Question mostly in the title. Looking to transport a 48" square folding table on my roof rack with crossbeams a total of about 400 miles, ideally at 70 MPH (speed limit most of the drive). I’ve got four ratcheting tiedowns each rated to 1,000 pounds work load, 50 feet of 3/8" rope, and a tarp for what’s basically 50 pounds of half-inch plywood. As far as I can tell, it’ll comfortably sit on the back of the rack two or three feet from the windshield. That said, I’m obviously still worried about it generating lift, especially at highway speed (and assuming it’s not also yet another windy day). Should I just be cautious and give up on this idea, or am I okay?
The ratcheting ties-downs may be rated to 1,000 lbf (tensile strength) but what about the rack you are tying down to?
Stranger
Would propping up the back a bit help generate some downforce?
About 150 pounds. Should be comfortable for a 50 pound load.
Well, that is the rated load for the weight is bearing down on the roof but how is it attached, and how much upward force can it take? OEM racks usually have some rails with structure built into the roof that distributes the load but a lot of aftermarket racks either screw into the roof or have clips that snug into the window sills. I’ve seen a number of those fail when someone puts a lightweight flat load on a roof rack. On a lightweight car you can also get enough lift at highways speeds to significantly reduce controllability of the vehicle. I’m generally not a fan of putting anything on a roof rack that isn’t heavy enough that it won’t lift away on its own or at least compact enough to not produce a lot of upforce (e.g. duffle bags). This table is basically a big wing and at even at 1/2 PSI of lift it is almost 600 lbf of force (assuming that your 48” table is folded in half).
Maybe; it would certainly be an negative angle of attack but if you get enough airflow under the table it could still generate a lot of upforce. Depending on the weight and mass distribution of the vehicle producing a lot of downforce could also cause controllability issues, especially if there are any crosswinds.
Stranger
The rack is OEM installed by Subaru, and the table really is basically just a 48x48x0.5 inch piece of wood. Doesn’t fold down any further, or I’d put it in the car. You can see why I keep worrying about a trip like this, as opposed to just driving it across town. I’d rather find a new solution than cause an accident or wreck my roof.
I’d be okay driving at surface streets with a load like that. At highway speeds I’d be concerned but then I don’t even transport stuff in the bed of my truck unless it is secured to an anchor or covered with a cargo net because I’m paranoid from an accident I witnessed (and narrowly avoided) when I just started driving. YMMV.
Stranger
I’ve driven many miles with kayaks and canoes atop various vehicles. I’m way more comfortable going 60 than 70.
The main concern is the connection between the rack and the car. When I drove trucks it was not uncommon to see a rack with luggage or golf clubs etc attached laying on the hard shoulder.
I used to run the straps through the car and close the doors on them. They don’t need to be super-tight, but they will give you peace of mind.
Is there no way to slide it into the back seat?
50 lbs. sounds heavy for the table you describe. Why won’t it slide into the back seat of your car? That may not make any difference here. How did you end up being in the position of transporting a folding table 400 miles by car? I don’t know what kind of special table this is but you can have Amazon deliver one to your destination for the about the same cost as the extra money you’ll spend on gas with that one tied to the top of your car.
Just as a visualization edge, I’m guess the OP’s table is something like this?
4x4, 1/2" wood, only the legs fold in, 43lbs, $141 with free Prime shipping (if that matters).
To be sure, I would use at least one cargo strap that crosses over the table down the side of the car and hooks under the car somewhere. Then a couple to attach to the roof racks. Might have some issue with the thing fluttering at the roof rack bars. So ratchet it tight at those points. The across the middle one will be to take the overall lift forces that might happen. Might trap your rear doors closed.
That’s the exact table, it’s actually 400 miles round trip, and it’s because I was looking to drive three people and a table to a small tabletop wargaming tournament where the proper table size is exactly 48x48 and was told if I could manage to bring it that’d be helpful. As it is, no car I have available can fit both it and a person in the back seat, and instead of risking the possibility of a catastrophe it’s not coming along.
Strapping the person to the roof will have a much lesser chance of being blown off by the wind.
Be sure to use those ratchet straps. Stop frequently in the first few miles to check your load, and retighten as necessary. For concern about generating lift, put something that blocks the front gap (between table and the car’s roof) and secure that to the table. A rolled up blanket can work well for this.
You should be fine. I’ve carried lots of different loads on my roof at highway speeds and up to 80 mph, 90 mph. You should be fine.
If you’re nervous drive on the freeway at 50 mph to start with. Stop after a few miles to check your load. Increase speed and stop to check every so often.
You should be fine.
It is important to note — and I’ll try to describe this adequately — that you need to be careful with how and where you attach the hooks for those ratchet straps. For example you do not want the tension of any given strap to be pulling on the rails of the roof rack laterally — or perpendicularly to the direction of the rack’s crossbars. Doing so could compromise the strength of your rack.
When you apply tension to the ratchet strap (also called a come-along) it can put severe pressure along the length of the strap. You don’t want that tension to be pulling your rack’s crossbars laterally, or sideways. You could rip the rack off of the roof.
You instead want the tension of the ratchet straps to be pulling downward towards the roof of the car, and also such that the tension applies a “squeezing force” on your load and the roof rack bars. Think of your hand holding a sandwich made with sliced bread. This “squeezing force” would be as if you were holding that sandwich with one hand where your four fingers are on top of the top slice of bread, and your thumb is below the bottom slice of bread, and you squeeze your fingers and thumb together to squeeze into the bread and into the sandwich.
For your roof rack load this is a good thing. This is what you want.
Again you should be fine. It can make you nervous the first few times you do it but once you get the hang of doing it, your confidence and skills will increase.
I have had Yakima racks on my cars since 1995. I’ve carried some sick loads for long distances at Highway speeds.
The ratchet straps are great. Don’t even bother with the rope to provide the binding strength to secure your load. The rope can be used to wrap your table with a tarp.
Ratchet straps are great for securing loads. I always keep a few in the trunk.
You should be fine.
ETA — sorry this is so long!
Reminds me of English musician James Blunt.
Did you really have your guitar strapped to the side of your tank when you went into Kosovo (5)?
Yes. I wanted to put the guitar in the safety of the tank and strap the soldiers to the outside, but my superior officer said I was not allowed.
Can you lower the rear windows a couple of centimeters and wrap a long strap all the way around the roof, through the passenger cabin? This is definitely a bodge, and it makes it impossible to open the rear doors so if you have anyone back there they’d need to get in first, but it would distribute a fair amount of the potential upward force away from the rack rails.
This does not at all address the concern about lift reducing vehicle control, of course, but the suggestion to jam a blanket or something underneath at the front to interrupt air flow will help a lot with that.
I have zero experience with roof racks, but this was my first guess , too.
Along with the suggestion to slope the table by propping up the back end.
Maybe instead of a rolled-up blanket, you could build a pointy shaped nose piece?. Take a piece of thin aluminum or sheet metal ,about 10 inches wide and 48 inches long. Fold it along its length to make a V shape, and put it on the front leading edge of the table.
I dunno-- I’m just guessin’ that the laws of physics should be applicable here. 
My dad used a roof rack and never had any problems. Like the OP, he covered it with a supplied canvas cover. I remember a few things getting wet when we drove through heavy rain.
I vaguely remember a box that my dad used to carry smaller items.I can’t recall much about it. It was probably aluminum and light weight.
Today a roof rack box would probably be rigid plastic.
OP, I’d rent this trailer for the trip. It’s an open trailer. You can still wrap your table in a canvas tarp. Ratchet strap it securely to the trailer.