roofs sometimes have lower limits. Check the handbook for your specific car rather than assuming.