Why do cracks matter when buying a house?
Cracks matter because the trivial and the serious kinds look similar at a viewing, and the serious kind is expensive. Almost every house has cracks, and most are trivial.
Subsidence repairs can run to five figures (figures given when this guide was first written), raise insurance premiums for years and complicate resale.
Buyers tend to make one of two opposite mistakes. Some panic at harmless shrinkage crazing. Others accept that "old houses all do that" and exchange contracts on active movement. Both come from not knowing which features of a crack signal a problem.
What is the difference between settlement, subsidence and heave?
Settlement is a building bedding down, subsidence is the ground beneath it sinking, and heave is the ground swelling and pushing it up.
- Settlement: the building beds down as the ground adjusts to its weight. It is most active in the first years after construction or extension, then generally stops. Historic settlement cracks are common, stable and rarely significant.
- Subsidence: the ground beneath the foundations loses volume and sinks, taking part of the building with it. The classic causes are shrinkable clay drying out, leaking drains and tree roots extracting moisture. Subsidence is progressive until the cause is dealt with, which is why it is treated far more seriously.
- Heave: the ground swells and pushes the building upward. It typically follows clay rehydrating after a large tree is removed.
Seen cracks at a home you are buying? Get a survey price for the home you are buying.
What causes cracks in walls?
Most wall cracks come from shrinkage and thermal movement; a smaller number come from structural movement, failed lintels or bulging walls.
Shrinkage and thermal movement
New plaster, render and masonry all shrink as they dry, producing hairline cracks and surface crazing. Temperature cycles add thermal movement, particularly on south-facing walls and around large openings.
These cracks are typically under 1mm, stay superficial and reappear harmlessly after redecoration. Left open externally, their only real risk is letting moisture in.
Cracked render on older walls
Render cracks alarm buyers out of proportion to their meaning. Rigid cement render is often applied over older walls bedded in soft lime mortar. The two expand and contract at different rates, and that thermal mismatch cracks the render even though the wall behind is sound.
Poor mixing, application in bad weather and freezing trapped moisture add to it. Render cracking still needs assessment, because it can mask genuine movement. On its own, though, it is a maintenance item rather than a structural verdict.
Structural movement
Movement cracks follow the forces that made them.
- Vertical cracks often relate to settlement.
- Diagonal and stepped cracks climb the mortar joints like a staircase and point to one part of the building dropping relative to the rest.
- Cracking concentrated above windows and doors frequently means a failed lintel (the beam spanning the opening), not the ground at all.
Each has a different cause, cost and urgency. Pattern matters more than presence.
Why are bulging walls a warning sign?
A wall does not have to crack to be failing. Sight along an elevation from the corner and it may belly outward or lean at the top. Three mechanisms dominate:
- Wall-tie failure: the metal ties binding the two leaves of a cavity wall corrode and expand. The rust cracks bed joints at regular courses and lets the outer leaf bow away. It is endemic in pre-1981 housing with early steel ties.
- Roof spread: a poorly restrained roof pushes the wall tops outward.
- Thermal bowing: long, poorly tied elevations bow over decades.
Bulging is lateral instability. It can progress to partial collapse and always warrants assessment.
When should you worry about cracks in your house?
Worry, meaning investigate rather than panic, when width, pattern, location and progression together point to movement. Surveyors judge a crack by those four things read together.
| Width and pattern | Likely significance | Sensible action |
|---|---|---|
| Hairline crazing (under 1mm) in plaster or render | Shrinkage or thermal movement. Cosmetic, BRE categories 0 to 1. | Fill and redecorate. Seal externally to keep moisture out. |
| Fine cracks 1 to 3mm, vertical or around openings, stable | Minor movement, curing or early lintel wear. Usually manageable. | Note it; have the survey confirm it is historic before purchase. |
| Cracks 3 to 5mm, or stepped through mortar joints | Movement worth diagnosing: settlement, lintel failure, tree or drain influence. | Level 3 survey before commitment; monitoring for an existing owner. |
| Cracks 5 to 15mm, diagonal or stepped, inside and out | Possible subsidence or significant movement, BRE category 3 and above. | Level 3 survey, then a structural engineer if recommended. Owners should notify their insurer. |
| Cracks 15 to 25mm, distorted openings, sticking doors | Serious movement; sections may need rebuilding. BRE category 4. | Structural engineer through the surveyor's referral; investigate the cause. |
| Over 25mm, wider at the top, or a bulging or leaning wall | Severe, with a potential stability risk. BRE category 5. | Immediate professional assessment. Do not ignore or decorate over. |
The combinations that concern a surveyor are consistent:
- Stepped cracking in brickwork.
