Tree roots are one of the major reasons for foundation problems and may cause a foundation to crack, tilt or shift up and down. When a foundation has movement it may cause damage as well as a number of problems with the home ranging from minor cracks up to structural failures, depending on how much movement occurs, the period of time the movement occurs over and the severity of the movement.
The biggest foundation problem with tree roots is not the upward pressure, but the downward movement of the foundation caused by the roots. This downward movement, called subsidence, is often caused by transpiration (roots pulling moisture out of the soil).
For us non-engineers
You may think of subsidence as a loss of support under a foundation. The most common example is when moisture in the soil, especially clayey type soils is reduced, causing the soil to shrink or go down. Transpiration causes moisture reduction in soils. Many people liken subsidence to settling.
Transpiration is the process by which moisture is carried from tree and plant roots to the leaves, where it changes to vapor and is released to the atmosphere. Interestingly, a large oak tree can draw 40,000 gallons of water a year up through the roots and evaporate that moisture through the leaves.
It’s not the roots pushing up
The pressure of tree roots growing against a footing or foundation, contrary to popular opinion, generally do not cause foundations to raise upward or crack from the pressure of the roots. What the roots do is usually draw moisture out of the soil under the foundation and this results in the soil shrinking and dropping down; as the soil drops a little there is less support for the foundation and this is what causes the foundation to crack.
Have you ever noticed that a tree trunk will grow around a rope tied to it rather than break the rope from the forces caused by the trees’ growth. Roots likewise will tend to grow around or under the foundation, rather than forcing the foundation upwards.
In reality a tree root may grow under a foundation but it will not put enough pressure on the foundation to crack it or move it upward.
Tree roots will crack concrete walks, driveways and patios
Tree roots do have a enough power to cause cracks in drives, walks and patios, for these do not have nearly the weight that a home puts on a foundation or are they structurally as strong; therefore, the walks, drives and patios will raise up and crack.
The reason roots crack and raise concrete walks, drives and patios relates to the fact that concrete is very strong when it comes to compression strength (it can carry lots of weight) but it is not nearly as strong when it comes to tensile strength; about 10 times stronger in compression than it is from a tensile strength standpoint. Thus when the roots get larger under a walk or driveway, they put the concrete into a bending type of tension situation where tensile strength is more important. It’s kind of like the roots are more solid or rigid than the soil on each side of them and the concrete is kind-of bending over the roots which requires a lot of tensile strength to keep them from cracking.
If the compressive strength is 3,000 psi, then the tensile strength is only about 300 psi, thus the concrete cracks easily, especially if only 4 inches thick and with no rebar or wire mesh reinforcement.