Facade Cracks: Diagnosis, Causes and Repair

A crack on your house facade is the question that keeps you up at night: is it just the render that has dried and shrunk, or is the house splitting in two due to foundation settlement? Between hairline crazing that is purely cosmetic and a large through-crack letting in daylight between two concrete blocks, there are four normalised categories (NF P 95-103), four diagnostic methods and four very different repair strategies. This guide teaches you how to distinguish a benign crack from an active one, how to measure its evolution with a crack width gauge, and how to choose between simple filling, reinforced crack bridging and structural expertise.

The four types of facade cracks

The NF P 95-103 standard (repair and reinforcement of masonry structures) classifies cracks by their opening width — the width measured perpendicularly to the line of the crack. Each category calls for a distinct treatment.

1. Crazing (hairline cracks, < 0.2 mm)

Crazing is recognised by its tight network of fine crackles, often in a star or spider-web pattern, visible only when the facade is wet or in raking light. Width invisible to the naked eye (< 0.2 mm), depth limited to the finishing render.

Cause: drying shrinkage of the render, particularly on a monocouche render applied too thick, too wet, or in excessively hot weather. Crazing appears in the 6 to 24 months following application.

Severity: purely cosmetic. No structural risk, no water ingress (the depth does not reach the substrate). Treatment is optional — a breathable microporous paint or a siloxane coating is enough to make crazing invisible.

2. Fine straight cracks (0.2 to 2 mm)

A single crack or small group, rectilinear, of consistent width between 0.2 and 2 mm, sometimes crossing the full depth of the render but generally stopping at the face of the masonry substrate.

Possible causes:

• Thermal variation: the south-facing facade heats to 50 °C in summer and drops to -5 °C in winter. Differential expansion between materials (concrete, render, stone) creates stresses that release as cracks.
• Late substrate shrinkage: concrete continues to shrink for 2 years. On new builds, this is the number one cause.
• Absence of expansion joints: a wall longer than 25 m without an expansion joint will crack systematically.

Severity: monitor. A fine crack may remain stable for 20 years or develop within 6 months. Diagnosis requires a crack width gauge (see below).

3. Stepped cracks (following the joints)

A crack that follows the block layout of the masonry: a vertical/horizontal zigzag exactly tracing the mortar joints of concrete blocks, bricks or stone. This is the pathognomonic sign of differential settlement — one part of the wall is sinking faster than another.

Causes:

• Undersized foundations relative to the load
• Clay soil that swells and shrinks with the seasons (clay shrinkage-swelling phenomenon, very common in France)
• Leaking pipe eroding the soil beneath one part of the house
• Tree root dehydrating the soil (poplars, oaks within 15 m)
• Recent excavation on an adjacent plot destabilising the underlying fill

Severity: structural expertise is mandatory. A stepped crack wider than 2 mm or an active crack must be referred to a structural engineer. If the cause is climatic (drought), the French CatNat (natural disaster) procedure can be triggered via your local town hall — relevant if you are building in France.

4. Large structural cracks (> 2 mm, often > 10 mm)

A large structural crack is wide (> 10 mm according to the standard), often through (you can see light passing through the wall), active (visibly opening from one season to the next). The line is diagonal, sometimes in a Y shape around openings, with a visible offset.

Causes: structural failure — sudden settlement, major foundation defect, excessive load above a lintel, earthquake, localised collapse. Sometimes also extreme clay shrinkage-swelling following a summer of drought (increasingly frequent in France since 2015).

Severity: URGENT. Large structural cracks signal an active structural problem. Temporary propping, immediate expertise, insurance mobilisation (structural damage cover if house < 10 years old, contractor’s decennial guarantee, CatNat if recognised drought).

The causes of facade cracks

Before repairing, you need to identify why the cracking occurred. Five main families of causes cover 90 % of cases.

Shrinkage and drying (cause #1 in new builds)

Concrete and mortars lose approximately 0.5 to 1 mm per linear metre during drying, which extends 12 to 24 months. On a 10 m facade, this represents 5 to 10 mm that must “go” somewhere — into expansion joints if they exist, otherwise into fine cracks.

