What subsidence really means in an older house

Subsidence is the downward movement of a building’s foundations, and in older houses it is rarely even. Victorian terraces, Edwardian semis and Georgian townhouses were often built on shallow foundations — sometimes just a metre deep — and they have been sitting in the same soil for a century or more. When the ground beneath one part of the house shrinks, shifts or washes away, that part sinks. The result is movement through the walls, which older brick and lime mortar buildings show in particular ways.

It helps to separate subsidence from settlement. Settlement is the small movement a new building makes as it beds in. Subsidence is later, progressive and usually uneven. Common triggers in older properties include clay soils shrinking in dry summers, tree roots drawing moisture from beneath foundations, leaking drains softening the ground, and old extensions or bay windows pulling away from the main house.

The cracks that should make you pause

Not every crack matters. Hairline cracks in plaster are part of life in an older home. The cracks that point towards subsidence tend to have a distinctive pattern and location.

• Diagonal cracks running at roughly 45 degrees, often stepping along mortar joints in brickwork.
• Tapered cracks that are wider at one end — perhaps 3mm or more — and narrow as they run.
• Cracks around openings, especially from the corners of windows and doors, where movement concentrates.
• Stepped cracks in external brickwork, following the zigzag of the mortar rather than cutting through the bricks.
• Cracks that appear suddenly or grow over a few months, rather than ones that have been there, unchanged, for years.

Measure any crack you are worried about. A crack wider than a 10p coin — around 3mm — is worth investigating. So is a crack that is visibly wider than it was last season. Take photographs with a ruler or coin beside the crack, date them, and keep a record.

Indoor clues: doors, windows and floors

Subsidence is not only about cracks. Movement in the foundations twists the whole structure, and that shows up in how the house works day to day. Doors and windows may start to stick, especially on the side of the house that is moving. A door that once closed with a gentle push might now need a shoulder, or catch at the top corner. Windows can jam, and cracks may appear in the plaster around their frames.

Look at the floors, too. Uneven floors are common in old houses, but a floor that slopes noticeably towards one wall, or a skirting board that has separated from the wall, is a stronger clue. Wallpaper may ripple or tear where the wall is moving. None of these signs proves subsidence on its own, but together they build a picture.

External signs to check around the building

Walk around the outside. Pay attention to the junctions where different parts of the building meet. Bay windows are a classic weak point in older houses: look for a gap or crack where the bay meets the main wall. Extensions and porches are similar. If an extension is pulling away from the original house, that joint will often open first.

Check the drains. A leaking or blocked drain can soften or wash away the soil under a foundation, and the resulting subsidence may be local to one corner. Look for damp patches, cracked drain covers, or subsidence in the path near the house. Trees and large shrubs close to the building are another factor, particularly on clay soils. A mature tree can remove huge amounts of moisture, and the effect is often worse in a dry summer.

Cracks that are probably not subsidence

It is easy to panic about every crack, so it is worth knowing the common false alarms. Fine vertical or horizontal cracks in plaster are usually caused by thermal movement, damp, or the natural drying out of old materials. A crack above a doorway or window may be a failed lintel or a rotted timber, rather than a foundation problem.

Older houses also move with the seasons. Clay soils swell in wet winters and shrink in dry summers, so a crack may open and close slightly over the year. That does not automatically mean subsidence, but if the movement is progressive — if the crack is wider this year than last — it needs proper attention. Do not simply fill and paint over a crack that is still moving. You will hide the evidence without solving the cause.

What to do if you suspect subsidence

Start by monitoring. Use a simple crack monitor, or a pair of glass tell-tales fixed either side of the crack, to see whether it is still moving. Record the date and the measurement. If you have any doubt, or if the crack is wider than 3mm, call a chartered structural engineer. They will inspect the house, assess the pattern of movement, and may recommend a drain survey, a soil investigation or monitoring over several months. A builder can carry out repairs, but is not the right person to diagnose subsidence.

If you are buying the house, do not commit to repairs until you have that advice. Speak to your insurer as well, because subsidence cover often has specific conditions and timescales. Many cases are manageable. Once the cause is identified — a leaking drain, a thirsty tree, a shallow foundation — the right repair can stabilise the house and protect its value. In an older property, look carefully, record what you see, and get expert eyes on it before you spend money on anything more than monitoring.