In your question you asked about centre of gravity. Here is a link that demonstrates the concept
BBC Forensic Engineering - Solve the Riddle - Centre of Gravity
If tipping of the vehicle results in the centre of gravity stepping outside its contact area i.e. the rectangle made by four tyre contact points it will tip over.
When considering centre of gravity in the context of loading a vehicle you will also need to account for moving/movable items. So, if you start to approach the tipping point and any of your load moves in the direction of lean you will suddenly get nearer to tipping over.
Fluids stored on a vehicle (fuel, water, spare fuel) will slosh about as you travel over bumpy surfaces. If you have a couple of alternating bumps followed by an extreme angle you may find all the fluids slosh to the side you're leaning towards and suddenly shift the centre of gravity, again tipping you over. Oddly, a full tank will do this less than one say half full.
Sloshing of fluids on roll-on roll-off ferries has been enough to flip them over with disastrous results.
Also having the centre of gravity too far forward or back in the vehicle will result in odd handling under extreme circumstances. Slamming the brakes on with a rear weighted vehicle may result in it penduluming and/or spinning round (swapping ends, a tendency seen in old VW beetles etc).
Years ago, before power steering, truck drivers would place loads at the rear of their load beds to lighten the steering. Not recommended!
So ideally you need to keep the C of G low, fairly central (or slightly front weighted) and as stable (non-moving) as possible. This means fluids, spare wheels, tools etc inside the vehicle on the floor and preferably tied down. With lighter stuff on the roof.
Plenty of people get away with more on the roof but they are limiting the off-road potential of their vehicle and also potentially affecting emergency handling on-road.
As always it will depend on what you want to be able to do.
I've yet to work out how two of us can sleep on the roof with the combined weight of rack and tent without collapsing the gutters on my 110, probably need to lose some weight?