Tree Roots vs. Foundations, Sewer Lines, Septic and Sidewalks

How do tree roots actually grow?

Outward, not downward. Colorado State University Extension describes most large roots as growing horizontally in the top 6 to 24 inches of soil, and the International Society of Arboriculture (ISA) notes that roots commonly spread many times the width of the crown and can extend farther than the tree is tall. Roots do not seek out pipes or walls. Purdue University Extension describes them as opportunistic: they grow where growing is easiest, in loose, moist soil, and when they meet a solid surface such as a pipe, curb or foundation they are typically turned sideways or up and over.

That explains most of what follows. Where a structure offers no moisture, air or soft soil, the root goes around it. Where a structure is leaking or cracked, the root moves in.

Can tree roots crack a foundation?

Purdue Extension's answer is that roots can damage a foundation, but only with an invitation. A root that meets sound concrete is redirected. If there is a breach or crack nearby, Purdue says, roots can and will exploit it in search of moisture. Purdue also notes that roots meeting the looser backfill soil beside a foundation can turn abruptly downward along the wall.

So a crack with a root in it is not proof that the root made the crack. Frost, settling, drainage and the age of the wall are all candidates, and a foundation contractor or structural engineer is the right person to say which. What an arborist can add is whether roots are present at the wall and whether they can be cut without destabilizing the tree. Purdue describes the method: careful excavation near the foundation to find the suspect roots, cutting them, and installing a barrier to discourage regrowth.

Do roots break sewer lines?

They enter sewer lines; they do not break sound ones. NC State Extension puts it directly: tree roots cannot create cracks in a sewer pipe, they can only exploit existing cracks or gaps, so roots inside a pipe mean the pipe was leaking before the roots arrived. ISA says the same thing from the utility side. New water and sewer lines made of steel or PVC are generally resistant to root damage, while older cement, clay or cast iron pipes that have degraded may leak, and that moisture attracts roots, which then penetrate and widen the cracks.

Older neighborhoods, in Missoula and the Bitterroot towns as anywhere, can still have pipe of those older materials under equally old trees. The practical conclusion is that removing the tree does not fix the pipe. A leaking line will draw roots from the next tree over, since, as NC State points out, roots can reach across a yard for moisture. The durable repair is to the line. NC State also explains why trees should not be planted directly over a sewer line: not because of root entry, but so the tree does not have to be cut down when the line needs work.

What about a septic drainfield?

Here the caution is real, and many properties outside the towns in Ravalli and Missoula counties are on septic. A drainfield is perforated pipe laid in shallow, moist, nutrient-rich soil, which is everything a root is looking for. The U.S. Environmental Protection Agency says tree and shrub roots can ensnarl and damage drainfield pipes, does not recommend planting trees or shrubs on the drainfield, and names native grasses and ground covers as the most appropriate planting over it.

EPA does not give one setback distance. Its advice is to plant trees an appropriate distance away and to ask a septic service professional what that is for your system and landscape. For exact local requirements, the county environmental health office that permits septic systems is the place to ask; this guide is general information. Thirsty, fast-growing species deserve the widest berth. Montana State University Extension lists poplar, aspen and willow among the trees that need the most water, and our guides to cottonwoods near houses and quaking aspen problems go into how those root systems behave.

Why do roots lift sidewalks and driveways?

This is the one place where the root itself does the damage. Iowa State University Extension explains that roots spreading near the surface simply grow under a sidewalk when they reach it. They start small, then thicken year after year, and that expansion pushes the slab up. The result is lifted panels, widened cracks and trip hazards.

Iowa State lists repairs that keep the tree: grinding down the raised edge, patching or ramping, re-pouring the walk so it rises over the roots or curves away from the trunk, switching to pavers, or building the new section on a thicker gravel base that encourages roots to stay lower. In the City of Missoula, boulevard trees sit in the public right-of-way and the city says all work on them requires a permit, so ask the Urban Forestry Division before anyone cuts roots. Our guide to Missoula tree permits has the details.

How much root can be cut safely?

Less than most concrete crews assume. Iowa State warns that removing major or buttress roots severs a large part of the root system and can lead to crown dieback or death, and can leave the tree structurally compromised and more likely to fail in high wind. Its guidance is to cut as far from the trunk as possible, avoid cutting roots over 2 inches in diameter, and make clean cuts with a saw instead of tearing roots with a backhoe.

Iowa State also gives a distance rule. Buttress roots should not be cut closer to the trunk than three times the trunk's diameter. Cut closer than that and damage becomes much more likely; at one and a half times the trunk diameter or less, it says removal is the more prudent option. A tree that has lost roots on one side should be assessed, as our guide to construction damage and root protection explains.

Do root barriers work?

Partly. Iowa State says barriers can direct roots downward and reduce the likelihood of lifting and cracking, but that they must be at least 18 inches deep with the top edge above the soil surface, and that roots may still find a way under. Treat them as a way to improve the odds with a new walk or a new tree, not as a guarantee.

When is removing the tree justified?

When the repair would require cutting so much root that the tree becomes unstable; when the tree is the wrong species in the wrong place and the conflict will repeat; when the tree is already in decline; or when it stands on a drainfield or directly over a line that has to be dug up. Iowa State makes the first point frankly: depending on the tree's condition, size and value, it may be better to remove it than to damage it during a sidewalk replacement and have it fail or die three to five years later.

It is not justified merely because a root was found in a pipe or near a wall. Our guide to deciding whether to remove a tree close to the house walks through that judgment.

How far from the house, pipes and pavement should you plant?

The sources give distances for some of these and not others. For pavement, Iowa State advises that a planting strip under four feet wide should not hold any tree, that small trees suit strips four to ten feet wide, and that large shade trees need at least ten feet. For gas lines, ISA notes that operators often keep the area directly over a pipeline and 10 feet to each side free of trees. For sewer lines, NC State argues that a few feet either way makes no difference to root entry; the point is to stay off the line itself.

For foundations, none of the sources we reviewed sets a single number, and we will not invent one. The sensible approach is to match mature size to the space available, which our list of the best trees to plant in western Montana helps with. More on these topics is collected in the Montana Tree Guide, and our tree services page describes how we assess a tree that is in conflict with a structure.

Call HJ Property Care & Tree Service at (406) 215-7319 or request a tree assessment and an ISA Certified Arborist will look at the tree with you.

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