- Any crack wider than 3mm, or wider at the top.
- The same crack inside and out, meaning it passes through the full thickness of the wall.
- Doors and windows that have started sticking.
- Cracks near drains or mature trees on clay soil.
- Above all, progression.
How is historic movement told apart from active movement?
Monitoring tells them apart. Calibrated tell-tales (crack-width gauges) are fixed across the crack and re-read over months, usually through a change of season. Stable readings downgrade the concern; widening ones confirm active movement. The RICS consumer guide to subsidence explains the principle for homeowners.
What to look for when viewing
Look for patterns worth photographing, and ask the seller and the solicitor the right questions.
Signs worth photographing
- Stepped or diagonal cracks in external brickwork, especially near corners and openings.
- Cracks that would take the edge of a £1 coin (about 3mm), anywhere.
- Bulges, leans or horizontal cracking at regular courses, seen by sighting along each elevation.
- Doors and windows that stick, or frames visibly out of square.
- Fresh filler, new render bands or one recently redecorated wall, since repairs can conceal history.
- Large trees or drain runs close to the walls, particularly on clay soil.
Questions for the seller or agent
- Has the property ever had subsidence, heave, landslip or structural movement?
- Has there been an insurance claim for movement, here or next door?
- Has any crack repair, repointing, re-rendering or wall-tie work been done, and when?
- Have any large trees been removed near the house in recent years?
Documents to ask the solicitor for
- The property's insurance claims history for movement or subsidence.
- Any underpinning guarantee, monitoring records or engineer's reports.
- Building regulations sign-off for structural repairs, and any Certificate of Structural Adequacy issued after a claim.
Which cracks are lower and higher risk?
Fine, old and stable cracks in finishes are usually lower risk; stepped, wide, through-wall or growing cracks are potentially higher risk.
| Usually lower risk | Potentially higher risk |
|---|---|
| Hairline crazing in plaster, paint or render finishes | Stepped cracks through brickwork mortar joints, especially near corners |
| Fine vertical cracks at plasterboard joints or above recent extensions (early settlement) | Cracks over 3mm, wider at the top, or visible inside and out |
| Old, stable cracks with dust and paint in them, unchanged for years | Bulging, bowing or leaning walls; regular horizontal cracking (possible wall-tie failure) |
| Render-only cracking on an otherwise straight, true wall | Cracks that reopen after repair or keep growing (active movement) |
| Movement signs on clay soil with mature trees or old drains nearby |
No single sign decides it. A 5mm crack that stopped moving decades ago may be a footnote. A 1mm crack that appeared since the marketing photos were taken needs answering.
Which level of investigation does a cracked house need?
The survey comes first in almost every buying situation, and the engineer follows only when there is a defined question to answer.
| Situation | Sensible course |
|---|---|
| Hairline crazing or fine decorative cracks on a modern, conventional house | Proceed; a Level 2 Homebuyer Report will confirm nothing sits behind them |
| Old, stable, documented cracks with repair paperwork | Proceed with survey confirmation that the movement is historic |
| Stepped cracking, any crack over 3mm, or cracks inside and out | Level 3 Building Survey before committing |
| Bulging or leaning wall, or suspected wall-tie failure | Level 3 Building Survey with close attention to lateral stability |
| Survey confirms significant or active movement needing calculation | Structural engineer (typically CEng, IStructE) on the surveyor's referral |
| Cause suspected in drains, trees or clay shrinkage | Specialist investigation: CCTV drain survey, arboricultural advice or soil analysis |
An engineer answers a defined question with calculation, such as designing a repair, certifying stability or specifying monitoring. Until an inspection establishes the question, there is nothing to calculate. Most cracked houses never need one.
