what part of "Depending on the roof line" dont you get. Also you need to read u p on what i have been saying. im sure i have put over the max load i should have on them, they are beefy. 800 lbs on any rack can easily snap the bolts right off if you were to have a collision.
You could probably put over 1,000 lbs on each point that they mount to the van, but the moment you hit the brakes HARD or bump into the rear end of some other car, any stopping force would be enough to shear the sheet metal off whatever its on.
I have been hauling loads on the top of my van, escape, my 10 foot, and my 25 foot trailer. i have been though a few road hauling safety training courses mandated by a previous employer. I know what im doing when it comes to hauling and making sure a load is as secure as the driver. I am telling you that if i put 850 lbs on the top of that van or ANY rack that was not DESIGNED to carry that in an impact that you are going to kill someone when your load comes flying off when you slam into a car in front of you, or have enough braking and traction to stop fast enough. those 40 lbs of 2x4x16, was only a few of them and the ones in the center FLEW OUT of the stack. i had 3x 3" ratchet straps holding them down properly. ones at the front and back go over and around the bundle compressing it. Roof racks can only get so tight on the straps before they bend or damage the roof. they were plenty secure. there was about 50 pictures that was taken as i have commercial auto and it must be well documented. i was cleared as having them secured in "overkill" as described by the adjuster. Still it flew out from the center and right into and though the back glass of the jerks car in front of me.
its not about weakness, its about a rack that can stop a load from shifting or ripping its self off the car.
The ones i have also are a tube in a tube design and they flex ALLOT.