It's been a while since I looked...

In a Scandinavian style roof, birch bark is the true watershed roof. The purpose of the turf, then, is largely to hold down and preserve the bark, while also providing a significant amount of insulation.

The turf on top is not soil, but is sod. In case you don't know, sod is grass cut from a field complete with the underlying root structure. It will contain a large amount of soil.

How thick you put it on depends on your climate. The turf needs to be thick enough to allow the grass to survive during the driest periods your roof would be subject to. In Norway, the roof covering is 2 layers of 3" sod, the first layer grass-down to help protect and shed water away from the bark, the second layer grass-up so that the grass can grow. Eventually the 2 layers will grow together.

Modern materials can be used in lieu of birch bark, which can make the roof last longer. The traditional lifespan of such a roof was said to be 30 years, which was dependent on the lifespan of the bark.

special care must be taken at the eaves and along the gables to help keep the sod in place, while still allowing suitable drainage.

In Iceland, things were done a little different.

The building would often times be a simple timber or post frame structure, which was covered almost entirely with turf, in later times wooden gables were left exposed and decorated.

here the birch bark is placed between layers of turf, if used at all.

Also, the walls consist of two thick layers of turf, with a filling of stone between them to provide drainage as well as stone underneath them for drainage.