Earth roof and tap root trees

I've never owned an earth roof home, nor do I know anyone who has. They intrigue me very much. One question that I've long wondered about it how to deal with tap root trees that take root. If I ever get to have one, I'd be for not mowing the roof. I don't want to be up there too often to be hunting for trees that could cause issues.

I don't know if it was Rob Roy or Malcolm wells who said something about tap roots not going where there is no water.

Am I making a mountain out of a mole hill?

One of the interesting things about root (and branch, for that matter) growth, is that it only happens at the very tip of the plant. So roots don't really grow into soil, but into spaces between soil particles.

I think a surface that won't admit water, also won't admit a root tip, and so the root will not be able to force it open when its diameter increases later on, the way it does to cracks in stone or to spaces between clay platelets.

Still, I think that if the weight of a plant increases quite a bit, the pressure it applies with its lowest roots might also increase, and puncture the seal. So the problem may not be tap roots per se, but heavy plants on stiff, irregular bases.

A dense cover of vegetation that isn't amenable to tree seedlings may be the best bet.

Might this be the reason that peasants with sod-roofed cottages used to graze a sheep or some geese on the roof from time to time? 

Kathleen