A roof box is a simple idea done badly very often. The box is only as secure as the bars under it, and the bars are only as secure as the mounting points on the car. Start with the handbook, then work upwards.
Bars first
Choose the bars before the box. The handbook gives the maximum roof load for the car, and that figure includes the bars, the box and everything inside it. A large box is no use if the car can only carry a small weight on the roof.
Check that the bars are listed for your car's mounting points: fixed points, flush rails or open rails. Check the fitment notes and compare the feet and clamps with the roof you actually have. A set that clamps to the wrong rail profile can work loose.
Size and shape
Capacity is quoted in litres, and the number is useful mainly for comparing boxes of the same shape. A long, low box suits skis and boards. A short, tall box suits bags and bulky soft items. Measure the roof and the garage or car park height before you buy, and remember the box adds height even when empty.
Think about how you will open it. A box that opens from one side only may be awkward if you park against a wall. Check the weight of the box itself as well as what it holds, and check the load limit stamped on the bars.
Fit the bars properly
Clean the roof and the mounting points first. Fit the bars to the handbook torque and check them after the first few miles, then at every fuel stop on a long trip. A bar that is not seated can move under load. If the car has a sunroof, check the handbook before fitting anything above it. Nothing here needs you under the car; if you are checking the underside or fitting a tow bar, use axle stands under the handbook points and torque every fixing to the handbook figure.
Lift with two people
A roof box is light but awkward, and a gust of wind on a driveway is enough to swing it into the paint. Lift it with two people, one at each end, and set it down on the bars rather than sliding it across the roof. Fit the mounting clamps to the bars, not to the roof, and tighten them in the order the instructions give.
Open and close the box once it is fitted. Check that the lid clears the roof and the boot, and that the key works before you load it. Load heavy items low and towards the centre, and spread the weight. Stop and check the clamps after the first few miles.
Roof box checklist
- Find the maximum roof load in the handbook.
- Check the bar type for your roof.
- Compare the bar feet with the mounting points.
- Clean the roof and the mounting points.
- Fit the bars and torque them to the handbook.
- Lift the box with two people.
- Clamp the box to the bars, not the roof.
- Check the lid, the key and the clearance.
- Load heavy items low and central.
- Recheck the clamps after a few miles.
The right combination is quiet, secure and easy to live with. The wrong one is a noise you cannot trace and a clamp you cannot trust. Take ten minutes to check the figures before you spend the money, and compare the bars with the mounting points on the car.
