Foundation damage by trees is a potential contributing factor to overall structural distress. This is why the homeowner needs to perform tree maintenance to prevent foundation failure. Are you thinking about planting a new tree? If so, it is recommended for it not be placed closer than half the canopy diameter of the mature tree. Maintaing at least 20 feet from foundations is typicaly recommended.
Any trees in closer proximity should be thoroughly soaked on a regular basis. At least twice a week during hot summer months, and once a week in periods of low rainfall is usually enough. However, more frequent tree watering may be required. You can have a better idea of the ground moisture by using a landscape hydrometer.
Experts agree both the presence and removal of large trees close to residential structures can cause foundation distress. This problem is aggravated by cyclic wet and dry seasons in the area. Foundation damage we have seen caused by adjacent trees provides evidence of foundation movement. Believe it or not, as much as three to five inches can be experienced.
This condition will be more severe during periods of extreme drought. Sometimes the root system of trees such as willow, elm, or oak can physically move foundations. Doing so can adversely affect the walls and other components as well and cause considerable structural damage.
Root barriers can be installed near the exterior grade beams to limit or prevent root encroachment on the foundation. Discuss options with a certified arborist. If used root blockers are typically installed a minimum depth of 36-inches when trees are left in place near foundations.
Foundation Damage By Trees Removed Before The House Was Built
Foundation damage occurs by tree roots seeking moisture forming under concrete slabs on the ground. There it tends to dehydrate the soil. Subsoil swelling can occur for several years after a tree has been removed.
Removal might have been prior to house construction, during the useful life of the house, or if the tree dies. Studies have shown this process can last up to 20 years in areas where highly expansive clays are present. That can affect Site Drainage And Foundation Performance.
However, this process does not occur in the areas where sandy soil is present. In areas of expansive soil, the foundation for the house should be designed for the anticipated maximum heave. Alternatively, the site should be left alone for several years. Doing so allows the moisture regime where roots used to be to stabilize to the surrounding subsoil conditions.
Foundation Damage By Trees Removed Near Existing Structures
Removing trees near existing foundations can be a challenge. But it is necessary to prevent foundation damage caused by trees. The Wildcat Inspections Team recommends you contact a qualified arborist before doing so. Tree removal can be safe provided the tree is no older than any part of the house.
This is because the subsequent heave can only return the foundation to its original level. In most cases there is no advantage to a staged reduction in the size of the tree. That means the tree should be completely removed at the earliest opportunity.
It is not advisable to remove a tree at a foundation where expansive soils and other factors exist. One other condition is where the tree is older than the house. Another is where there are more recent extensions to the house. In those cases, there is an elevated danger of damaging heave. This is true unless the foundation was designed for the total computed expected heave. In the areas where non-expansive soils are present, no significant foundation distress should occur because of the tree removal.
Tree Removal Might Not Be An Option
It is common knowledge tree roots can damage a foundation. The question is how to prevent foundation damage by trees. There are other tactics to prevent foundation failure when tree removal might not be an option.
One choice is root pruning. The pruning process is usually performed by excavating a trench between the tree and the damaged property. The trench should be deep enough to cut most of the roots. It is imperative for the trench not be so close to the tree it jeopardizes its stability. Otherwise, the action could wind up with you in court. The tree will grow new roots over time to replace those cut when correctly done by a professional. However, in the short term there will be some recovery as the degree of dryness in the soil under the foundations reduces.
A return to a normal weather pattern may prevent further foundation damage by trees where the damage only appeared during dry weather. In the meantime, keep it well watered.
You may find there is opposition to the removal or reduction of an offending tree. For example, it may belong to a neighbor or the local authority. It might even have a Tree Preservation Plan filed on it. Permission from the local authority is required before pruning the roots of a tree with a preservation plan on it. In such cases there are other techniques that can be used within your own property.
Other Options To Prevent foundation Damage By Trees
Maintenance to prevent foundation damage by trees is especially important where clay soils exist. In the areas where excessive heave might occur with tree removal some kind of pruning, such as crown thinning, crown reduction or pollarding should be considered. Pollarding is often mistakenly specified. Pollarding is where most of the branches are removed, and the height of the main trunk is reduced. This mistake is because much published advice links the height of the tree to the likelihood of damage. In fact, the leaf area is the most important factor.
Crown thinning or crown reduction, in which some branches are removed or shortened, is therefore generally preferable to pollarding. Pruning should be done in such a way as to minimize the future growth of the tree without leaving it vulnerable to disease (as pollarding often does) while maintaining its shape. This should be done only by a reputable tree surgeon or qualified contractor working under the instructions of an arbor culturist.
Root barriers are a variant of root pruning. You should not simply fill the trench with soil after cutting the roots. There are a couple of options for filling the trench. One is to fill it with concrete. Another is to line it with an impermeable layer to form a “permanent” barrier to the roots. Some barriers have a life expectancy of about 20 years. Also, the roots might be able to grow around or under the trench and barrier. Root barriers serve as a bio barrier root control system and appear to perform satisfactorily. The design of the root barrier system should be developed with a qualified arborist or geotechnical engineer. That way you can assume long-term performance of the structure if tree maintenance is observed to prevent foundation failure.
