Root Heave: What It Is, Why It Happens, When To Act

If you have noticed the ground around a tree lifting, cracking or becoming uneven, there is a good chance you are looking at root heave. 

It is one of the most common tree-related problems affecting gardens and properties across the UK, and whilst it can sometimes be managed, it can also cause serious and costly damage if left unaddressed. Here is what root heave is, why it happens and what you should do about it.

What Is Root Heave?

Root heave is the upward movement of soil, paving, paths or other surfaces caused by the growth of tree roots beneath them. As roots expand in diameter over time, they exert significant upward and outward pressure on whatever lies above and around them.

This pressure can lift paving slabs, crack concrete, buckle tarmac and cause the ground surface to become increasingly uneven.

Root heave is distinct from subsidence, which involves the downward movement of the ground as soil shrinks or is displaced. Whereas subsidence causes surfaces to sink and crack downward, root heave pushes surfaces upward. Both can cause significant damage to hard surfaces and structures, but they require different approaches to resolve.

Why Does Root Heave Happen?

Root Growth Patterns

Tree roots do not grow straight down into the ground as many people assume. The majority of a tree's feeder roots grow relatively close to the surface, typically within the top 600mm of soil, where oxygen, water and nutrients are most readily available. 

These roots spread outward in all directions from the base of the tree, often extending well beyond the canopy above.

As these roots increase in diameter year on year, they gradually push upward against whatever is above them. Hard surfaces such as paving, concrete and tarmac provide resistance, but over time the force exerted by a growing root system is considerable and the surface will eventually give way.

Species Most Likely To Cause Root Heave

Whilst any tree has the potential to cause root heave if planted close enough to hard surfaces, certain species are more commonly associated with the problem. 

Trees with vigorous, shallow root systems (such as willows, poplars, silver birches and some varieties of cherry and plum), are particularly prone to causing surface disruption. Large, mature specimens of almost any species can cause problems if they are growing in close proximity to paved areas, walls or structures.

Proximity To Hard Surfaces

Root heave is most likely to occur where trees have been planted, or have self-seeded and established, close to hard landscaping. A tree planted several metres from a path or driveway may never cause a problem, whilst one planted immediately adjacent to a paved surface is likely to cause issues as it matures.

What Damage Can Root Heave Cause?

The most visible damage caused by root heave is to hard surfaces. Lifted and cracked paving slabs, buckled tarmac and uneven paths and driveways are all common outcomes. 

Beyond being unsightly, uneven surfaces can present a trip hazard and may affect drainage, causing water to pool in areas where the surface has been displaced.

In more serious cases, roots growing beneath or alongside walls and fences can undermine their foundations, causing leaning, cracking or partial collapse. 

Where roots grow beneath a building's foundations, they can contribute to structural movement, though this is less common than subsidence caused by moisture extraction.

What Can Be Done About Root Heave?

Root Pruning

In some situations, selectively pruning the roots responsible for the heave can provide a solution without requiring the removal of the tree. 

This is specialist work that must be carried out carefully to avoid compromising the tree's stability or health. Root pruning too close to the trunk or removing too large a proportion of the root system can destabilise the tree and create a safety risk.

Tree Removal

Where root heave is causing significant and ongoing damage, or where the tree is too close to structures for root pruning to be a viable long-term solution, removal may be the most appropriate course of action. 

Once the tree has been removed and the stump ground out, the affected roots will gradually decay and the pressure on surrounding surfaces will be relieved over time.

Installing Root Barriers

Where a tree is valued and removal is not desirable, root barriers can sometimes be installed to redirect root growth away from vulnerable surfaces. 

These are typically used during landscaping projects rather than retrospectively, as installing them around an established root system is complex and may not be fully effective.