outbackjoe

a camping trip of ridiculous proportions

Roof Loading and Roof Top Tents

last updated 28/09/2016

WARNING: Never exceed manufacturer’s specifications. Doing so could cause injury or death.

This article is concerned with whether it’s safe to be using a roof top tent on a vehicle. Is it safe just with the weight of the tent and supporting racks? What about when there’s people in the tent?

Putting stuff on the roof raises the vehicle’s center of gravity and changes how the car handles. With load on the roof, it means the car will sway sideways more when under the effects of cornering, driving across a slope or driving over uneven terrain. This impairs road handling and increases the risk of rollover. Manufacturers specify a maximum roof load to maintain handling characteristics and ensure the increased rollover risk is kept within reasonable limits. Also taken into account by the manufacturer’s maximum load specification is dynamic loading – how much force will the roof have to support when driving over bumps, corrugations, etc. If you put more than the specified maximum roof loading onto your car and drive, you will experience poor handling, increased rollover risk and increased roof damage risk, more so than allowed by the vehicle’s design criteria.

The manufacturer’s specified maximum roof load is for dynamic conditions – whilst driving the car. It takes into account vehicle handling, rollover risk and dynamic loading. What about loading the roof of the car whilst it’s not moving, as in the case of sleeping in a roof top tent? I have not seen any manufacturer specify a static roof load capacity. It’s a fairly niche parameter and is irrelevant to most people and could mislead some into overloading their roof under dynamic conditions. So it’s up to us to decide whether it’s safe to put a couple of hundred kilos of people and tent on the roof of a parked vehicle. Lets investigate so we can come to some reasonable conclusions.

Road handling is meaningless when not driving. So for a car that is not moving, we can disregard road handling. Rollover is also irrelevant whilst not moving. Loading up the roof of a parked car is not going to make it spontaneously roll onto its side. So rollover risk does not need to be considered. Both road handling and rollover risk require acceleration to come into effect. When not moving, acceleration is zero. All we are left with to consider is the roof’s load capacity. Is the roof strong enough to hold the weight?

Lets consider how strong the roof needs to be to support the manufacturer’s specified load limit. Dynamic loads can be significantly more than static loads. So we know that the roof of a car can hold a lot more than the load rating, because that rating takes into account hitting bumps and other stressful forces that take place when driving. So most people just assume the roof is strong enough to sleep on. Rather than just assuming, lets do some calculations on dynamic load to give us a guide on the instantaneous loading of a roof under dynamic conditions. This will give us some confidence in the roof’s strength. I’ll use my Hilux as an example. The calculations are based on a series of assumptions and of course will not be perfectly accurate. The point isn’t to provide an accurate number, but rather to give us an idea of what sort of weight the roof can endure.

WARNING: The following section contains maths. Skip to avoid boredom.

The manufacturer’s maximum roof loading for my model Hilux is 70kg. Lets assume we’re loaded to 70kg and we’re driving at 80km/h over corrugations that are 1m apart. Lets assume the corrugations are 4cm from trough to peak and the tyres and suspension absorb three quarters of that and that the roof is moving by a distance of 1cm.

At 80km/h the frequency in cycles per second (Hz) would be:

frequency = 80 * 1000 / 3600 / 1 = 22.22Hz

Assuming a sinusoidal profile, 1cm peak to peak displacement corresponds to an amplitude of 0.5cm and the displacement as a function of time would be:

displacement = 0.005 sin (2π x 22.22t) where t = time in seconds and displacement is in meters.

Taking the first derivative, we get velocity:

velocity = 0.005 x 2π x 22.22 cos (2π x 22.22t)

Taking the derivative again, we get acceleration:

acceleration = 0.005 x 2π x 2π x 22.22 x 22.22 sin (2π x 22.22t)

Peak acceleration occurs when sin (2π x 22.22t) = 1. So peak acceleration is:

peak acceleration = 0.005 x 2π x 2π x 22.22 x 22.22 = 97m/s/s

To calculate peak force, use the formula Force = mass x acceleration, using a mass of 70kg which is the maximum roof load of my Hilux:

peak force = 70 x 97 = 6790N

The force applied by the static weight is 70kg times acceleration due to gravity (9.8m/s/s):

static force = 70 x 9.8 = 686N

So the peak force is nearly 10 times greater than the static force. Converting the peak force into an equivalent static weight:

equivalent static weight of peak force = 6790 / 9.8 = 692kg.

