If you have stuff on the roof, you either have the wrong car, too much stuff, or are not solving the space problems correctly.
Lets look at this in detail:
1. Buy the correct vehicle: A Defender 90 is probably not the correct vehicle for a family of four on a six month trip. Buy a bigger vehicle, like a 110, or a TLC 78, etc. If the D90 is all you have, and cannot afford to replace it, then understand it will be a compromise in safety and performance. If possible, use a trailer
before a heavy roof load. Once the roof load exceeds about 120lbs. on a narrow-track vehicle, go with the trailer. Never use both a heavy roof load AND a trailer on technical terrain, and even on mild terrain, do not exceed GCVWR.
2. Think lightweight: Stay under GVWR. With some vehicles, just adding the trail accessories and passengers puts them over GVWR. Go with smaller, compact camping gear if you are driving a smaller vehicle. If it is two people in a Suzuki Samurai, you must pack like a backpacker. If two people can go around the world on a KLR650, it is certainly possible to reduce or eliminate a roof load in a Land Rover Discovery with two people.
3. Solve the Space Problems: Get the weight down low and forward if possible. Use frame-mounted tanks for water and fuel if possible. Remove the rear seats if possible (or necessary), which is a weight and space saver. I often see a family of four in an SUV, with the roof loaded to the gills, but nothing in the rear passenger footwells, their kids feet dangling over precious unused and ideally located space. Put the water down there, tools and other heavy items, then make sure they are properly lashed in place to protect the occupants. On my JKs, I am able to stow all tools and most of the heavy recovery items under the driver and passenger seats.
Of course there are times when a roof load is unavoidable. If you do run a roof load, do so with caution, and make sure everything possible has been done to get the weight down (like noted above). Roof loads are easier managed with lower, wider vehicles. Land Rovers are neither. In the Camel Trophy, they flopped those trucks at alarming rates, it just never made the highlight reel (I have talked with the directors of the event over beer - oh the stories). There are also times when you are traveling on known routes on paved or easy roads and traveling at low/conservative speeds. In those cases, heavier roof load risks are more easily managed or mitigated. My strong opinion on roof loads is because I search out the most technical, unknown, abandoned, eroded, exposed, dune filled tracks I can find in developing countries. Do a few vehicle rollover recoveries in the Sahara and it will shape your opinions on roof loads. . .
This:
Or This:
Remember, anything on the roof is a compromise to safety and performance. Sometimes, that compromise is unavoidable, but you should do it with eyes wide open to the potential risk.