When it comes to assessing infrastructure damage in urban areas, trees often take the blame! As urban forestry consultants, we frequently receive questions from our clients about whether tree roots can:

  • Crack building foundations;
  • Cause buildings to sink in clay soil;
  • Break water supply pipes;
  • Block French drains and other underground pipes;
  • Break any other rigid structures installed on the ground.

But what’s the real story? Some of these claims are false, some have a grain of truth, and others are very real, but things are generally not black and white—rather, they’re nuanced.

 

Destructive roots

First and foremost, it’s important to understand that roots seek out water and will not damage a structure simply by being there. A tree near a building will not necessarily cause damage to the building, unless the trunk or base of the trunk is very close to the building (to the point of physically touching it). A tree with a wide canopy, such as a maple, oak, or any other large species, whose trunk is 3 meters from a building’s foundation can very well thrive without damaging the building’s structure. There are three main factors to consider when assessing the risk of damage to a building from a tree:

  1. Eastern cottonwood in conflict with an outbuilding (Photo : Trame-Verte)

    The distance between the trunk and the structure, specifically whether the tree’s trunk or branches have enough space to grow freely (see the photo of the obstruction opposite);

  2. The species and size of the tree, as certain species are known to have more invasive root systems, which increases the risk of damage as the tree matures. Often, municipalities prohibit the planting of these species with extensive root systems, such as willows, poplars, silver maples, etc.;
  3. The condition of the structure you wish to preserve. Removing a tree located near an old structure will not necessarily eliminate the risks associated with the structure’s deterioration. For example, the loss of effectiveness in an old French drain is often linked to the infiltration of soil and other substrates into the drain. By the end of their useful life, the gaps in the French drain, filled with soil and water, provide an opportunity for root growth, hence the myth that the roots are the cause of the problem. On another level, a building may show cracks and other signs of ground movement, but the nearby tree may not be to blame when construction defects or weak soil are present. Trust us, this is more common than you might think!

These criteria also apply to buried pipes, which—if they are as watertight as intended—will not be penetrated by roots. A leaking water inlet is a problem in and of itself; any roots that may find their way there only did so after the leak had already developed.

It is also important to note that in all soils, but particularly in clay soils, the vast majority of tree roots are found within the top 30 cm of soil, due to a lack of oxygen in the deeper soil layers (see photo opposite). This characteristic of root systems, generally overlooked by both the public and professionals across all fields, helps us better understand the real impacts that trees can have on underground infrastructure.

Soil profile (Photo: Trame-Verte)

 

Roots: the cause of clay subsidence

The phenomenon of clay soil subsidence is nothing new and is very much a reality in the St. Lawrence Lowlands region, although it is often more frightening than it actually is. The phenomenon stems from the fact that urban development, by directing water into drainage systems and increasing the amount of impervious surfaces (streets, sidewalks, parking lots), has significantly reduced the amount of water absorbed by the soil, making it vulnerable during periods of drought. Trees are therefore not responsible for the sinking itself, since sinking can occur even without trees, but they can sometimes contribute to exacerbating the phenomenon. However, we are referring here only to mature trees with highly developed root systems.

Generally, issues with subsidence on clay soils are limited to certain areas, and these areas are usually already known to residents and local professionals. On sensitive clay soils, the best preventive measure is to schedule regular watering of the lawn. Watering will help stabilize the site’s water reserves, regardless of the presence, size, or distance of a tree from the building. The internet is full of information on the subject, but be sure to consult reliable sources, such as the comprehensive publication by the Canada Mortgage and Housing Corporation on the topic:  Comprendre l’interaction des arbres, du sol d’argile sensible et des fondations et agir en conséquence. 

In short, trees are rarely the primary cause of structural damage, and before blaming a tree in such situations, it is best to investigate the structures themselves and the surrounding conditions. Of course, there is nothing like the advice of a tree expert to accurately identify potential conflicts between trees and surrounding structures!