Common planar surfaces of failure include joints or bedding planes, especially where a permeable layer overrides an impermeable surface.

Impermeable surface area was kept to a minimum and many of the buildings have green roofs.

Aside from their obvious advantage over straw roofs in terms of fire retardance, the impermeable surface also protected the adobe walls below from the damaging effects of rain.

Floods can also occur if water falls on an impermeable surface, such as concrete, paving or frozen ground, and can not rapidly dissipate into the ground.

Legislation will be introduced to require planning permission for impermeable surfaces in front gardens in 2008.