	Trees & foundation damage

Relationship between trees and foundation damage

Trees are often considered to be the cause of damage to building foundations and, as a result, are usually removed. However, trees are not always the cause of the problem.

Damage to building foundations is often a result of the behaviour of the soils under the foundation. Uneven soil shrinkage can lead to uneven settlement and foundation damage. The Ottawa area is dominated by a soil called Leda clay — a marine silty clay with a high water content. When water is removed from Leda clay, it shrinks and does not regain its original volume.

In the Ottawa area, shrinkage of Leda clay is a major contributor to building foundation damage.

Factors that contribute to Leda clay shrinkage include:

Long periods of dry weather.
Lowering of the groundwater table. The groundwater table level is reduced by deep trenching for underground services or housing development.
Lack of storm water absorption into the soil due to a large area of hard surface.
Soils become dry from the high water demand of trees, shrubs, hedges, grass and flowers.

Mature trees found within 10 metres of a building foundation that is constructed on Leda clay soils are often identified with soil shrinkage leading to the uneven settlement of the foundation.

Responsibility of the City of Ottawa

As a responsible corporation, the City of Ottawa is committed to protecting its natural environment and resource base. In an effort to preserve mature trees and be fair to its citizens,

the City has instituted a four-phase procedure for assessing the circumstances related to mature municipal trees that are suspected of causing damage to building foundations.

A four-phase assessment process:
Are trees the cause of your foundation damage?

Phase I
Confirm the Existence of Leda Clay

Action: The complainant is responsible for determining the presence of Leda clay on the site of the damaged foundation. The City will consider reviewing any complaint that is accompanied by a report indicating that Leda clay is the dominate soil of the site.

If there is no evidence of Leda clay, the City of Ottawa's position will be that the municipal tree at risk is unlikely to be the main cause of the foundation problem.

If Leda clay is present, the City will conduct Phase II of the assessment process.

Phase II
Apply First Level Screening

Action: The City of Ottawa's representatives will conduct an assessment to determine the probability of the tree being the cause of the foundation damage. The assessment will be based on five key indicators of soil shrinkage. The tree will not be considered as the probable cause of damage if:

Deep trenching has occurred within 10 metres of the building foundation
The tree has a low demand for water
The site has a large amount of other plants, such as shrubs and trees
The front yard surface is highly impermeable
The foundation is found to be substandard

If any of the above indicators are positive, the investigation will be terminated. The complainant will be notified of the reasons and of any available recourse.

If none of these indicators is present, the City of Ottawa's representatives will conduct Phase III of the assessment to determine the relative contribution of the mature tree to the foundation damage.

Phase III
Detailed Site Review and Evaluation
Action: A detailed inspection of the site and the circumstances of the case will be completed by the City of Ottawa. This includes an investigation into the type, age and condition of the building foundations, site drainage patterns, quantity of vegetation versus the area of hard surface on the site, an assessment of the size, health and species of the municipal tree that is considered to be the problem, and a general assessment of activity that has occurred in the area over the last few years.
Once the damage has been assessed and the cause confirmed, the City will conduct Phase IV of the process.

Phase IV
Best Management Practices
Action: The City of Ottawa will work with you to determine the best options to mitigate the damage, first and foremost without removing the tree. It is understood that sometimes tree removal is unavoidable.
Options that Work - A Few Landscape Design Measures

Maximize your lot's surface water permeability by increasing the green area;

Maximize use of rainfall by directing storm water for use by the trees, keep area around trees permeable, and redirect roof runoff towards green space and away from sewers;

Augment surface and ground water content by watering trees during droughts;

Reduce the water consumption of new trees by planting species that have low water demands;

Only remove a tree if no other measures are effective or appropriate.

Note: There is no evidence that tree removal eliminates the risk of future soil shrinkage and foundation damage.

Did you know?

Just a few facts about tree roots

Tree roots are usually located within 60 cm of the soil surface.
Tree roots may span a diameter equal to the height of the tree.
In urban settings, trees are most often located within 3 to 7 metres of building foundations.
Tree roots generally grow along the path of least resistance.

The only force exerted directly by a tree root is that caused by its growth.
Larger trees demand more water than do smaller trees.
Trees take water from the soil through their root system.
The rate of tree root growth slows during summer months.
Some tree species require more water than others.

Water demand of various tree species
High demand	Moderate demand	Low demand
Elm Oak Poplar Willow Silver Maple Manitoba Maple	Cherry Ash Hawthorn Hornbeam Other maples (Sugar, Red) Mountain Ash	Beech Birch Mulberry Cedar Fir Pine Spruce

Who to contact

For further information related to trees and building foundation damage, please contact the Operations Branch of the City of Ottawa at

3-1-1

Contact the Ottawa Forests and Greenspace Advisory Committee
Contact City of Ottawa Forestry Services

Trees & foundation damage