Subsidence Cracks vs Normal Cracks

Your home has cracked. Now you’re trying to determine if you’re looking at a five-figure subsidence repair or a £50 filling job. The difference matters enormously, both financially and for your home’s value.

Learning to distinguish subsidence cracks from normal cracking saves panic and helps you act appropriately when genuine problems develop.

Pattern Recognition

Subsidence creates diagonal cracks wider at the top than the bottom. They run through walls at roughly 45-degree angles, often following the mortar between bricks in a stepped pattern. These cracks typically start narrow at ground level and widen as they rise.

Normal settlement cracks run vertically along weak points. You’ll find them at doorframes, window corners, and where walls meet. They maintain consistent width throughout their length rather than tapering.

Thermal movement causes vertical cracks that open and close seasonally. They appear wider in winter when materials contract, narrower in summer when expansion occurs. Measure them across seasons to identify this pattern.

Location Provides Clues

Subsidence affects areas closest to the foundation problem. Cracks concentrate on one corner of a building if tree roots desiccate clay under that section. They appear along one wall if a drain leak has washed away supporting soil beneath it.

Normal cracks distribute randomly across the building. They appear at stress points regardless of foundation condition. You’ll find them scattered across different elevations and walls without forming patterns that relate to ground conditions.

Ground floor external walls show subsidence damage first. The problem starts underground and works upward. Upper floors might crack later as movement progresses, but initial damage concentrates low down where foundations have dropped.

Width Measurements

The Building Research Establishment classification system helps:

– Category 0: Hairline cracks under 0.1mm – aesthetic only
– Category 1: Fine cracks up to 1mm – easily filled
– Category 2: Cracks up to 5mm – need monitoring
– Category 3: Cracks 5-15mm – structural concern
– Category 4: Cracks 15-25mm – urgent repairs needed
– Category 5: Cracks over 25mm – may need partial rebuild

Subsidence typically produces Category 3+ cracks. Normal settling rarely exceeds Category 2. This guideline isn’t absolute, but it provides useful reference points.

Measure crack width at the widest point using a crack gauge or ruler. Check multiple locations along each crack. Record measurements monthly to track progression.

Speed of Appearance

Subsidence cracks develop over weeks to months. You notice them getting progressively worse. Last month’s hairline crack is 3mm wide today and shows no signs of stabilising.

Settlement cracks appear quickly then stop growing. A crack opens during initial settling, reaches its maximum width within days or weeks, then remains static. Six months of stability indicates the issue has resolved itself.

Seasonal cracks change width predictably. Summer heat closes them partially. Winter cold opens them wider. Annual cycling without progressive worsening suggests thermal movement rather than subsidence.

Internal and External Correlation

Subsidence creates matching cracks on internal and external wall faces. Check both sides. Finding corresponding cracks in identical locations confirms through-wall damage affecting the structure itself.

Surface cracks affect only plaster or render. External walls might look perfect whilst internal plaster shows extensive cracking. This pattern indicates plaster problems rather than structural movement.

Cavity wall construction can show external cracking without internal symptoms temporarily. The outer leaf cracks first. Internal damage appears later as movement progresses through the wall tie system.

Associated Symptoms

Doors sticking suddenly in previously functional frames suggest subsidence. The building has moved enough to throw frames out of square. You need to lift or push doors to close them properly.

Windows becoming difficult to open accompany subsidence. Gaps appear around frames where they’ve pulled away from surrounding masonry. These gaps often match the direction of diagonal cracks visible on the same wall.

Floors developing noticeable slopes indicate foundation problems. Place a spirit level across floors in multiple directions. Drops exceeding 10mm over 2 metres suggest structural movement.

Rippling wallpaper or tiles cracking without obvious cause shows wall movement. The wall surface deforms slightly as foundations shift. Decorative finishes can’t accommodate this movement and fail.

Testing for Active Movement

Tell-tales cost £10-15 from builder’s merchants. These glass or plastic strips bond across cracks. Continued movement breaks the tell-tale, providing clear evidence of active subsidence.

Install tell-tales in multiple locations across major cracks. Leave them for at least three months, ideally six. A single broken tell-tale might indicate minor adjustment. Multiple failures across different walls confirm systematic movement.

Photograph cracks monthly with a ruler for scale. Date all images. This visual record documents progression more compellingly than written descriptions. Insurers and surveyors value photographic evidence.

Subsidence Risk Factors

Clay soils shrink and swell dramatically with moisture changes. Properties built on clay face higher subsidence risk, particularly during extended dry periods. Trees extract soil moisture, causing clay to contract and foundations to drop.

Trees within 20 metres of buildings on clay soil create subsidence risk. Poplars, willows, and oaks have particularly aggressive root systems. These species can affect buildings up to 30 metres away in extreme cases.

Drain leaks wash away soil supporting foundations. A small leak might remove tonnes of material over months or years. Foundations undermined by soil loss drop suddenly once support disappears.

Properties built near slopes face different risks. Soil creep downhill can destabilise foundations. Retaining walls failing allow soil to shift, removing support from buildings above.

When Normal Cracks Mimic Subsidence

Lintel failure above windows creates diagonal cracks similar to subsidence patterns. The crack radiates from the window corner but relates to inadequate support above the opening rather than foundation movement.

Wall tie failure in cavity walls produces bulging and cracking. Corrosion causes ties to expand, forcing the outer leaf away from the inner leaf. The resulting cracks look concerning but stem from wall construction rather than ground movement.

Thermal movement in long walls without expansion joints generates impressive cracks. Buildings expand and contract with temperature. Long uninterrupted walls can’t accommodate movement and crack at weak points.

Professional Diagnosis Methods

Structural engineers assess crack patterns, measure levels, and investigate causes. They use precise levels to determine which areas have dropped. They check for tree roots, drain leaks, and soil problems.

Trial pits excavated next to foundations reveal foundation depth and condition. Engineers inspect soil bearing capacity and check for voids where material has washed away. This invasive investigation provides definitive answers.

Drainage surveys using CCTV cameras identify leaking pipes underground. Blocked or broken drains often cause subsidence by saturating and washing away soil. Fixing drains halts further movement.