I was driving by a house on my way to a foundation inspection yesturday and saw signs for a local contractor installing root wall in a yard.  I had some time so I thought I would look to see the quality of the work.  I found the condition shown in the photo above.  The root wall was burried under the grass.  This will allow the tree near this wall to jump the top of the barrier.  If the top of your root wall is not visible it will NOT work.  It should also extend at least 30 inches deep.  See our recommendations page for more information and keep an eye on those contractors.