House Subsidence: Signs, Causes & What to Do

This guide covers everything a homeowner needs to understand about house subsidence in Ireland: what it is, how to recognise the signs, what causes it, how it is distinguished from normal settlement and heave, and what you should, and should not, do if you suspect your house is affected.

What Is House Subsidence?

Subsidence, in structural engineering terms, is the uneven downward movement of the ground beneath a building, causing the building's foundations to move differentially: one part sinking more than another. This differential movement is what causes the visible distortion in the building, such as cracks that are wider at the top than the bottom, doors that suddenly stick, and floors that slope.

Subsidence is distinct from settlement and heave. Settlement is the gradual, uniform compression of soil beneath a building's weight. Normal settlement occurs in all new buildings and is generally symmetrical, meaning the whole building settles evenly. Settlement is expected and, unless severe, does not cause structural problems. Heave is the upward movement of clay soils, typically when previously dry clay rehydrates, for example after a tree is removed. Heave can cause upward cracking in floors and can lift foundations.

The critical distinction is differential movement. Subsidence causes one part of a building to move more than another, creating the structural distortion that damages the building fabric.

Signs of House Subsidence

• Diagonal Cracking Near Door and Window Openings The most characteristic sign of foundation subsidence is a diagonal crack, running at approximately 45 degrees from the corner of a door or window opening. These cracks follow the line of least resistance in the wall structure, moving diagonally from the weakest point outward, and are typically wider at the top than the bottom, indicating that the foundation beneath the cracked area has dropped relative to the rest of the structure. A crack more than 3mm wide, roughly the width of a 10 pence coin, should be assessed by a structural engineer. Cracks wider than 5mm indicate significant movement.
• Cracks That Reappear After Being Filled If a crack has been filled or plastered over and reappears, particularly if it reappears in the same location and with the same orientation, this indicates ongoing movement. Historic subsidence (movement that has ceased) does not typically generate new cracking in the same location.
• Sticking Doors and Windows When a building's structure distorts due to foundation movement, door and window frames distort with it. Doors that previously operated normally may suddenly stick or jam at a corner, and windows may crack at the corners of the frame or refuse to open or close properly. If this is a new development rather than a longstanding issue with the building's age, it may indicate active movement.
• Sloping Floors A floor that was previously level but has developed a perceptible slope, particularly at the junction with an external wall, may indicate that part of the foundation beneath that wall has dropped. Sloping floors are more common in older properties on clay soils.
• Gaps Between Walls and Skirting Boards A gap developing between a skirting board and the floor, or between a door or window frame and the surrounding wall structure, indicates relative movement between these elements, consistent with foundation movement.
• Cracks Visible Both Inside and Outside A crack that is visible on both the internal plaster and the external masonry of the same wall indicates through wall movement, a more significant indicator than cracking in plaster alone.

What Is Not Subsidence

Most cracking in Irish homes is not subsidence. Understanding what to look for, and what to distinguish, is important before you conclude that you have a subsidence problem.

What Causes House Subsidence in Ireland?

Clay Soil Shrinkage: Tree Root Moisture Extraction

This is the most common cause of house subsidence in Dublin and the surrounding area. Dublin's southside is largely underlain by shrinkable clay soils. These soils expand when wet and contract when dry. During dry summers, trees and large shrubs extract moisture from the clay through their root systems, causing the clay to shrink and the ground surface to drop. Foundations sitting on this shrinking clay drop with it, causing differential settlement in the structure above.

This type of subsidence has a seasonal pattern: cracks open during dry summers and partially close during wet winters. Over time, if the cause is not addressed, the movement accumulates and the structural damage becomes permanent.

Leaking Drains: Sub-Base Erosion

A leaking drain beneath a building gradually erodes the sub base material supporting the foundation. As the fine particles wash away, voids develop beneath the foundation, and eventually the foundation loses support and drops. Leaking drains are a common cause of subsidence in older Irish housing where clay drainage pipes have cracked or lost their joints due to ground movement.

Drain related subsidence is typically less seasonal than tree root subsidence and may be faster acting if a significant leak develops. A CCTV drain survey is usually the first step where drainage is a suspected cause.

Poor Fill or Made Ground

Some properties in Ireland are built on sites that were previously filled, such as former quarries, excavated ground, reclaimed land, or simply poor quality fill placed during site preparation. Fill material that has not been properly compacted, or that contains degradable organic material, can settle or collapse unevenly beneath a building, causing subsidence.

Overloading

Adding significant loading to an existing structure, such as a heavy extension or a new floor on an existing building, without appropriate foundation strengthening can cause the existing foundation to settle under the increased load.

What to Do If You Suspect House Subsidence

• 1. Do Not Fill or Repair the Cracks Cracks are evidence: they show the direction, timing, and extent of movement. Filling them destroys this evidence and can prejudice any insurance claim.
• 2. Photograph the Cracks Document all visible cracks with a scale (ruler or coin) in shot, and note the date.
• 3. Contact a Chartered Structural Engineer A Chartered Structural Engineer is the professional qualified to assess whether what you are seeing is subsidence, determine the likely cause, and advise on the next steps. Contact Banrach Consulting Engineers.
• 4. Notify Your Insurer If you suspect subsidence, notify your home insurer promptly. Delayed notification can create complications with the claim.
• 5. Do Not Commission a Builder to Carry Out Repairs Until the cause is known and a structural engineer has specified the repair, any remedial work carried out by a builder may mask the problem and will not constitute a permanent fix.

Frequently Asked Questions

• What are the first signs of house subsidence?

  The first signs of house subsidence are typically diagonal cracks near door or window openings (wider at the top than the bottom), sticking or jamming doors or windows, and gaps developing between the skirting board and the floor or between window frames and the surrounding wall. See the first signs of subsidence for a detailed breakdown.
• Is all cracking in my house a sign of subsidence?

  No. Most cracking in Irish homes is cosmetic: hairline plaster cracks from shrinkage and thermal movement. Subsidence is indicated by diagonal cracks wider than 3mm near door or window openings, cracks that reappear after filling, sticking doors, or sloping floors. If you are uncertain, a Chartered Structural Engineer can assess the significance of the cracking.
• What is the difference between subsidence and settlement?

  Settlement is the normal, gradual, and generally symmetrical downward movement of a new building under its own weight. Subsidence is differential downward movement, meaning one part of the foundation moves more than another, causing the structural distortion that damages the building. Settlement is expected and generally harmless; subsidence requires investigation and remediation.