What to do next if a survey finds significant cracking
- Establish the mechanism: settlement, subsidence, heave, lintel failure, wall-tie corrosion or render movement.
- Establish whether it is active, from historic evidence, monitoring or both.
- Commission the further investigation the report directs: engineer, drainage CCTV, or tree or soil advice.
- Cost the repair properly, with real quotations for repointing, lintels, tie replacement or underpinning rather than guesses.
- Ask the solicitor about claims history, guarantees, certificates and what the insurer will need.
- Renegotiate, proceed or withdraw on evidence. A diagnosed, costed defect is a fair basis for a price reduction.
An example from practice
A buyer viewing a 1930s semi on London clay noticed a filled diagonal crack beside the bay window. The Level 3 inspection mapped the cracking as stepped and historic, with the paint over the repair unbroken. It also flagged a mature lime tree within six metres of the bay and an old clay drain running along the same side.
The surveyor recommended targeted enquiries rather than an immediate engineer's report: the claims history and a CCTV drain survey. The drains proved sound, and the paperwork showed a monitored, insurer-closed episode from a decade earlier. The buyer proceeded at a modest price adjustment.
What a survey can — and cannot — tell you
A building survey provides an informed assessment of the visible condition of a property. Inspections are non-intrusive and limited to areas that are accessible at the time of inspection.
Where cracking is a concern, the surveyor reads the building as a whole. A Level 3 report explains the likely mechanism, whether the movement appears historic or active, and what that means for cost and negotiation.
- Every elevation, sighted for bulge and lean.
- Crack patterns, mapped against openings, mortar joints, lintels, render, ground levels, drains and nearby trees.
- The same cracks traced internally through the wall thickness.
- Openings checked for distortion and floors for slope.
- The roof space checked for spread.
However, there are limitations to what a survey can determine without intrusive investigation.
- A survey is a visual inspection on a single visit, so it cannot measure movement over time.
- It cannot open up the fabric of the building.
- It cannot excavate foundations.
- Only investigation confirms subsidence, because other mechanisms mimic its crack pattern.
Where higher-risk indicators are identified, further specialist or intrusive investigation may be recommended.
Frequently asked questions
Are cracks normal in old houses?
Yes. Period properties on shallow foundations have usually moved, settled and been altered over a century or more, and stable historic cracking is part of that record. Old, dusty, paint-filled cracks are generally reassuring; fresh, clean cracks need explaining.
What do subsidence cracks look like?
Classically, diagonal or stepped cracks following the mortar joints, wider at the top, near corners and openings, and visible on both faces of the wall. They often appear or widen after dry summers on clay soil, sometimes with sticking doors and sloping floors.
Will insurance cover cracks or subsidence?
Buildings insurance normally covers damage from subsidence, heave and landslip, usually with a higher excess. It does not cover cosmetic cracking, wear and tear, or defects predating the policy. A subsidence history usually means higher premiums and specialist insurers.
Do sellers have to declare subsidence?
Sellers are likely to be asked about movement on the TA6 Property Information Form, and a seller who answers dishonestly risks a misrepresentation claim. The solicitor should raise it explicitly in writing, with the claims history and any repair guarantees, rather than the buyer relying on a verbal "no problems" at a viewing.
Who assesses cracks, a structural engineer or a surveyor?
In a purchase, the surveyor comes first. A Level 3 Building Survey reads the crack pattern in context and identifies whether an engineer's calculation is genuinely needed.
Is it sensible to buy a house with cracks?
Often, yes, on evidence. Most cracked houses are structurally fine, and a diagnosed, costed defect is a negotiating position rather than a reason to withdraw. Be wary where movement is active and unexplained, repairs lack paperwork, or a seller resists investigation.
