Root damage: warning signs

Root damage looks different from a drainage failure. Roots push a section of wall out of line near the tree, heave the soil on the tree side, and leave cracks radiating from the wall. A lean that affects the whole wall is more likely water; a bulge or displacement concentrated around a tree is likely roots.

What roots do to a wall

Roots grow where there is water, air and space. A gravel backfill behind a retaining wall offers all three, which is why roots find their way into the wall zone.

As a root thickens, it lifts the soil above it and pushes sideways. A wall in the way gets displaced, usually over years rather than months.

Roots also block drains. A root mass in the gravel can stop the perforated pipe, which produces the water pressure that causes a lean. So a wall can be damaged by roots in two ways at once, and the second is often the one that causes the sudden failure.

The signs

The local pattern is the key. Drainage problems tend to affect a length of wall; root problems tend to affect a point or a short section. That is the first thing to establish before anyone quotes a repair.

Local displacement versus a lean

A lean rotates the whole wall about its base. The top moves more than the bottom, and the joints open gradually along the length.

Root displacement moves a section outward at a specific height. The units above and below may be unaffected, which produces a step or a bulge rather than a uniform lean.

Measure both. A level against the face shows the lean; a straightedge along the wall shows the bulge. A photograph from each end of the wall on the same day each year makes the comparison much easier.

Soil heave on the tree side

Lifting soil on the tree side is one of the clearest signs. A root growing under the surface pushes the soil up into a mound.

The mound often has cracks radiating from it, and the wall face opposite the mound shows a matching bulge.

If the mound is on the wall side of the tree, the root is between the tree and the wall, which is the worst position. If it is on the far side, the root may be growing away. The mound also shows where a root barrier would need to go if the tree is kept.

Roots at the face

A root visible in a joint is a late sign. It means the root has grown through the wall or the backfill and reached the face.

At that point the root is already displacing the units around it, and the drain is likely affected. The repair has to deal with the root, not just the wall.

What to do before cutting

Cutting a large root can destabilize a tree, and a tree that dies or falls creates a bigger problem than the wall.

The sequence is deliberate:

1. Photograph the wall and note exactly where the displacement is.
2. Identify which tree the root belongs to and how far it is from the wall.
3. Ask an arborist whether the root can be cut without harming the tree.
4. Ask an engineer whether the wall needs a redesign around the root.
5. Install a root barrier during the rebuild if the tree stays.
6. Monitor the wall and the tree for a year after the work.

An arborist can assess whether the root can be cut without harming the tree, and an engineer can assess whether the wall needs a redesign around the root.

A root barrier installed during a rebuild can deflect future growth, but it has to be placed on the tree side of the wall and deep enough to matter.

When the damage is urgent

Root damage becomes urgent when a section of wall is displaced enough to lose support, when the drain is blocked and water is at the face, or when the tree is leaning toward a structure.

A wall that has lost a section of face has lost its ability to retain soil at that point. Keep people and vehicles away until it is assessed.

Any wall above the local height threshold with visible displacement should have an engineered assessment, whether the cause is roots or water.

Repair costs

Drainage and backfill at $5 to $15 per square foot is part of any repair. On a 100 square foot section at $10, that is 100 × $10 = $1,000.

A rebuild at $25 per square foot on the same section is 100 × $25 = $2,500.

Tree work is separate and depends on the size and access. A root barrier adds a line item to the rebuild.

For the repair methods, see root damage repair. For prevention, read root damage prevention, and for the cost detail, check the root damage cost guide.