Thermal variations

Exterior render undergoes thermal swings of 50 to 70 °C between winter nights and summer days. Materials expand (concrete: 0.01 mm/m/°C), contract, and each annual cycle fatigues the assembly. South- and west-facing facades crack three times more than north-facing ones.

Differential settlement (cause #1 in structural pathology)

If one part of the house rests on more compressible soil than another, it subsides further. The wall cracks at the boundary between the two zones. A proper G2AVP ground investigation before construction drastically reduces this risk.

Clay shrinkage-swelling

Clay soils lose up to 10 % of their volume during dry periods and regain it during wet periods. Shallow foundations move in step with the seasons. In France, 48 % of the territory is classified as “high” or “medium” risk — a G2 ground investigation has been mandatory since 2020 in these zones.

Connection defects and stress concentration points

Cracks almost always appear at opening corners (at the top and bottom corners of windows), at the junction of two materials (masonry/concrete, render/insulation), or at the junction of a new wall with existing construction. These zones concentrate stress.

Diagnosis: measuring and monitoring

A stable crack and an active crack are not repaired the same way. Diagnosis consists of measuring the opening at time T then monitoring its evolution over several months.

Measuring the opening with a gauge

Three tools, from the simplest to the most precise:

The crack width card is a transparent graduated rule, applied directly over the crack. Dated photograph, the reading is archived.

The tell-tale crack gauge (VectorFit type) consists of two translucent overlapping plates, fixed on either side of the crack. A millimetre grid allows you to read the evolution in two axes (opening + shear) at each visit. This is the professional method used by structural surveyors — also recommended by the Royal Institution of Chartered Surveyors (RICS) for crack monitoring.

The plaster tell-tale is the traditional technique: apply a strip of fresh plaster 10 cm x 3 cm across the crack. If the plaster cracks in the weeks that follow, the crack is active. Simple, nearly free, binary (open/closed, without measurement).

Monitoring period

A crack must be observed over at least one complete annual cycle (cold winter + hot summer + dry season) before making a judgement. Monthly readings with the gauge, dated photograph in consistent lighting.

• Stable crack over 12 months: variation < 0.3 mm → cosmetic repair possible
• Oscillating crack (opens in winter, closes in summer): purely thermal origin, reinforced bridging
• Continuously opening crack: structural origin, seek expertise

Decision tree

Repairing a surface crack (DIY)

For a stable, fine (< 2 mm) and non-active crack, DIY repair is accessible to any self-builder. Allow half a day for a 3 m run.

Tools and materials

• Triangle scraper or angle grinder with fine diamond disc
• Wire brush, site vacuum
• Flat brush and microporous roller
• Acrylic sealant for static cracks (Sika AcryliFlex, Parexlanko)
• Fibred exterior filling render (Toupret, Polyfilla Facade)
• 8–10 cm stainless steel filling knife + sponge float
• Microporous facade paint or siloxane

Steps

1. Open the crack in a V with a scraper or angle grinder to 5–8 mm depth and 3–5 mm width. The aim is to create sufficient section to accommodate the sealant.
2. Dust thoroughly with a wire brush then the vacuum. No residual dust, otherwise adhesion fails.
3. Dampen slightly with a brush (on concrete or clay substrates: dry = too absorbent).
4. Inject the acrylic sealant with an extrusion gun, filling the full groove. Smooth with a wet finger or spatula.
5. Leave to dry 24 h at 20 °C (longer in cold or damp weather).
6. Apply fibred filling render in two coats, each with a stainless steel knife, the second 4–6 h after the first. Sand after 24 h drying.
7. Microporous facade paint in two crossed coats, diluted on the first pass. Observe temperature limits (8–25 °C, not within 48 h before rain).

Repairing an active crack by reinforced bridging

For an active crack (seasonal oscillation) of non-structural origin, reinforced crack bridging is the professional solution suited to self-build. The principle: an elastic sealant absorbs the movement, a reinforcement mesh distributes the stress over 10 cm on each side of the crack. Service life: 15–20 years.

