If you have a roof rack you need to watch this

I think I might have mentioned my dislike of putting heavy things on the roof before.
Roof tents are wrong for so many reasons. I always laugh at those who drive around with them on the roof 365 days a year. I love camping but for the 345 days a year I'm not using my tent I like to keep it in a cupboard.

It not just dead weight, it's dynnamic load that causes things to fail. A 20Kg load weighs 20Kg but exerts a (approx) 200KN force (weight x acceleration due to gravity). So if your 20Kg weight comes to a sudden stop, like going over a bump it can put 200KN through the fixings, that is why it breaks the bolts/plastic pieces. You are likely to get a lot of bumps and sudden stops when off road, hence the lower rating. The same is true when going round corners, especially at speed.

Wind loading can be a factor as well. I used to windsurf a lot and a couple of windsurf boards don't weigh that much but certainly catch a lot of wind, so much so it would stop my Fiesta [at the time] from pulling top gear when it was windy, which was all of the time I wanted to go sailing. Imagine how much force was going through the roof bars and fixings, that's why I used Thule bars with a good solid fixing.

I saw an update from Ronny D saying they had found out the roof load limit was only 60kg for his vehicle, makes the Jimnys 30kg look quite good in comparison.