Question
How deep do the roots go for thuja green giants?  (At full height?)

Answer
The root systems of arborvitae are generally shallow depending on the variety and may spread to the dripline or edges of the branches, according to North Dakota State University. Green Giant Arborvitae are not known for having invasive or aggressive roots. The dept is usually in the first 2-3 feet of soil. This will depend on the type of soil.