A guide to protecting valuable trees from construction damage.
Thomas J. Story
Written byKathleen N. Brenzel and Julie Chai Updated On September 3, 2021
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During a remodel, bad things can and often do happen to trees. Among the mishaps we’ve observed recently in neighborhoods around the West: A lovely, large Japanese maple, dug up during the heat of summer, left for a month atop the soil with its rootball exposed, then planted just a few feet from a new foundation.
Young oaks, also dug up during summer, replanted next to a sidewalk; they died by fall. The tops of tall liquidambars lopped off to make way for an expanded roof line. Magnolia branches, broken by trucks, left hanging from stubs. And root zones cut in half to dig basements or trenches for computer cables.
Such damage is no surprise: When a 1950s ranch house is razed and replaced with a mini-mansion that fills much of a suburban lot, or a house is expanded outward, trees are often in the way. Designing a house addition to nudge or hug a large tree is not the answer; this solution does not allow for the tree’s future growth, or for its movement during strong winds.
The time to protect mature trees on your property, if you decide to save them, is during the planning stage, before construction starts. Otherwise, you could end up paying many thousands of dollars to replace trees that are injured or killed.
There are a number of steps you can take before and during a remodel to protect your trees. Following them will pay off in the long run; indeed, trees are your landscape’s most valuable assets, adding beauty and cooling shade around your house.
Save, Relocate, or Remove?
Before finalizing house expansion plans that may involve areas near established trees, consult an arborist certified by the International Society of Arboriculture (ISA). The arborist can evaluate the tree’s health, age, present and future size, and any hazards it may pose.
If the tree is healthy and in a strategic location such as a front yard, saving it may be worth the effort. If the tree is in the way of proposed construction, ask whether it should be removed, or, if it’s young and healthy, if it can be moved. Also ask the arborist how far any solid paving should be from the tree’s trunk to avoid suffocating the roots.
For a large, established tree such as an oak or redwood that will remain in its present location, avoid building within its root zone. “If you stay outside the canopy, we can almost assure the tree’s survival,” says arborist Jim Clark of HortScience in Pleasanton, California. As a guideline, allow at least 1 foot of space between the trunk and the structure for every inch of trunk diameter measured at 54 inches above soil level. To further protect roots, learn where any new underground lines will go, and if possible, reroute them away from trees. If it’s not possible for contractors to work outside the root zone, up to one-third of a healthy tree’s roots can be removed without severely harming the tree.
If you decide to remove a big tree, make sure you are allowed to. Without prior approval, some communities prohibit the removal of trees whose trunk caliper (diameter) is more than a certain number of inches.
Before Breaking Ground
It’s a good idea to have the arborist consult with your contractor to find out exactly where the construction will take place and how to coordinate the building process with the tree’s preservation. That way, the arborist can perform any pruning needed to allow clearance for moving heavy equipment onto the site.
Hang signs or otherwise prominently mark the trees to be saved so there’s less chance of them being seriously injured by trucks or heavy equipment.
If a tree is to be relocated, make sure the new site has well-drained soil, proper light exposure (whether sun or shade), and enough room to accommodate future growth of roots, canopy, and trunk. Ideally, the tree should be moved when it’s dormant, before buds break, then planted so that the rootball sits 1 to 2 inches above its previous grade level.
Water the transplanted tree deeply, making sure that moisture penetrates the original rootball, not just the soil around it. Allow a hose to trickle slowly and move it periodically over several hours. Or lay a soaker hose on the soil around the tree between its trunk and the drip line.
Any trees to be removed should be cut down and their stumps ground up (not pulled out) to avoid injuring roots of nearby trees.
During the Remodel
“Half of the tree is below ground, so it’s really important to (protect) the root system,” says Robert Tate, executive director of ISA’s Western Chapter.
When roots are compacted by heavy equipment or severed by trenching, chances are you won’t see the damage immediately. But injured roots are often unable to take up water, air, or nutrients, resulting in the decline and eventual death of the tree, even years later.
Disease organisms or pest infestations that enter unhealed wounds can also, in time, kill the tree.
To ensure that the tree comes through the remodel with its health intact, protect its roots, trunk, and canopy.
If heavy equipment must be moved over the root zone, cover the area from the trunk out to the drip line with a 12-inch-thick layer of wood chips, then top the mulch with interlocking sheet-metal plates or plywood sheets to minimize soil compaction.
Make sure your contractor knows your wishes regarding your trees and will convey them to workers; the best way to do that is to spell them out in the remodeling contract.
Leading Causes of Root Damage
Trenching. Digging trenches for utilities too close to the trunk can seriously injure or sever tree roots.
Solid paving. Nonporous paving under the canopy can prevent water and air from reaching roots.
Grade change. As little as a couple of inches of fill dirt, if it does not have good drainage, can smother roots.
Soil contamination. Spilling wet concrete, paint, and solvents within the root zone can poison the tree.
Compaction. Heavy equipment or materials such as pallets of pavers can squash a tree’s shallow surface roots.
The Value of Trees
It’s hard to put a price on beauty, but property values of homes whose landscapes include mature trees are 5 to 20 percent higher than those without them.
Located where they’ll shade windows from hot afternoon sun, mature trees can lower cooling costs by 10 to 40 percent.
One such tree can cool a house as well as five average room air conditioners running 12 hours a day. Trees planted to block winter wind can reduce heating bills by 10 to 50 percent.
