Why it's important not to carry too much weight on your roof

If you’re struggling to find enough space to squeeze all of your cargo into an SUV, dual cab ute or commercial van, a roof rack can solve a lot of problems.

In the same way a home owner will add an extra storey when they need more space, the roof of your vehicle offers a similar solution.

However, just because it’s long, wide and fairly flat doesn’t mean that you can load it up in the same way you might load a vehicle’s designated cargo area. While the roof may offer ample space, that’s not the same as offering ample load, because it doesn't have the same weight-bearing strength as the vehicle beneath it.

Fact is, overloading your roof is dangerous and illegal. It can also provide a valid reason for your insurance company to reduce or reject a claim in the event of any damage caused. So, here’s some practical tips to ensure these dramas can be avoided.

Roof weight rating

All vehicles have a maximum roof-load rating expressed in kilograms and these ratings can be found in the owner’s manual, sales brochures, specification sheets published online or by contacting the manufacturer.

Most SUVs, dual cab utes and vans can legally carry up to 100kg on their roofs, but some have higher ratings. Whatever this figure, the key thing to remember is that it’s a ‘dynamic’ rating, or how much load the roof is designed to carry when the vehicle is being driven.

For example, a roof with a load rating of 100kg can withstand higher loadings when stationary. So, if you have a platform-style roof rack for example and you need to walk around on it when loading and unloading, or use it as a temporary work/viewing platform, or perhaps sleep in a roof-top camper, that is usually within the roof’s stationary or ‘static’ load-bearing capacity.

Vehicles will have a ‘static’ load-bearing capacity.

By comparison, roof loads subjected to gravitational forces when cornering, accelerating and braking during road use can result in higher loadings and therefore place greater strain on roofs and roof racks. This can lead to structural failures if the roof load limit is exceeded.

4x4 vehicles usually have separate dynamic roof load ratings for on-road and off-road use. That’s because the rigours of off-roading inflict even higher stresses on chassis and body structures, particularly the pillars supporting the roof, which is why the off-road load rating can be as little as half that of the on-road rating.

Roof rack weight rating

Roof racks, be they cross-bars or platform-type, also have maximum load ratings as determined by their manufacturers. If you don’t pay attention to these numbers, you could end up with a roof rack that’s not going to fulfil your needs.

For example, if your vehicle has a roof load rating of 100kg and you fit a roof rack designed to carry only 60kg, then the rack becomes the determining factor in how much load you can carry on your roof.

However, that could still exceed your 100kg roof load rating if the rack weighs more than 40kg and you load it with 60kg, because roof load is the combined weight of the rack and its cargo load. So, knowing the unladen weight of your rack is equally important in these calculations.

Roof load is the combined weight of the rack and its cargo load.

Centre of gravity

A critical factor affecting your vehicle’s handling is the location of its centre of gravity (CG). This is where the vehicle’s weight or mass is centred. You can't see the CG. It's a point in space between the front and rear wheels and its height is determined by a number of factors.

The CG is where centrifugal force acts on your vehicle when cornering. The higher the CG, the more leverage it can exert, resulting in more weight transfer from the inside tyres to the outside tyres. This top-heavy effect creates more body roll and raises the risk of a vehicle tipping over. It also increases weight transfer from front to rear when accelerating and rear to front when braking, making it more unstable all round.

An overloaded roof can make your vehicle dangerously unsteady on the road.

Therefore, an overloaded roof’s CG can not only make your vehicle dangerously unsteady on the road but also when negotiating rugged off-road terrain, because any sudden sideways tilt can create enough of a ‘pendulum’ effect at roof height to topple a vehicle on to its side.

In this context, pay particular attention to the weight of your roof rack. Quality aluminium racks can be just as robust as steel equivalents yet weigh far less, resulting in a lower CG and allowing more equipment to be carried upstairs within the legal limits.

Conclusion

Overloading your vehicle’s roof can lead to serious legal, performance and safety issues. So, always try to minimise the amount of weight carried upstairs and ensure that roof and roof rack load ratings are never exceeded.