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Method of placement is to trench down to a naturally occurring horizontal zone which is rootproof, place the root barrier in one continuous piece into the trench with the root barrier top finishing above the ground.
Development of root barriers, their construction and applications has progressed since 1992 when technology progressed from a concrete barrier which cracked and failed, to specific plastics with the capacity to handle major stresses and loads created by matric suction, soil and moisture movement.
Initial development of this product was based on stopping trees from affecting buildings, but as the research evolved it was discovered that what was actually being done was stabilizing moisture in reactive clay. A flexible waterproof cut off wall placed around a building will allow the building foundations to float on a block of stable clay and moisture, reducing the need for piers and other structure and proving to be very economical.
Normal placement of the barrier is to locate it around the structure, out from and parallel to the footings of the structure. Try not to surround the tree. The preferred method is placing the root barrier along beside the building, path, road etc. so that the tree roots cannot gain access to the structure. Root barrier works as a waterproof seal protecting the soil under the structure from moisture loss laterally. The structure prevents loss of moisture vertically and so the moisture content of the soil can be stabilized and will stay constant. After installation the soil under the building can be rehydrated if necessary to return it to the moisture content that it was when the building was built.
Arboricultural considerations, working in from the drip line, the closer you get to the trunk the higher the risk of damaging or destabilizing the tree. About 50% of the distance from the drip line to the trunk is regarded as the closest you can cut without major risk to plants health. If it is necessary to trench closer than halfway towards the trunk, it would be advisable to engage the services of an arborist to assess the tree prior to the work being carried out, and to help nurse the tree through the period of installation.
Area of good soil that the plant will require to live a healthy life may be calculated by multiplying the radius of the mature plant canopy by p r2 x .3m. The answer will give you the cubic volume of good soil required. If works require the ground surface area is not available for the plant, special pits filled with quality soils, drainage etc. may provide the answer.
Root barrier applications
Civil works; Back of kerb, bio swales, protecting infrastructure. New houses; Stabilizing reactive clay under foundations, Established houses; Buildings cracked by clay shrinkage can be restored by isolating and stabilizing the moisture under the building. Waterproof cutoff walls; Contamination migration through the soil prevention. Landscaping applications; Root group separation, hardscape protection.  
- Application of a new vertical moisture barrier construction method for highway pavements, R. P. Evans, J. C. Holden and J. K. McManus. Published Vol 5, No3, September 1996 Road and Transport Research
- Expansive Clay soils and Vegetative influence on shallow foundations, ASCE Geotechnical special publication number 115