When a tree is identified as the cause of clay shrinkage subsidence, the instinct is to remove it. It seems like the obvious solution: remove the cause, stop the damage. But on clay soil — which underlies much of Hertfordshire — removing a tree can trigger a new and equally damaging problem called heave.
This article explains the mechanism, the risks, and the safer alternatives.
What Is Heave?
Heave is the opposite of subsidence. Where subsidence is downward movement caused by soil shrinkage, heave is upward movement caused by soil swelling.
Here's what happens:
- A mature tree extracts moisture from the clay soil, keeping it in a dried, shrunken state
- The building's foundations have adjusted to this dried state over years
- The tree is removed
- Without the tree extracting moisture, the clay gradually rehydrates
- The clay swells, pushing the foundations upwards
- The upward movement causes cracking, distortion, and structural damage
The cruel irony is that heave damage looks almost identical to subsidence damage — diagonal cracks, sticking doors, sloping floors. But the direction of movement is reversed, and the repair approach needs to be different.
How Serious Is Heave?
Heave can be more damaging than the original subsidence, for several reasons:
| Factor | Subsidence | Heave |
|---|---|---|
| Direction of movement | Downward | Upward |
| Duration | Ongoing (seasonal) | Can continue 5–25 years |
| Magnitude | Typically 10–50mm | Can exceed 100mm |
| Insurance coverage | Usually covered | Often excluded or limited |
| Repair complexity | Well-established methods | More complex; may need piling |
The long duration is particularly problematic. Clay rehydration is a slow process — it can take decades for the soil to fully re-establish its equilibrium moisture content. During this time, the building continues to move.
When Should a Tree Be Removed?
Tree removal is only appropriate when:
- The tree has been confirmed by investigation as the cause of subsidence (not just assumed because it's nearby)
- A heave risk assessment has been carried out by a qualified engineer
- Engineering solutions (root barriers, underpinning, ground stabilisation) are not suitable
- The tree is dead, dying, or structurally dangerous regardless of subsidence
- Your insurer has been consulted and agrees
Never remove a tree on clay soil without professional advice. This includes your neighbours' trees — if a neighbour's tree is contributing to subsidence at your property, the correct approach is negotiation and professional tree management, not unilateral removal.
Better Alternatives to Removal
In most cases, tree management is preferable to removal:
Crown reduction — reducing the canopy volume by 30–50%. This significantly reduces the tree's water uptake while keeping the root system in place. The clay stays in its current state, and no heave occurs.
Pollarding — cutting branches back to the trunk. Suitable for certain species (lime, willow, plane). Reduces water demand dramatically.
Root barriers — physical barriers (usually rigid plastic sheet) installed vertically between the tree and the foundations. These prevent roots from extending towards the building while allowing the tree to continue extracting moisture from the other side.
Ground stabilisation — resin injection can stabilise the ground beneath the foundations, making them resistant to future clay movement regardless of what the tree does. This is often the most practical solution when the tree provides value (amenity, screening, conservation protection) but its roots are affecting the building.
Trees with Conservation Protection
In Hertfordshire, many trees are protected by Tree Preservation Orders (TPOs) or are within Conservation Areas. Removing or significantly pruning a protected tree without consent is a criminal offence.
If a protected tree is causing subsidence:
- Apply to your local council for consent to carry out works
- Provide evidence that the tree is causing damage (your subsidence survey report supports this)
- Propose the minimum intervention necessary (crown reduction rather than removal)
Councils generally grant consent for works to prevent structural damage, but they want to see evidence and proportionate proposals.
The Insurance Angle
Before any tree works:
- Notify your insurer. If you remove a tree without insurer approval and heave damage results, you may not be covered for the subsequent claim.
- Get it in writing. Any agreement about tree works should be documented.
- Consider future insurability. A property with a heave claim can be harder to insure than one with a subsidence claim.
What If a Tree Has Already Been Removed?
If you've recently removed a mature tree from clay soil and are concerned about heave:
- Monitor for signs — upward cracking (wider at the bottom than the top), doors lifting in their frames, humping floors
- Install monitoring — a structural engineer can fit tell-tale gauges across vulnerable areas
- Consider proactive stabilisation — resin injection or underpinning can be installed preventatively if the heave risk is assessed as significant
The sooner potential heave is identified, the more options are available.
Concerned about a tree near your property? Get expert advice →