Tree roots system vs Building foundations

Understanding foundation systems and soil types are key for anyone looking to build or extend their home. The cost of building foundations is one of the biggest variables, so it is important to work out the most appropriate foundation system to use as early as possible in order to keep control of the budget and schedule.

Foundation systems and soil types go hand in hand when it comes to choosing the right foundation solution for your site. Identifying the most cost-effective solution will depend largely on the ground conditions on-site, including the type of soil, aggregate or rock and its key constituents. The best way to achieve this prior to starting work on site is to instruct a geotechnical study, which involves digging or boring trial holes around the site of a proposed new building, principally to establish the load-bearing capacity of the soil at differing depths.

Samples are collected and tested for plasticity (shrinkability), pH levels, sulphates, moisture contact (seasonal water table) and other factors that will allow the foundation solution best suited to the site to be determined. The selection of foundation type for a given structure is also determined by a number of factors other than the soil type, for example, past site usage, adjacent construction, size of the development process, limitations, etc.

Trees

Most of us like living and working near greenery, but there are major issues to consider when designing buildings that will be close to trees.

If you’re planning to build close to trees, it’s important to conduct a tree survey and dig trial pits to assess the condition of the ground. Then you can establish with the structural engineer whether a full site investigation is required. All this should be done at an early stage of the project so that the correct foundations’ depth is allowed for from day one.

In simple terms depending on various factors the closer a tree is to a new building the deeper the foundations must be taken down. The minimum depth for any foundation is 900mm; however, this depth could be greatly increased, depending on how close the tree is to the new extension. The basis of design is to ensure that the foundations are taken down to such a depth, that they will not be affected by tree roots. The trees themselves do not necessarily have to be large varieties, such as Oak or Beech, even small trees, such as Plum or Apple, could have an effect on foundations. Also, should you have an existing tree in your garden, having it felled will not mean that the foundations will not be affected. Heave in clay soil can take place when it takes up moisture and swells, after the felling or removal of trees and hedgerows.

Trees adjacent to a building are often the cause of structural problems such as cracking and sometimes movement. Trees can cause problems for nearby foundations in three primary ways:

1. By physical contact with the foundation

When there is physical contact between a tree and a foundation, the damage, if any, may vary greatly according to the species of tree, the site conditions, proximity to the foundation, the type of foundation, the portion of the tree contacting the foundation, and the tree’s size. The tree species is important because a larger, older, longer-lived, shade tree is a greater concern than a smaller, younger, shorter-lived ornamental tree or shrub.

This is due in part to the extent of the root system involved with shade trees. Most trees have root systems contained within the top 4 to 5 feet of soil, with the absorbing roots in the top 12 inches. The roots can extend outward beyond three times the height of the tree. Thus, a tree with a height of 50 feet, could have roots that grow outward 150 feet from the trunk. If your tree has limbs near your house, you are likely to have roots growing under or along your foundation. If you do not water around the foundation, research shows that roots will have a tendency to congregate and develop more rapidly under the foundation simply because of the increased amount of available moisture underneath.

The type of foundation is important because, as a general rule, a pier-and-beam foundation can tolerate nearby trees better than a slab. To understand, consider the portions of the tree that contact the foundation and the forces exerted. A root, root crown, or root flare running under a foundation can exert an upward pressure as the contacting portion expands in diameter. The root, root crown, root flare, or trunk, can also exert an inward pressure if they contact the outer perimeter of the foundation as they expand in diameter. Since a pier-and-beam foundation gains its support from piers rather than just concrete on top of soil, they can more readily sustain minor pressure from nearby trees. If a large shade tree is 5 feet from a foundation, physical contact and pressure exerted against the foundation is likely. If the tree is 15 feet away, physical contact is likely and pressure exerted against the foundation is possible. If the tree is 30 feet away, physical contact is still possible, but not as likely to exert much pressure against the foundation.

2. By affecting the moisture content of the soil under or near a foundation

Trees can affect the moisture content of the soil under and around foundations. A large shade tree can transpire as much as 200 gallons of water per day. Trees “transpire” or release water into the air in the form of vapour. The water for transpiration must come from the soil by way of the roots. This is why trees are known to “pull” water from under a foundation, but only when water in the surrounding soil is inadequate or unavailable. Since water naturally moves through the soil from high moisture areas to low moisture areas, simply not watering at all can cause foundation problems regardless of nearby trees. Trees can add to the problem but are usually not the primary cause of moisture loss from soil.

3. By causing air gaps and shifting soil due to decaying roots under or near a foundation (when a tree dies or is removed)

When trees have died or have been removed, the roots that remain under or near a foundation can create problems. As the root system begins to rot and decay, air spaces and gaps occur in the soil to replace the roots. This results in a soil that can shift to fill the void. Many years ago, foundation professionals would see a larger shade tree that is close to the house and automatically recommend removal. Today, research shows that the removal of the tree is not always the answer. Since many trees and foundations have existed together for many years, any changes that occur do so over time and they can be minor, compared to the removal of the tree. If a large shade tree, that is close to a foundation is removed, more dramatic changes can occur to the soil and foundation than if the tree was left alone, in most cases. Consideration must be given to the full, long term effect of removing an existing tree near a foundation.

The roots of all vegetation take water from the soil to make good the water lost from the leaves. If the soil contains clay it will shrink as it is dried or swell if it is allowed to rehydrate. If the shrinking or swelling extends below the foundations, the foundations will subside or heave respectively. If the movements are in excess of those that can be tolerated by the building, damage is likely to occur.

Damage is best avoided by increasing foundation depth to below the level where significant changes in moisture content are likely to occur.

The extent of risk depends on the

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The nature and depth of the foundations is a particular consideration. The species of tree, the distance between the building and the trees, the extent of the root systems and the type of soil all affect decisions about footings.

The aim is to ensure there’s no risk to the structure as a result of heave during wet winters or settlement during dry summers. It’s sobering to reflect that problems associated with trees account for the highest number of home insurance claims annually.

Reach out to us at?OraLabora?to see how we can assist.