From Clark Griswold in National Lampoon’s Vacation to your father swearing under his breath as he tied a spider’s web to the top of the family station wagon, generations of men have asked, How much weight can I put on my car roof? But in the rush to assemble the family, strap items into a cargo box, and pull out of the driveway on schedule, many are more likely to research bridge clearances than running a quick Google search asking, “How much weight can roof racks hold?” Car roof weight capacity if often overlooked or assumed, and this can lead to disastrous (and dangerous) results. If you’ve ever wondered — or if you’ve got your own end-of-summer vacation quickly approaching with more gear than interior storage — figuring out how much weight a car can carry is critical. (And no, just because it all fits in a cargo box isn’t a foolproof rule of thumb.) So here it is, the ultimate guide to answer an age-old question: How much weight can a car roof hold? The answer might surprise you.

Bad writing aside, the answer, of course, depends on your vehicle, whether you have a roof rack or carrier, and, from a legal standpoint, what the laws are in your state, or those states you’re driving through. According to the National Highway Traffic Safety Administration (NHTSA), there were 51,000 crashes, 10,000 injuries, and 440 known fatalities in 2010 as a result of debris flying off cars. The AAA Foundation for Traffic Safety turned up similar findings in a 2016 study, noting that “about two-thirds of debris-related crashes are the result of items falling from a vehicle due to improper maintenance and unsecured loads.” Granted, it’s not exactly incontrovertible proof that too few men are asking, “How much weight can I put on my car roof?” But it certainly suggests it. That report also identified 200,000 crashes, 39,000 injuries, and more than 500 perished between 2011 and 2014 as a result of road debris.

To identify how much weight you can safely load on top of your car, you must first pay attention to your car’s dynamics, as well as the regulations that govern your state. Here’s what you need to know about safely lashing stuff to the roof of your car.

Understand Your Car’s Dynamic Rating

The first consideration all motorists must take into account is what kind of car they’re driving. As you might imagine, the archetype of vehicle has a specific range of weights common with in the category. An SUV? You’ll hold more. A two-seater coupe? Likely much less.

Once you’ve found your specific car’s requirements, make sure you look for two numbers: a dynamic weight limit and a static weight limit. Dynamic capacity is the most important, as it’s the magic number for how much the roof can hold when the car is driving down the road. Hint: It’s not nearly as much as you’d think. On average, most cars, trucks, and SUVs can carry only about 165 pounds. Smaller cars have even less. And despite broad generalities regarding capacity, there are plenty of exceptions: A 2017 Toyota Rav4, a four-wheel-drive SUV you’d expect to be able to haul all kinds of crazy gear, is only rated to 100 pounds.

“Each vehicle will differ,” says AAA’s Tamra Johnson, “which is why drivers should consult their vehicle’s driver manual to determine weight restrictions.”

“Most vehicles are pretty limited; a couple hundred pounds max,” adds automotive writer Aaron Miller, who’s quick to point out that you also have to take into consideration the weight and shape of the cargo. “A kayak is light and aerodynamic. A mattress isn’t. Air can catch the mattress and cause lift, making the vehicle very unstable.”

The static rating, by the way, is how much the car’s roof can hold when the vehicle is standing still. It’s generally much greater than the dynamic rating because: a) the cargo weight is dispersed over the entire frame of the vehicle; and b) car roofs are by design built pretty tough (they do, after all, have to protect passengers in the case of a rollover).

But unless you’re planning to install a rooftop tent, you really don’t need to worry about your vehicle’s static rating. When it comes to hauling gear, the dynamic capacity is the figure you need to know ⏤ so once again, don’t forget to check that manual.

Your Roof Rack or Cargo Box

A roof rack obviously helps spread the weight across the car’s frame, as opposed to directly on the roof, but it doesn’t mean you can carry more stuff when the car is in motion. In fact, just the opposite. If you’re using a roof rack or cargo carrier, you need to subtract its weight from your total rating. For example, if your car roof is rated to 165 pounds, and your roof rack weighs 30 pounds, you can really only load 135 pounds of camping gear on top. The same is true of any crossbars you’ve installed to help fit your cargo box ⏤ they count too. Thule, the popular Swedish purveyor of all kinds of car and bike racks, puts this simple equation on its website: “Maximum roof load = load rack weight + any fitted rack accessories + the weight of the load itself.”

What’s equally important is to never exceed the weight limit of the weakest link in your roof-rack system. For example, “The maximum capacity of your roof-rack will be dependent on the lowest-rated part of the system,” writes etrailer, an online retailer of custom-fit towing accessories. “So, if you have a roof-mounted cargo box that has a weight capacity of 150 lbs., roof-rack crossbars that are rated at 200 lbs., and these are on a roof rated at 100 lbs., then you cannot load more than 100 lbs.”

This means you can buy the biggest cargo basket on the market, but if you’re thinking it will increase your roof’s weight capacity, you might as well be lighting your money on fire. The same is true of that sweet rooftop tent and its Instagram photos. If your car’s top is rated for 165 pounds and your tent along is pushing 100, it’s unlikely you’ll be able to do more than set it up and look at it.

Know the Law

When it comes to the law, the question isn’t, “How much stuff have you tied to the roof?” but, “What are the odds of it flying off?” Only 15 states have statutes regarding improperly or unsecured loads, while all 50 states have laws making it illegal to drop things from your vehicle.

“Currently every state has laws that make it illegal for items to fall from a vehicle while on the road,” says Johnson, “Most states’ penalties result in fines ranging from $10-$5,000, with at least 16 states include possible jail time as punishment.”

So, yes, a police officer who notices you driving a Smart Car with a grand piano strapped to the top probably will pull you over. But assuming it’s properly tied down and doesn’t slide off as they approach your driver’s side window, you’re not likely breaking any laws. However, if you’re driving through Montana and that same piano smashes in the passing lane, get ready for up to a year behind bars.

Which is why it’s important to check the road debris laws in your state, and any that you’ll be driving through, before you start lashing old furniture onto the roof, if only to know what you’re in for if you screw up.

Learn How To Tie A Damn Knot

Or, at least how to take the proper precautions when securing cargo to your car. According to AAA, you should make sure that everything is properly “tied down with rope, netting, or straps, and that larger objects are secured directly to the vehicle.” They also suggest covering the entire load with a tarp to help secure items from possibly flying off while in motion.

Not sure if your granny knot or bunnies’ ears will securely hold? Let us be the first to say that there’s no f—ing way. Learning proper knots doesn’t require a return to Boy Scouts or the Naval Academy; rather, you can easily find a few on YouTube that, with practice, will become second nature. Our advice? Use the “Truckers’ Hitch Tie,” which will quickly and securely lash your cargo to you roof with ease.

If you’re using a cargo carrier, make sure you distribute the weight across the box. Heavier items should go inside the car, not on top. “From a safety standpoint, you’d want the cargo as low as possible,” Miller says. “Keep the free weights inside the vehicle, pack the clothing on top.” Thule also notes: “With a load on the roof, the vehicle’s driving characteristics, its braking performance and its sensitivity to side-winds may change.” The reason is simple: With heavier items on top, your center of gravity is higher than normal, making turns more dangerous. It also exposes your rack and roof the greater stress and potential damage.

And finally, regardless of whether you’re helping a buddy move a mattress or are taking your collection of rare porcelain ballerina figurines to the park, check your cargo before you pull away. Then check it again. Tug on the knots and ensure your cargo box is locked. And if you want to play it safe, do as Thule suggests: “Check it after a short distance, and thereafter at suitable intervals.”

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This article was originally published on Aug. 18, 2017

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