Note when the vehicle is moving up in its bump cycle the peak force would approximately equal 692kg + 70kg (total 762kg). The roof is not only speeding up the mass, but it is working against gravity. The peak force when moving down will be much less, since the roof is working with gravity and the rate it can fall is somewhat limited by acceleration due to gravity, although the recoiling suspension and associated unsprung mass will pull the vehicle down faster than gravity. The result is a sine wave with a skew towards shorter, steeper up cycles and longer, shallower down cycles.

So under the assumptions we’ve made, with 70kg of load, the roof would be experiencing a peak dynamic load of around 700kg when driving over corrugations. The assumptions may not be perfect but it does give you an idea of how dynamic load escalates significantly compared to static load and what sort of stress your roof is under when driving over a rough surface.

WARNING: Never hit your head with a hammer.

An experiment you can do to demonstrate how much greater dynamic load is compared to static load, is to grab a hammer and gently rest it on top of your head, allowing your head to support the full weight of the hammer. Note how much it hurts. Next, hold the hammer 10cm above your head and drop it onto your head. Don’t force it down. Just allow it to drop under its own weight. Again note how much it hurts and compare with your first results. I predict you’ll have a sore head! When stuff moves and you start getting accelerations, forces grow rapidly. It’s common sense physics.

According to the calculations above, the roof of a Hilux can hold at least 700kg. But we shouldn’t read too much into the number itself, it may not be very accurate. What we can say though is the roof can hold a static weight many times that specified by the manufacturer. How much can it hold? Not sure exactly, but quite a bit! This is further re-inforced if you’ve ever seen a car resting on its roof after a rollover. If the rollover is gentle, the roof is largely intact whilst supporting the entire weight of the car.

13 toyota prado 1998 telegraph road near moreton

Upside-down Prado with it’s roof largely in tact. Photo taken on Cape York Peninsular.

Lets calculate how much weight will be on the roof when sleeping in the tent.

Tent weight = 45kg

Roof rack weight = 10kg

People weight = 2 x 80kg = 160kg

Total weight = 215kg

With the ladder extended on fold out roof top tents, around 1/3 of the weight is supported by the ladder. The weight on the roof is:

total weight on roof = 215 x 2/3 = 143kg

The total weight on the roof is 143kg – several times less than the 700kg we calculated. So although the 700kg is rough, the gap to 143kg is big enough that we can say we have a good safety factor.

Consider the shape of road corrugations, bumps and pot holes. Are they nice smooth sinusoidal profiles like we assume in the calculations above? If the bump has a sharp profile then the acceleration is higher and the load is higher. The sinusoidal profile is underestimating the peak load that the roof has to endure. The roof is even stronger than we think. What if the obstacle is a large bump bigger than typical corrugations? I could try to roughly calculate the load from a sharp bump based on some linear piecewise functions, but I don’t want scare off bored readers.

Another thing to note is that under dynamic conditions loads are transferred in all directions depending on for example whether the vehicle is turning, speeding up, slowing down, hitting a bump on just one side or both sides, up hill, down hill etc. So the roof is enduring elevated forces from all directions. With a static load, the force is straight down, evenly loading the roof structure and avoiding any twisting forces that would otherwise concentrate stress. So we can be even more confident that for static loads the roof is far stronger than the manufacturer’s dynamic load rating, since under dynamic conditions it must be able to tolerate elevated forces as calculated above, plus tolerate those forces under twisting, uneven distributions where certain parts of the roof may experience an overload condition with the majority of the stress concentrated at certain points.

If you are loading your roof top tent onto a support that is completely separate to your roof, as is my case of placing it above the canopy, then you need to ensure the support system’s rating exceeds the weight of the tent. The support system will need to endure the same dynamic loads as the roof would have to, so it will have the same capacity to support a static weight far greater than its specification for dynamic conditions. Note, when driving, your total weight at roof level summed across both the roof and any other support arrangements should not exceed the vehicle manufacturer’s maximum roof loading. Even though some of the load isn’t on the roof, it behaves the same when it comes to vehicle handling and rollover risk. Center of gravity is raised the same amount regardless of whether the load is placed on the roof or on some other roof level support. So for me I have 45kg over the canopy, that leaves me with just 25kg available to place on the roof before exceeding the 70kg limit.

Roof top tents typically weigh in at around 45kg. Add on top of that the weight of your roof racks. If the total is within the maximum roof loading and / or rack loading as specified by the vehicle and rack manufacturer, then you are good to load up your roof top tent. When it comes to sleeping on it, I’m fairly certain that under static conditions the roof will handle the load fine. Maybe if you are an extra cautious person and you’re having special cuddles with your partner then take care not to create excessive impact loads!

