Its not a hard and fast rule, but for many plants the adult height will give you an approximate idea of the eventual root depth. In your case, therefore, I'd stick to stuff that doesn't get much taller than 2 feet for example. In general stuff that spreads by runners will need less soil than those with large taproots.

I'd have to disagree - there is no correlation between plant height and root depth (root spread perhaps, but not depth). Root depth is ultimately a factor of soil conditions - loose, sandy soils allow deeper root penetration, heavier compacted or clay soils do not. But for a general rule of thumb, most plants, trees included, will have the bulk of their root system located within the top 24" of the soil and many will have most of these just below the soil surface. In most cases, roots tend to grow laterally rather than straight down. The exception will be plants that develop tap roots - you may find it easier to search for those that exceed your desired depth rather than the other way around. The list is much shorter :-)

Interestingly enough, this exact question has popped up on a number of forums recently and by different posters. Is this a class assignment of some sort or does everyone have issues with underground pipes and utilities? Planting over septic fields is a whole 'nother issue and should be addressed separately.

I agree that soil conditions will probably be the biggest factor on root development and hence eventual depth.

While you are correct that eventual size is not a reliable indicator of root depth, especially for large specimens like trees (a 100 foot tree won't have its roots 100 ft down!), most low carpeting plants have roots that don't penetrate more than a couple of feet. It was this in mind that I thought of simplifying the task by keeping to low growing plants.

YMMV.

Vouts

Most roots need air, roots that go deep tend to be special ones that make up a minority of the root system. When a tree growing on heavy soil falls over it is typical to find that most of the roots had made a mat just below the surface.

There are certain desert trees and prairie grasses that have amazingly extensive and deep root systems.

Robert Kourik has some amazing information in his book on Edible Landscaping on root spread and depth. His book is out of print, but some libraries might have it on hand, and it's available used through Amazon.

Here is a link that might be useful: Kourik's book at Amazon

Why do you want to know? If you have a specific reason, perhaps someone will be able to offer better advice.

For instance, for green roofs, several people have researched which plants do best, and one of the main factors is amount of water and maintenance required.

If you are planting in hardpan or raised beds or planters, there again, people have specific experience. Plants whose roots go deeper than 2 feet can often tolerate and even thrive with container culture -- tomatoes are a notable example..

what plants can we plant over a leach pit? we live in the high desert, freezing nights in winter, scorching days in summer...
looking for root depths of different plants.

Have you checked out container gardening books? Most of what I've checked into have a depth needs.

Marla