Specific materials

• Elastomeric sealant polyurethane (Sikaflex-11 FC+) or SMP (MS Polymer) — elongation at break > 400 %, price £8–15/cartridge
• Fiberglass reinforcement mesh (4 x 4 mm weave, 100 mm wide) — 50 m roll: £30–50
• Embedding render type Weberthan or Parexlanko Armabande
• Universal bonding primer

Steps

1. Remove the existing render over 10 cm on each side of the crack, down to sound substrate.
2. Open the crack in a V as for simple filling.
3. Inject the elastomeric sealant bead to the full depth of the groove. Smooth flush with the substrate.
4. Apply bonding primer over the entire stripped zone.
5. Spread a first coat of embedding render (3–4 mm) over 20 cm width.
6. Bed the fiberglass mesh: unroll the mesh centred over the crack, press with a stainless steel float chasing out the air. The mesh must be fully embedded in the render.
7. Second coat of render to fully cover the mesh. Smooth, feather into the existing render.
8. Finishing render floated or scraped to match the existing facade texture.
9. Microporous facade paint two coats.

When to call an expert

Six warning signs require calling a professional without delay:

1. Crack > 2 mm measured with a gauge
2. Through crack (visible from inside or letting light through)
3. Stepped crack on a load-bearing wall
4. Y-shaped crack around an opening with visible offset
5. Rapid development — opening of more than 1 mm in 6 months
6. Multiple cracks appearing simultaneously on several facades

Which professional?

Available insurance cover

• Structural damage insurance (dommages-ouvrage, France): mandatory from completion, covers structural failure for 10 years. Claim notification by recorded post within 5 days.
• Contractor’s decennial guarantee: parallel recourse if the house was built by a company. See decennial guarantee.
• Guarantee of perfect completion: in year 1, the contractor must remedy reported defects free of charge. See completion and biennial guarantees.
• Home insurance — natural disaster cover (CatNat): triggered if a natural disaster order is published in the French Official Journal (drought, earthquake, ground movement). Excess of €1,520 for private individuals.

Indicative repair costs

2026 price range (French facade contractor)

A new house with two stable fine cracks can be repaired for €100–200 in materials as a self-build. A confirmed settlement requiring underpinning with micro-piles regularly exceeds €100,000 — hence the critical importance of a preliminary ground investigation and structural damage insurance.

Prevention and maintenance

Four measures drastically reduce the risk of cracking:

1. G2 ground investigation before construction — mandatory in clay zones (48 % of French territory). Budget: €1,500–3,000 upfront, potentially saving €100,000 in remediation.
2. Respect concrete curing times: 28 days for full set, 6 months before exterior render on new substrate.
3. Expansion joints every 25 m maximum on masonry, every 7–10 m on exterior render.
4. Water the perimeter during extreme drought (heatwave summer): a 1.5 m band around the walls maintains soil moisture in clay ground and limits shrinkage-swelling.
5. Keep water-hungry trees away (poplars, oaks, willows) at a distance of at least 1.5 times their adult height.
6. Annual inspection of existing cracks at the first spring (condition after winter).

Checklist before acting

Useful links

• To redo a full facade after remediation, see monocouche render application or traditional 3-coat render depending on your substrate.
• Before any foundation work, a G2AVP ground investigation is the best insurance against cracking.
• Understand when and how to install expansion and movement joints to prevent cracking in new builds.
• On the insurance side: decennial guarantee, completion and biennial guarantees and structural damage insurance.
• Foundation waterproofing limits differential movement linked to groundwater.
• Official resources: Agence Qualite Construction (AQC) for building pathologies, Georisques for clay shrinkage-swelling zoning and CatNat orders, CSTB for DTU 42.1 (facade waterproofing) and NF P 95-103, Sika and Parexlanko for technical sealants and renders, Royal Institution of Chartered Surveyors (RICS) for structural crack assessment guidance.