A Note About Going Offroad

Some vehicle manufacturers, and some roof rack manufacturers, de-rate their maximum roof load for offroad conditions. This is a consequence of the extreme dynamic loads that can occur when traversing uneven terrain. Plus there’s the extra detrimental effect on handling and rollover risk when the vehicle is being thrown around on rough terrain. Even if your vehicle and rack has no de-rating for offroad conditions, you should aim to come in well under the maximum load rating if planning to use the vehicle offroad, and you should drive slowly and cautiously when in rough conditions.

See also:

Ebay Roof Top Tent Review

Hilux Canopy and Canopy Roof Racks for Roof Top Tent

How to Catch Barramundi

back to 4WD, Touring and Camping

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41 thoughts on “Roof Loading and Roof Top Tents

  1. hmmm, very interesting….but what about when i’m getting ‘dynamic’ with the Mrs in our rooftop tent! More calculations required!!

  2. Just waiting for the new MU-X to be delivered. Roof rating is only 60 kilos. I was really unsure about this stuff. Fantastic article. Thanks heaps OutbackJoe….

    • No worries dude happy camping, hope you enjoy your new 4WD and exploring the outback.

      • we have a mux and isuzu have told us that it won’t support our 48 kg roof top tent along with our weight (about 150 ) trying yo get them yo confirm that the manual wright is dynamic is impossible they say it makes no difference if some one has some more info or are using a tent on mux with no problems plz respond

      • Hi Sharon. Some sales dude at a dealership isn’t gonna put his cock on the block and give you the ok to go against the vehicle specification. If your vehicle is ok to drive with 60kg on the roof then you know the roof is pretty strong and should be fine with a bit more weight under static conditions.

  3. Thanks a lot for your time to write this. I didn’t bother checking your math (or was not able to!) but I was thinking the same sorta thing in regards to dynamic vs static loads. I’m keen to put a tent on my vw tiguan with 100kg load rating. But would also consider mounting on Hyundai load work truck with around 70kg rating. You have made me a lot Moore comfortable to do so! Thanks again.

    • Yo David glad the article helped. Don’t worry about the math, all it does is help put on paper what you already knew – dynamic loads vs static loads. Have fun in you’re roof top tent.

  4. Wow, mate. I don’t reckon I’ve ever read a post about four wheel driving and physics! Interesting read; I try not to put anything on my roof that is heavy, and it scares me when I see troopies rolling down the road with half a house on top of the roof

  5. Hi I have a Toyota Hilux 05 Current shape i phoned the 4×4 center in my town , i told him i want to put my roof tent on some racks he said you cant go past the Rhino roof racks ,he said they will handle a roof tent no worries at all, do you think they would do the job as there any other roof racks on the market that would do the same job but a bit cheaper in price some one said Rola racks any help would be appreciated Regards Anthony.

    • Hey Anthony, Rhino are good, that’s what I have. I don’t know about other brands. If you are going offroad then the clamp on version of any brand, rhino or anything else, isn’t very good. The racks move under vibration and cause damage. For offroad you need the racks that permanently attach to the roof.

  6. Hi thank you for the reply one 4×4 place told me gutter mount ones would be ok , phone another 4×4 place he said no you have to use channel mount racks you have to pop rivet the mount in the channel does that sound right regards Anthony

    • yes, for offroading, you need to get the one that rivets into the channel. I have first hand experience using the clamp on gutter ones offroading and when on corrugated tracks they moved around, damaged the paint and damaged the rubber door seals where they clamp under the door frame.

  7. Thank you very much for your help much appreciated regards Anthony.

  8. so im assuming, if i have a load of 50kg for my zook (which may only leave the road to push up a well built track to get to a surf break) that by staying on the road, and driving like a civilised person, there will be NO problems with me rocking a roof top tent, rather than having to buy a surf van 🙂

    thanks heaps for this article mate.

  9. Hi, we are considering a Roof Top tent for our trip to the Simpson and wondered whether there is one suitable for a NT 2011 Pajero.

  10. Great article, thank you, Rhino roof racks give there loading at 75Kilos. With a roof top tent at 60kilos then add 160 kilos of human when static, should the roof rack at that static load be OK.

