When I had my foundation inspected, one of the recommendations from the engineer was to put in a root barrier. The purpose of root barriers is to stop the roots of nearby trees from getting under the concrete slab, sucking up water, thus causing the foundation to sink a bit. This typically happens in regions where homes are built on expansive shrink-swell clay soils as in the Dallas and Houston areas of Texas.
The engineering report called for the root barriers or "root walls" to be made of concrete 10 inches thick and three feet deep, or plastic with a minimum thickness of 20 mil buried to the same depth. These are also sometimes referred to as "root caps." As you'll read later, arborists are not big fans of them.
The contractor I hired uses corrugated vinyl sheet similar to what you would use for patio or awning roofing. It is flexible enough to bend into the semi-circle design two of the root barriers called for yet thick enough to deflect new root growth.
The crew hand dug the trenches with "sharp shooter" shovels, and used clippers and axes to create a 8 inch or so gap with the tree roots cleanly cut on both sides of the trench. The vinyl sheeting was placed in the ditch and held upright while the trench was back-filled. The top portion of the sheeting was trimmed to ground level and also put in the trench.
In all, we had four root barriers (root caps) installed. Three are to divert the roots of trees on our property and a fourth one to keep a neighbor's tree roots at bay.
We also had a row of Nellie R. Stevens holly shrubs removed that had been planed right along the foundation on the west side of the home. That type of bush quickly grows into small trees if not kept pruned back. The contractor thought it would be good for the foundation if they were dug out. I would have to trim them back two or three times a year, they are prickly and I'm allergic to them so it was without tears that I bid them elsewhere.
To be sure, there is debate between tree lovers and engineers as to how much damage tree roots can cause a concrete slab foundation built on expansive clay soil. Many arborists do not like root barriers because they inhibit the tree's ability to take up water and nutrients and view the claims of damage to foundations as exaggerated.
Engineers will tell you a good sized tree can remove hundreds of gallons of water from the soil through transpiration and contribute to foundation settlement. They see root barriers as a compromise to removing trees entirely. If installed properly root barriers can create an atmosphere where tree and slab can remain pals.
Finally, the contractor re-worked the soaker hose system. He thinks that with the root barriers, removing the large shrubs along one side of the foundation, faithful use of soaker hoses, and a decent amount of rain, we can reverse the settling and get some up-lift in the slab.