I think it’s mostly only going to be a danger if the live roots are growing up into the soil, which will be prevented by the landscaping barrier.

Here is a good resource for you:
http://www.wvu.edu/~agexten/hortcult/fruits/blkwalnt.htm

Don’t use plastic!

Use a professional landscaping type of weed barrier. You can get it at any nursery or garden center or hardware store. Weed barriers are designed so that water can drain through.

http://www.backyardstyle.com/landscapefabric.php

You can make your own biodegradable weed barrier out of thick layers of wet cardboard or lots and lots of wet newspaper layers. It will last for quite a while, and you can just keep adding more compost to the top of the raised bed over the years as the soil settles.

1. You’re really sure that the roof can support the weight of the soil, any water that it will absorb, and all of the plants, along with the structure that holds it? That’s what I would worry about.

2. How do you water it and take care of it? With a ladder?

3. If the roof is sloped, how is it going to work with both watering, and with the soil? Deeper at one end and shallower at the other, so it’s level? I’m wondering if the soil would tend to slide down with gravity toward the bottom. And I’m afraid that the water will run down so that things will always be drier at the top, shallow end.

But hey, what do I know? It might just work. Plants can grow anywhere there’s soil, sun and water.

Please come back and send me some photos during the season, and I’ll see about writing a blog post about it!

Thanks!

The best thing to do is remove the grass. You can use a tiller if you have one, or you can try and remove it with a shovel.

When I put in my raised bed garden over a grassy area, I also put down weed barrier after taking out the grass, to prevent anything else from coming back up.

Good luck!