Disco 2 Roof load - custom roof rack

I'm considering making a custom roof rack, but I'm a little confused by the maximum roof loads. I see it quoted as 50kg for normal driving, but most racks seem to be pretty much that already. Depending what thickness of steel I use mine could be anywhere from about 40 to 60kg, securely attached to the rain gutter. Given that it's mostly for vanity anyway, and it'll rarely have anything on it - a bike or some garden waste now and again and a couple of chairs and a suitcase for camping - plus I don't really off road, how much of an issue will it be?

I haven't managed to find anything regarding rain rail weights. The weights quoted in the manual are for the standard through roof racks