The root tips offer no resistance against obstacles, whether a small rock, a piece of wood, a foundation wall and even a hard and compacted soil. The elongation of the roots will follow the direction where the ground condition is ideal.
When roots encounter crushed stones, under the parking lot or street, or when they reach the reinforcement of pavement or any other barrier, they grow in another direction.
For the same reasons, a parking lot that has a properly structured foundation under the asphalt, or the paving stones or slabs, will create a barrier against the roots. When construction standards are respected and soil is stabilized to prevent the movements of freeze-thaw cracks, no root damage is possible. If after several years and for different reasons (soil compaction, freeze-thaw, water infiltration or other), the construction deteriorates enough for a root to infiltrate, cut the root and resurface the parking lot.
Research shows that the presence of roots in pipes is due to dislocation or loss of a seal in the connecting joints. This opening permits root infiltration.
The opportunist roots take advantage of water availability, nutrients and oxygen. As long as the conduits are not fixed and impermeable, roots will grow, ramify and colonize the place.
The joint opening is due to a set of defect factors, often combined with age, such as the type of pipe used, the mechanical motion of the soil (packing or freeze-thaw), and the quality of the bedding or of the foundation when installing pipes, the type of filling and the type of connecting joints between the pipes.
Roots take advantage of a weakness or of a default existing in the conduits. Repair of underground pipes solves the problem permanently. (People used to use concrete piping (or clay) of a length of 4 feet; and today, they use PVC plastic pipes or fibreglass FRP in longer sections.)