Construction In The Critical Root Zone

Posted on Dec 28, 2015 by
Trees Protected Around Construction

Trees are such an essential part of our landscape. They not only beautify our world, but nourish us as well. They provide oxygen, fruits, and nuts, shade, and shelter for birds and small critters. So it behooves us to take the best care of them possible. This is especially true when trees are in a construction zone, whether it is new construction or a remodel job. Great care must be taken when a construction crew is working around trees. Particularly sensitive is the Critical Root Zone (CRZ) of the trees.

In many cities there are regulations in place to ensure the safety of trees within the construction zone. Sometimes a tree must be removed from a construction site, but generally it’s best to work around them. If a tree is already diseased or dying, this is a good time to remove it. Occasionally a tree can be removed and relocated to a more appropriate site.

What Is The CRZ?

Just what is the Critical Root Zone anyway? And why is it so important? How is it determined? The CRZ contains most of the tree’s roots that are vital to its health and survival. If the construction invades too far into the CRZ, the integrity of the tree may be at risk and the tree may become a hazard. If 30 percent of the tree’s CRZ is impacted, it could be deemed beyond probable recovery.

Before beginning any construction on a site that has trees, first locate the tree(s) and determine where the roots lie in relation to the building footprint. Between 90 and 95 percent of the tree’s root system lies within the top three feet of soil, with more than half of it being in the top one foot. Care must be taken to not damage them.

To determine where the CRZ is, measure the tree’s diameter at a height of four and one-half feet above the ground. Convert the measurement to inches and allow one to one and one-half feet of critical root radius for every inch of the tree’s diameter. Example: the tree’s diameter is 10 inches; therefore its critical root radius is 10–15 feet. Another calculation is to take the measurement in inches of the tree’s diameter and multiply it by 18. This will give you the radius of the Critical Root Zone. This is the minimum area to protect; a larger area is even better.

All CRZs must be so noted on the project plans so that review personnel can determine the impact construction will have on the trees. The tree type and size must also be noted. Any tree that must necessarily be removed will need to be replaced at some point.

Use construction fencing to enclose the tree and its CRZ before beginning any construction work at the site, including the moving in of materials and equipment. This precaution will protect the trees and save the construction boss problems later on. This fencing must remain in place until the construction is completed.

A tree in a construction zone will benefit from the CRZ being covered in a good organic mulch. This will go a long way to insulate the soil from the changes that will be taking place.

Construction crews may not perform any of the following within the CRZ:

  • Equipment parking
  • Spilling of any fuels, chemicals, or any other toxic or harmful substances
  • Grade alterations
  • Storage of any materials, debris, or fill
  • Washing of anything that contained concrete or cement

Place signs designating the area as a protected tree area approximately every 20 feet all around the protective fencing.

Some municipalities require the construction company to hire a Project Arborist to oversee the protection, alteration, or removal of trees on the site. If there are limbs that are potentially in the way, the PA must determine if they can be protected or if they will need to be removed. The PA is responsible for all limb removal.

The same is true for the tree’s roots. If there are roots that significantly interfere with the proposed construction, the PA will be the one to determine whether or not the root(s) can be safely cut back out of the way, or not.

The root must be cut with a sharp saw to ensure a clean, even cut. The remaining exposed root must be properly cared for by covering it with wet burlap or loam. Any roots that are left exposed must also be cared for by covering them with wet burlap, backfill, or mulch. The affected tree must be monitored carefully and watered weekly to ensure their health and preservation. Your municipality will have guidelines for you to follow in all these aspects. There are more guidelines for digging or tunneling to install utilities.

In some urban areas it is not possible to fence off a tree’s entire CRZ due to lack of space. In a case like that it is important to work with the PA to minimize the impact on the tree and work to preserve it if at all possible. Trees, especially young, healthy trees, may overcome construction stress and damage if they are taken care of properly both during and after construction.

It is a good idea to place a copy of your city’s preservation policy on the project site for continual reference. Initially it should be read, discussed, and understood at your construction crew’s meetings. Any damage or injury to any tree on the construction site should be reported to the Project Arborist immediately so that the PA can take steps to preserve the injured tree. Often, trees on a construction site will be monitored for several years after the construction is complete to evaluate the tree’s health.

Trees and construction equipment and crews can survive one another with some carefully planning and forethought. If the tree is treated as a valuable resource, then caring for it responsibly during the construction process will be considered an honorable duty rather than a nuisance. When the construction project is complete and the trees add to the aesthetic beauty of the property, it will be seen as an inconvenience well worth the extra effort.

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