
Anyways, back to the topic at hand... zombie apocalypse

. Your best bet if you're sticking with a roof rack, particularly a large one, is to put the lightest, bulkiest items on top (think blankets, bedding, freeze-dried food, clothing, etc) sealed from the elements in bags or by tarp. You would also want to put any gasoline outside the vehicle in case of fire.
As has been said earlier, a cargo tray on the rear can allow you to store more stuff and make room for the 5 people you're considering. Even better would be a trailer (like what Teebes has). Either the tray or trailer could be removed if no longer necessary or practical and abandoned if necessary.
Key to consuming as little space as possible is versatility. You want most everything in your vehicle to have at least 3 functions. Remember that blankets can be shelters, storage bags, and in the most exceptional of cases, fuel for a fire. Tarps can serve as rainwater collection devices, hammocks, and more. Tools should be multifunctional, but not flimsy like Leathermans and the like.
Communications should range from simple (ability to produce fire and smoke) to complex (SPOT, CB, FRS, or SatPhone).
And don't forget your vehicle as a tool as well... it can serve as a heater (provided you have sufficient fuel and route exhaust out), oven and grill, generator, mode of transportation, shelter, pack mule, water heater, and the list goes on.
Simplicity is key. For every complex system you have, you need replacement parts, maintenance items, and tools for repair. Make sure that you attach nearly everything to your vehicle with the same slotted hex head screws (dual driving options). Pack replacements for non-repairable items and electronics, and repair kits and tools for repairable items.
As far as weapons, you want the most durable, efficient weapons you can find. Guns are great for defense, but ammo is always a concern, and they are complex items. Much better items include compound bows or crossbows, machetes, hatchets and axes

, knives, and the multipurpose GI shovel. These items can be reused again and again with a minimum of maintenance.
On top of everything, you just need to remember to provide for the basic human needs for however many people you have. You will need a livable temperature, shelter, food, water, protection, and basic medical facilities. Your goal should be to be as self-sustaining as possible while balancing that capacity with simplicity and ease of maintenance, setup, and takedown. Anything you need to obtain to maintain should be relatively easy to get or at least to produce.
This is, of course, a long-term view of survival. Short-term situations allow you much more flexibility in what you do and don't need.
Hope this is enlightening and useful for some,
-Phil
