Good analysis. To fully nerd you should have used a D20 though.
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Lol, that would be a lot closer to statistically valid probably!
I have the same rack system for two boats on a VW sport wagon. In addition to roof support hold downs I like to have a tie downs at the front and rear. I haven’t had any trouble at any speed and consider myself cautious
Someone was killed a few days ago when someone else’s rack came off the car and a kayak went through his windshield. If your towers and crossbars attach to the factory rack there’s nothing to prevent the factory rack from pulling right out of the car’s roof. Bow and stern lines should be used and will help with the side forces which can be greater than the wind from the front. Think of how your car gets pushed when a truck passes it and how much more surface area of the boat is exposed to side wind. Bow and stern lines give a bit more safety and and indicator if something may be loosening up.
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You bet. I don’t go anywhere without bow and stern lines.
(I can’t remember, but I may have posted an image earlier without stern lines, but those boats were completely untied and ready for removal; I snapped the pic to show a friend that they fit up there side by side 
Factory roof rails shouldn’t pull out under stress. I know how the rails bolt to structure under the roof sheet metal in my German SUV; I expect that some vehicles aren’t built quite as robustly. In fact, after market roof rails are available for my SUV that use not much more than sheet metal screws and spit. A used vehicle purchaser may not know the difference.
My mom’s car was hit by a poly kayak that got loose from a pickup truck on Suicide Alley (Route 6) on Cape Cod. Fortunately it hit the hood of her Volvo and bounced off without causing any significant damage. She was very lucky!
Maybe that’s why I don’t like poly kayaks!
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Perhaps I’m missing something here. So far no one has mentioned the difference between how much weight a RACK can carry and how much weight the ROOF can support. Those are two different things. My understanding is that the weak point is the roof itself, not the bars. Maybe PaddleDog’s quote above alludes to this. Link: How Much Weight Can Your Car Carry? - Forbes Wheels
Not roof exactly but the security of the mount into the structural component that lies front to back along the outside edge of a car roof. That will take a lot of weight, but how things are bolted into it may not be as robust.
Most car manuals will give you the weight capacity of your roof, as will third party rack manufacturers. For the car they may list a static load limit as well as a dynamic load limit. The static load limit is when the car is standing still and generally is only relevant for things like roof mount tents. The dynamic weight limit is for when the car is moving, like carrying a boat. The dynamic weight limit has a safety margin, but assumes a balanced well distributed load. Wind and rough roads can further reduce the safe load limit and common sense must prevail. Auto manuals may have further information depending on how the load is carried. Loads carried on roof rails generally have a higher load limit than loads carried directly on a bare roof.
Add-on rack systems usually will have a higher load limit than the car roof. In this case the lower weight should be followed unless there is an exception in your car manual.
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The discussion about the car roof gets increasingly confounding as more manufacturers go to flush rails with holes set up to take third party racks. Talking about the roof without specifying the specific structural elements is not all that helpful.
What part of the roof? I know that the middle of my car roof can flex when I climb up on the roof. People who are 5 ft 3.5 inches sometimes have to climb up there to get a rack thing fixed up, But nothing budges when my weight is over the structural rail.
We first had Sable and Taurus station wagons. For those, the Yalima rack footing mounted into a rail that ran above the frame element of the structural roof rail. There were screws or something that tightened down into the track above the structural rail.
Was this as secure as the more current systems that bolt into holes in the structural rail, like the Thule Fit Kits? I don’t know. Because the weight bearing in those rack feet on our Sable and Taurus wagons was actually on the track above the structural rail. Not in the thing itself.
Then we had a series of cars with raised rails, with depending on the vehicle potentially tremendously strong attachments to the raised rails. So now the question was how well anchored to the structural rail are the manyfacturer’s raised rails. Some vehicles were better than others.
Throughout this there have existed systems that make no reliance on the structural rail at all. They clip onto the side of the car in various ways, or onto the raised cover over the structural rails. I don’t know anything about these, having always been onto or into the structural rail.
Personally I miss rain gutters. They were created from the weld between the roof and side panel sheet metal. At least one was folded over so they were 3 layers thick, stuck up from the rest of metal, and bent so nothing could slip off. They were probably the strongest feature of the car body. You could get $20 roof racks at Kmart that clamped onto those gutters and they held just fine.
If you can work with wood, here’s a $50 rack:
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My roof rack on the Soul is also DIY and the soul came with just a plain room with no rails of any kind. What I found were 4 plastic covers that snapped out and under the covers 6mm threaded holes. The holes are part of a bent clip that has a threaded insert as part of it and it is then welded into the roof structure. Once I figured out the thread size thru some internet research I screwed some bolts in to get a feeling for how strong they were. I was impressed they seemed pretty substantial so I used them as my anchor points.
I think the holes are there if you wanted to add factory rails but I didn’t see the point in adding another layer of complexity to it. I made 4 tapered riser blocks from PT 2x4 and then attached a PT 2x4x48 as a cross bar times 2 and held down with long 6mm stainless bolts into a counter bored hole with large washers so everything is flush on the top. For a single canoe that all I needed and for a double I made 2x4x84 extensions that bolt to the cross bars with 5/16 bolts.
The bars that stay on full time I painted black to match the car and they run very quiet. The extensions I didn’t paint except the ends are blaze orange as a warning as they are wider than the car.
I run bow and stern lines and double cam straps on each boat and feel very safe with this DIY setup compared to some of the rigging I see people here using. I’m likely about 170 pounds on the roof on what I would call a very light small car. I keep my speeds very reasonable and haven’t noticed any side wind issues. But then again have driven in mostly fair weather avoiding major highways.
Our other car is a Kia Sportage and it has raised rails and after market cross bars that clamp on I got off line. She uses it for just her 40# rec-kayak inverted and 2 cam straps and bow and stern lines when going out without me.
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I see many posted use bow and stern lines. If you really want to secure it I suggest two lines for bow and stern so 4 lines in total for each boat. Yes a lot of lines BUT I think its worth it if you worried about a complete failure of base rack. I have added a picture But I didn’t have any with the lines I use so i drew in two lines to show what I mean. This way a heavy wind from either side direction is countered buy a line. With only one line it cant do that. Plus if the base fails with two lines it will hold the boat in place much better . One line say the right one boat falls over and smashes passenger side of car.
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I agree. With my setup and two good size canoes my preference would be a single bow and stern line when I’m driving locally. That is 95% of what I do and distance is under 10 miles and speed is under 40MPH. If I was to take any kind of highway trip with speeds up around 60-65 MPH I would make the extra effort and go with the double bow and stern for each boat. I have enough cam straps to do that. 
Don’t absolutely tneed wo lines to handle cross winds, though for a long trip two is better than one. For my canoe which I swear will see more water this spring than it has this season I use one with a knot in the middle where it ties to something inside. Then the bow line runs from one of my bow loops to the one on the other side.
not safe.
Having a boat come off your vehicle is really dangerous.
Deadly as we have seen. See how a lawyer will dissect you in court if being sued if overloaded.