What is tree subsidence? 

Subsidence happens when the ground beneath a building's foundations gradually sinks downwards unevenly, causing the structure above to move and crack. Tree subsidence is a specific type of subsidence that’s caused by tree roots drawing moisture out of clay soil. When soil loses moisture, it shrinks. If this shrinkage beneath a property’s foundation is uneven the house can settle at different rates in different places. Over time, this can lead to structural damage such as cracking.  

Tree subsidence differs from other causes of subsidence such as leaking drains because it’s triggered by a natural biological process rather than by an external influence. This is important as it helps identify the most appropriate remediation works. 

How trees cause subsidence in properties 

Clay-rich soils naturally expand when wet and contract when dry. Trees exaggerate this shrink-swell process because their roots extract large volumes of water, especially during hot dry months when moisture is scarce. How much risk a tree can pose to a nearby property, depends on several factors. 

Prolonged periods of drought accelerate the risk of tree-related subsidence because soil moisture is drawn out faster than it can be replenished. 

Signs of tree subsidence 

Some of the most common warning signs to watch out for include: 

• Diagonal cracks that are wider at the top than the bottom

• Cracks that appear around doors and windows

• Doors and windows that suddenly begin to stick or become misaligned

The key thing is timing. If you notice any of these signs during long spells of dry weather and there is a large tree nearby, then it’s more likely that the damage is occurring due to root-related soil shrinkage than general settlement. 

The key thing is timing. If you notice any of these signs during long spells of dry weather and there is a large tree nearby, then it’s more likely that the damage is occurring due to root-related soil shrinkage than general settlement. 

Which trees are most likely to cause subsidence? 

Some trees pose more of a risk than others. Larger trees with extensive, thirsty root systems such as oak, willow and poplar are generally higher risk, especially when planted in clay soils near a property. For a more detailed background of species and their relative risk, see our guide to the worst trees in the UK for subsidence. 

How to repair tree subsidence 

Traditional underpinning has been the go-to fix for subsidence, but it’s often disruptive, costly and slow, with works typically requiring excavation around the foundations and lasting weeks or in some extreme cases, months. Modern technologies used by Mainmark, such as resin injection, root barriers or screw piling offer a faster, less invasive alternative, which minimises disruption to occupants.  

Does removing trees stop subsidence? 

It’s reasonable to assume that removing the tree will solve the problem, however, this can make it worse. When a mature tree is removed, the soil beneath it can quickly rehydrate and swell, causing ground heave. This can lead to other forms of structural movement that’s more damaging than the original subsidence.  

Another issue is that trees may be protected by a Tree Preservation Order (TPO) or are located within Conservation Areas. That means you must seek permission from your local authority before removing them. If you suspect a tree is causing subsidence on your property, you should always seek professional advice from an arborist or tree surgeon. 

Preventing subsidence from trees 

In instances where the tree must remain on the property, root barriers offer an effective, long-term solution. Root barriers are specially designed membranes made from copper, engineered fabric or heavy-duty plastic, which are installed underground to redirect root growth away from the property’s foundations. The roots are encouraged to branch out away from the foundation’s “zone of influence”, which is the area of soil most likely to affect a building’s support while the tree itself is left undisturbed.  

Mainmark specialises in the design and installation of tree root barrier systems and can install them to significant depths and lengths depending on the species and soil conditions.  

Getting professional advice 

Although subsidence caused by trees can sound alarming, the issue can be addressed and managed if spotted early. Mainmark's ground engineering specialists can visit your home to assess the ground conditions, identify the cause of movement and recommend the most appropriate solution. 

If you’ve spotted any of the warning signs or have any concerns regarding tree root subsidence, get in touch with our engineering team today.