Earth berm home question

Concerning earth berm homes that are built into a hill side: If the roof is covered with earth, what method of construction is normally used for the roof? This weekend, I went to look at a very remote, very "distressed" piece of property that is being sold for the value of the land only. The listing was for land only, but digging deeper into the photos and description, the land has an "earth berm" home on the property. A call to the realtor told the story. The people built the home about 16 years ago, then had to move out of state to take care of relatives. The home has been abandoned since then. And I mean really abandoned. There are a hundred small pine trees growing right in front of the house. So many in fact, that I could not even see one end of the house. I cautiously entered the house through the front door which had been opened by vandals years earlier. The entire house is trashy, but the basic structure is intact. As I made my way through the dark, spooky interior, I was surprised to find what I originally thought were 2 huge rooms. But when I look toward the wall, I saw 2 garage doors! The house has a huge underground garage! There were so many pine trees grown up in front of the house that the garage doors could not be seen from the outside even if you were standing 20 feet away from them.
My main question is this: The roof looks like it was made from steel support beams. Not solid "I" beams, but those zig-zag type beams that are use to support flat tin roofs on industrial buildings. If this house used a tin roof supported by steel beams, how could the roof have ever been made TOTALLY water proof? It would seem like an exercise in futility to think that earth and rock laid over a tin roof would ever remain waterproof. Even if the tin were waterproofed with tar or some type of coating. Would there be another substance between the tin and the earth layer to give it waterproofing properties? The suspended ceiling looked to be falling down everywhere, but that could have been the work of vandals instead of leaks. I really didn't take too long to eyeball the situation because I was by myself and literally did not like turning my back to anything except a solid wall. It look like the kind of place that homeless crystal meth cookers would be camping inside. It wasn't in a bad area, it had just been abandoned so long that limited vandalism and mother nature had taken its toll.
So what's the word on the roof structure of these homes? Did somebody use the wrong method to build a roof on this house?

 

Might ask these guys - http://www.earthship.org/
DOE http://www.eere.energy.gov/consumerinfo/factsheets.html
Links galore - http://www.liquefaction.com/berm/index.htm
(including link I've spent a bit of time "researching" - Monolith domes)

 

There is probably some kind of waterproof membrane between the roof and the fill above it.

I'm not sure what it would be made of, some kind of rubber, probably. I'm remembering a local story - there's a park in downtown Milwaukee with a parking garage underneath, they are thinking of tearing down the garage and re-doing the park space, because the waterproof membrane above the garage is starting to leak, and it would cost too much to repair it. I think the article said that the membrane had a 50-year life expectancy.