Clay, roots and cracks: what St Albans homeowners should check before blaming the tree

A crack appears in a wall. There is a large tree in the garden. The two get connected, somebody suggests taking the tree out, and by the time an engineer gets involved the evidence has gone and the ground has started doing something else entirely.

This guide sets out the order to do things in. The short version: the tree is the last thing you touch, not the first.

Why this happens here in particular

St Albans sits on shrink-swell clay, and Hertfordshire is a recognised subsidence area for exactly that reason. Clay soils take up water and expand in a wet winter, then give it up and contract in a dry summer. The ground under a house is therefore not still, and it moves whether or not there is a tree within fifty metres of it.

Trees make that seasonal movement larger where they are close enough for their roots to be drawing from the same soil that supports the foundations. High water demand species make it larger still: oak, poplar, willow, elm and eucalyptus are the usual names, with lime, ash and sycamore behind them. A shallow Victorian footing on clay next to a mature oak is a genuinely different risk from a modern trench-fill foundation next to a birch.

But "clay plus tree plus crack" is a hypothesis, not a diagnosis. Plenty of cracks in this district are nothing of the kind.

What your crack might actually be

• Thermal and shrinkage cracking. New plaster, new render and modern blockwork all move as they dry and as they heat and cool. Hairline cracks that do not change over a year are usually this.
• Historic movement. Older houses have almost always moved at some point. A crack that has been painted over three times and has not opened since is a scar, not an injury.
• A leaking drain. A cracked drain run washing fines out of the ground under a footing causes localised settlement that looks exactly like tree-related subsidence and has nothing to do with the tree. This is far more common than people expect and it is cheap to check with a camera survey.
• Heave. The opposite of subsidence: ground swelling upward, often after a large tree has been removed and the soil it was drying out rehydrates over several years. Removing a tree can cause this.
• Genuine tree-related subsidence. Real, and worth taking seriously, but it needs proving rather than assuming.

The pattern that points at real movement: cracks wider at the top than the bottom, running diagonally, often stepping through mortar joints, wider than about 3mm, showing on both the inside and outside of the same wall, with doors or windows binding. Cracks that open in late summer and close again in winter are the seasonal signature.

What a claim actually needs

Here is the part almost nobody publishes. The council sets out what evidence it requires before it will consider a claim relating to one of its own trees, and the list tells you everything about how these are assessed:

Read that carefully and notice three things.

Twelve months of monitoring. Both level and crack width, continuous. That is a full seasonal cycle, because a single snapshot cannot distinguish seasonal movement from progressive movement. There is no way to shorten this, and any process that skips it is not a claim, it is an opinion.

DNA root identification. Roots found in a borehole near a footing have to be matched to a specific tree. Roots travel a long way and several trees can be candidates. This step is what turns "there is a tree nearby" into evidence.

Nowhere does it say fell the tree and see. That is not an oversight.

Why felling first is the expensive mistake

Three reasons, and each one is enough on its own.

It destroys the evidence. Once the tree is gone, the root investigation, the DNA matching and the arboricultural assessment all become far weaker or impossible. If there was a claim, you have just damaged it.

It can cause heave. A mature tree in clay has been drying that soil out for decades. Remove it and the clay slowly rehydrates and swells, lifting whatever sits on it. Heave damage is often more expensive to repair than the subsidence would have been, and it can take years to appear, by which point nobody connects it to the tree that came out.

The tree may be protected. If it carries a Tree Preservation Order or stands in one of the district's 19 conservation areas, felling it without consent is an offence carrying a maximum fine of £20,000 per tree. Subsidence is a reason to apply, not an exemption from applying. See our guide to Tree Preservation Orders.

The order to do things in

1. Photograph and date the crack. Something in frame for scale, a ruler or a coin. Repeat monthly. This costs nothing and is worth more than any opinion you will be given in the first month.
2. Rule out a drain leak. A camera survey is inexpensive and rules out one of the commonest causes.
3. Tell your insurer. Buildings insurance normally covers subsidence, and insurers want to be involved from the start. If you commission works first they may decline to cover them. This is a phone call, not a commitment.
4. Let the structural side lead. The insurer will usually appoint an engineer, and monitoring runs from there. If you are proceeding privately, the engineer comes before the tree surgeon.
5. Get the arboricultural report. Species, size, distance from the affected elevation, water demand, condition, and an honest view on whether the tree is a plausible contributor at all. This is our part, and it is written to sit alongside the engineer's work rather than argue with it.
6. Then decide about the tree. Which is often not removal. A managed reduction on a cycle can reduce water demand without triggering heave, and that is frequently the recommendation where the tree is otherwise sound.

If it is not your tree

If the tree belongs to a neighbour, the council or Hertfordshire County Council as highway authority, the evidence bar is the same but the process runs through them and, usually, through their insurer. You have a common law right to cut back roots and branches to your boundary, but doing so on a protected tree still needs consent, and severing a major root can destabilise the tree and land the liability on you. Get the position in writing before anyone digs.

What we will and will not tell you

We will tell you whether the species, size and distance make the tree a plausible contributor, and we will say plainly when they do not. We will tell you if the crack pattern does not look like subsidence at all, even though that means no work for us.

What we will not do is write a report to a conclusion you have already chosen. A report that bends is worthless the moment an insurer or a surveyor reads it, and it is your money that gets wasted, not ours.

More on what a report contains and who asks for one on our tree reports and surveys page.