Raised beds for poor drained soils.
It is common in heavy clay soils for water to collect in any low spots. Many plants need well drained soil. To produce this environment one can put a plant bed on a slop.
Or one can put in a raised bed.
The raised bed will allow water to drain out of the soil. The soil in the bed will provide a environment for those plants that require a well drained environment. Putting a raised bed over tree roots can present a problem. Not only can roots grow into the bed. The tree roots below the bed can also die from lack of Oxygen.
All roots require a minimum level of Oxygen. In heavy soils Oxygen is a limiting factor. Minimum levels of Oxygen may only exist down to a foot or so in clay soils. If you place two feet of soil on top of the roots, those roots may no longer receive enough usable Oxygen. The roots will now die. Not only will these dead roots no longer provide nutrients and water to the tree, they will provide a path for disease to enter the tree. If the roots are not Structural or major transport roots it may be better to cut the roots under the raised bed. This will result in a small area of disease exposure and new roots can grow from the site of the cut.
