Typically the roof load figure provided by manufacturers refers to the dynamic load, namely the maximum weight that can be carried on the roof while the vehicle is in motion. This is restricted to ensure that the vehicle remains safe when turning, accelerating or braking, as an excessive load could cause the vehicle to become unstable. The maximum static roof load is normally several times this amount, as it is dependent on the strength of the roof itself - which must be sufficient to support the entire weight of the vehicle in the event of a rollover - and the roof bars themselves, so as long as you choose roof rails and a tent box that are designed specifically for your vehicle you should have no issues.