Ask Mr. Smarty Plants

The US Forest Service database says:  

"Eastern redcedar generally has a shallow, fibrous root system, though roots of mature eastern redcedar trees may penetrate 25 feet (7.6 m) and lateral roots may reach 20 feet (6 m).  Eastern redcedar seedlings have penetrating taproots and may later develop a lateral taproot system.  The deep, early taproot is usually replaced by an extensive, shallow root system with age.  Even 1st year seedlings begin developing a long fibrous root system, often at the expense of top growth.  The root system may be deep where soil permits, but on shallow and rocky soils eastern redcedar roots are very fibrous and tend to spread widely.  The development of a lateral taproot with age may also enable eastern redcedar to persist on outcrops and shallow soils."

The US Forest Service Silvics Manual says that the soil condition determines whether the tree develops a deep tap root or spreading lateral roots.  In thin rocky soil fibrous lateral roots will be predominant; but, if the soil permits, the root system may be a deep penetrating tap root.  So, if your soil is good, you shouldn't have great concern about lateral roots.  

You should probably not plant the trees a distance from your wall that is less than half the width of the mature tree.   Kansas Forest Service recommends a planting spacing for redcedars of 6 to 10 feet and University of Missouri Extension recommends a spacing of 8-12 feet.

Cultivars have been developed that are smaller; for instance, according to the University of Connecticut there is a cultivar, 'Emerald Sentinel', that is a pyramidal, columnar shape that grows to 20 feet high and only 8 feet wide. If this tree sounds as if it is tall enough for you, it could probably be planted 3 to 4 feet from your wall.

Juniperus virginiana

Juniperus virginiana

Juniperus virginiana

Juniperus virginiana