The roof is, even more than the walls, the place where nature and our building meet. I haven't figured out how to make an edible, green roof without plastic yet.

Our goal is to make our home with nature, not against nature. Loose earth washes off a roof, but earth with roots does not. With a stepped roof under the sand, this becomes a little easier. Wooden boards are allowed to rot away as soon as there are enough roots to hold the sand. But a stepped roof is more difficult to waterproof because you have more edges and corners.

Often under a green roof a partially poisonous roof is laid with something green on top. Plastic in the sun is also polluting. Plastic such as EPDM under the earth of the roof may be OK, as well as carrot cloth (made of plastic) to protect the plastic.

However, a real green roof on which vegetables are grown has a lot to endure. Someone can walk on it. There might go a shovel in the ground, touching the bottom.

A traditional flat roof with EPDM or other film, and something heavy and green on it, is not wrong, not even for a dismountable house. But if you want to be able to move the greenery on your roof with you, then something with demountable bins is better.

A solution is to use geotextiles or coconut mats to create a kind of bag-like or stepped structure that retains the earth. Perhaps there are also plants or fungi that can do this? The roof would then consist of 30 cm wide U-gutters, covered with plastic, containing geotextiles and drainage material such as stones, shells, or something made of plastic. The geotextile can be attached to the rim of the U-gutters at the top if it is covered with a waterproof material such as plastic foil or a plastic profile. It can also be attached to this profile. All this has to be covered with so much earth that the plastic does not get into the sun.

Probably you can't walk on the ground on this roof because this can damage the plastic underlay and because the geotextile cannot endure this. When the roof is steep then this will be intuitively clear. A flatter roof will need instructions.

Another idea is to make the roof, just like floors, out of removable bins. In the case of demountable roof components, a slight slope or other height difference between the bins forming the roof is desirable for the drainage of water. If the roof is highest in the middle, then it can easily be made stronger there, making the whole as lightweight as possible.

You can make each roof element like a container filled with earth which drains its water. But a drain can clog, so that's vulnerable. You may be able to use the linking beams between the boxes as gutters, preferably but not necessarily with a small slope. It's useful to think of something to easily clean the gutters, such as a brush that you can pull through with a rope.

You can also make an overflow at the bottom of each container to drain into a lower laying container. The trays must then be deeper than they are high, so that they protrude above an underlying tray.

If you make this completely tight then you don't need a water repellent sealing below. In case of small leaks, ventilated wood should be sufficient. The wood can be laid overlapping (as with roof tiles) or with foil underneath, so that any leakage can be drained off downwards.

A simple roof with built-in linking gutter seems to be a corrugated roof sheet. With the right paint (but not ecologically) you can make a nice cold roof in case there is no earth on it. But if the corrugated sheets are fixed to the roof elements in advance, it can be quite difficult to fit them together. A slightly different-than-standard profile could solve this.

An advantage of corrugated sheets is that they are relatively easy to attach because they are screwed on the roof frame at the highest points of the waves. On this you can. for example, screw a planter that is just as slanted at the bottom as the roof is. You can also make "upside-down feet" on the corrugated iron mounting and put planters on them.

The disadvantage is that everything above the corrugated sheets must be weather and moisture resistant.

If you make the waves of the corrugated iron tens of centimetres high, you will automatically get planters. Combine that with "pockets" of geotextiles.

With ordinary corrugated iron, this is probably also possible. If you put a lot of earth on this, say 30 cm, then the highest point of the corrugated sheets is still above any water level that may occur. You may have a little more chance of leakage, but with ventilation under the roof this will not be a problem.

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