  11. Would like to know how you clamp, bolt, or tye rooftop tent to your car roof.thankyou

  12. That is an amazing piece of maths and common sense outbakjoe. You have satisfied my quarms about rooftop tents. Thanks heaps. Bronco Bob

  13. Hey dude,

    I love your calculations!!
    Care to do some maths for me? If I was to put a roof top tent on an 80 series at 2.2m heigh roughly. Can you calculate how much less angle I can get before rolling? My biggest fear is what that extra 80kgs with the rack is going to do to my tipping angle as to how adventurous I can get offroad.

    Cheers
    Brad

  14. Hey Joe, great reading, you’ve been around a tad. Hey mate I have a set of ryno roof bars, flush mount on my 2009 triton, with a super cheap pretty solid roof rack on top, 75kilos I know is how much I should have on top- We are about to start travelling oz but mainly the coast- no corrugated roads I hope. How much weight do you think I could put on top in your opinion and still be pretty safe travelling at 100 or 110km on highways and steady going on beach tracks! I’m not a mad 4×4 driver ( yet )

    • Hey Rickie It’s all on a spectrum. More weight = more risk. I reckon the manufacturer’s specification is a good limit to stick to. A lot of people exceed that and have no problems. Depends how unlucky you are. Even at the manufacturer’s specification there is still increased risk of roll over etc. Keep the weight on the roof as low as you can.

  15. If your assumptions show 7g of vertical acceleration caused by 1G of gravity, your assumptions are wrong.

    • Ah yeah, good point, except that it doesn’t make sense. Care to use any science or logic to validate it?

      The acceleration isn’t caused by gravity. What you been smoking? What article you reading? It’s caused by hitting corrugations.

  16. Thank you for the great information on rooftop weight limits…with especially great points made regarding the differences between dynamic and static rooftop loads. Here is some additional info…

    The Insurance Institute for Highway Safety (IIHS)…see link below…offers scientific testing results data (take a look and see if there is data on your specific vehicle). The IIHS tests “roof strength” for a given vehicle by placing a large plate on one side of the roof (thereby simulating initial rollover forces) and applies a steady force until the roof starts to crush, then the peak force prior to roof crush is recorded, that force is then compared to the curb weight of the vehicle via a strength-to-weight ratio. A “good” rating means that the vehicle’s roof strength-to-weight ratio is at least 4:1…so, for example, if a vehicle’s curb weight is 3000 lbs (sorry for the imperial units) and its roof has a strength-to-weight ratio of 4:1, then the roof can support at least 12,000 lbs before it will crush (the IIHS website also defines “marginal, acceptable, and poor” roof strength-to-weight ratios). Take home point, if a given vehicle’s roof has a “good” rating, then the total weight of a rooftop tent + gear + two people = a fly speck (in relative terms). 😉

    Link to the IIHS website (the link is specific for my vehicle, a Chevrolet Malibu, so you’ll need to search for data on your vehicle) —
    http://www.iihs.org/iihs/ratings/vehicle/v/chevrolet/malibu-4-door-sedan/2016?
    (then click on “roof strength”)

    • Yo John heaps of interesting data on that site. Even some of the big SUVs have a ratio of over 4, that’s impressive. Yep roofs are pretty strong. It makes sense that they can hold a few times the weight of the car if they’re gonna have a chance of withstanding a rollover.

  17. Excellent article. I did have to skip over some of the math but, I’m happy to see my plan of using yakima crossbars on factory roof side rails should support the 150lb roof top tent i’m looking to get (150lbs is the load limit for our suv).

  18. Thanks mate

  19. Trying to understand if my 2012 Xterra can safely hold the new iKamper roof tent.

    The Nissan Owners manual states: Always evenly distribute the cargo on the roof rack. The maximum total load including the gear basket is 150 lb (68 kg) evenly distributed. The maximum total load for the gear basket is 30 lb (13 kg) evenly distributed.

    The iKamper specs state: The dynamic loads exerts varying amounts of force upon the Skycamp while driving (i.e. wind speeds on any given day, the speed in which you are driving at, and other external factors). The dynamic weight capacity for any roof rack that we suggest, should have a dynamic weight limit of 165 lbs or more. While the vehicle is in motion, many of these external factors will have a role in having an effect on the dynamic weight limit of your roof rack.

    Not sure if Nissan is talking about static or dynamic?

    • Vehicle manufacturers always give dynamic rating. They are telling you what you can safely drive around with on the roof. It sounds like the ikamper might be heavier than the rating of your roof.

  20. Thank you very much for the reply and information.

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