I agree with Martyn, you want to keep things light up there on the roof, if you put a kayak up there, it can be really high. That being said, if you have a vehicle that can handle it. I have a couple suggestions.
I don't recommend a kayak on top of a rooftent, the Maggiolina Extreme can do it, but I think you could be subjecting your kayak to a lot of abuse up there.
The idea of the kayak or kayaks next to the rooftent is very good, but most rooftents are wide so there isn't much room for them. We have a rather obscure rooftent, actually one of my favorites called an OverCamp. It is a folding tent, but it opens like a book so it is long and thin. It is what we consider an expedition tent, has features that make it very good for expedition use but don't make it as appropriate for family use. Namely, on this one, the whole sides roll up so it works well in the tropics. As a few people have mentioned, you could roll over and fall out of this tent. There are doors and screens to keep you in when you are sleeping but you need to use them. The other drawback is that the OverCamp is designed to be very basic and honest, no alloys, everything basic and easy to repair, but this makes it heavy. The small one is supposed to be 95 pounds but we beef them up so they are more like 115 pounds, the medium one is huge but weighs about 135 pounds. They work very well on trailers, and aren't that expensive.
Rich @ AutoHome
www.autohomeus.com