THE SHORT ANSWER

A crack is a symptom, not a repair instruction. Check for movement, moisture and deeper damage before filling it. Photograph the crack with a ruler and date. A dry, stable cosmetic plaster crack may be suitable for compatible filler, followed by the specified cure, primer and paint. Growing or displaced cracks, cracking through masonry, sagging, or new sticking openings need professional assessment before cosmetic repair. Width alone cannot establish safety. If the building seems unsafe, keep people away and seek urgent professional help; do not wait to monitor it.

Regional note

This guide is written for South Africa. Always check local codes, product specifications, licences, and site conditions.

At-a-glance visual

What you see: the first sensible action

These are screening clues, not a structural diagnosis. A qualified professional must assess suspected movement; a photograph or a crack-width rule cannot certify a wall as safe.

01

Fine line in dry, firmly attached paint or plaster

No dampness, loose material or associated warning signs are apparent.

Next check

Photograph and monitor, then use a compatible cosmetic filler only once the surface is confirmed stable. Ask a competent professional if its depth or cause is uncertain.

02

Damp, blistering or crumbling plaster

Water stains, salts, mould, lifting paint or hollow and loose plaster accompany the crack.

Next check

Trace and correct the moisture or adhesion problem before patching. A surface filler cannot reattach an unsound plaster layer.

03

Line at a frame, ceiling junction or movement joint

The crack follows a junction between materials rather than the middle of a plaster surface.

Next check

Identify the joint system. Use an approved flexible detail if suitable, not generic hard filler. Never close a designed movement joint with a rigid cosmetic patch.

04

Widening, displacement, masonry cracking or sagging

Steps through brick or block, changing floor levels, new sticking doors or windows, or several affected surfaces.

Next check

Stop cosmetic repair and request prompt professional assessment. Keep people clear if the area may be unsafe; urgent danger is not a monitoring exercise.

Preparation

Do these checks before buying anything

This guide covers cosmetic cracks in painted cement or gypsum plaster, not structural repairs, masonry reinforcement or engineering diagnosis. Preserve useful evidence before changing the surface.

  • Photograph the full wall and a close-up with a ruler and date. Mark the ends on the photograph, not a decorative surface. Record changes in length, width and alignment.
  • Look for damp after rain, stains, salts, mould and blistering paint. Identify and correct the moisture source before repainting.
  • Note whether the crack follows a ceiling junction, frame, opening corner or masonry joint. Check whether adjacent doors and windows have recently changed alignment.
  • Do not chip into masonry to investigate the depth. A crack continuing through brick or block, or a large area of loose plaster, needs assessment by a competent builder or structural engineer as appropriate.
  • For recently built or renovated homes, retain dated photos, plans and builder records. Obtain advice before disturbing evidence if warranty or defect rights might apply; no particular outcome is assumed.
  • Read the actual filler and paint technical and safety sheets. Protect floors, ventilate and use the specified skin, eye and dust protection. Do not sand, scrape or disturb suspect hazardous old coatings or materials until assessed.
Why the details matter

Deeper guidance

01

Work out what has cracked

A line in paint or skim coat can have a different repair from a crack in the plaster layer, at a material junction or through masonry. Drying and temperature changes are possible explanations, not a diagnosis from a photograph. Hollow, lifting or powdery plaster may require a wider repair by a competent tradesperson. Do not cover evidence of deeper damage simply because the line looks narrow.

02

Choose a filler that matches the surface

Interior and exterior products are not interchangeable. Dulux’s interior technical sheet lists cement and gypsum plaster but excludes exterior use and structural masonry defects. Its multipurpose sheet includes exterior applications, while still excluding structural repairs. These examples illustrate why the actual product sheet matters; they are not endorsements. Plascon’s ready-mixed Polyfilla listing also excludes structural cracks. Confirm the latest preparation, exposure, depth and finishing requirements for the product you buy.

03

Why a repaired crack may return

The surface may have been patched while a joint continued moving, plaster remained damp or poorly bonded, or the wrong product was used outdoors. Painting before a filler has cured can also compromise the finish. Adding mesh, widening a crack or applying more compound is not a remedy for foundation movement. A neat painted surface does not prove that the cause has been solved.

The method

Step by step

01

Confirm the wall is dry and sound

Resolve active leaks and let the wall reach the chosen coating system’s moisture requirements. If dampness persists, plaster is lifting, or the crack changes, postpone cosmetic patching. A wall that merely looks dry is not necessarily ready for paint.

For moisture clues, compare rising damp, condensation and leaks before planning the cosmetic repair.

02

Remove only loose material

For a confirmed cosmetic defect, use a small hand scraper or putty knife to lift loose coating or old filler. If the selected product requires opening the crack for adhesion, work gently within the sound plaster finish only. Do not cut into masonry, conduits or hidden services. Stop if the work reveals deeper damage or significant loose plaster.

03

Dust off and prepare the substrate

Remove loose particles without spreading dust. Follow the filler instructions on cleaning, priming, sealing or pre-wetting; these differ by product. Do not apply filler over dusty, glossy, powdery or poorly bonded paint and assume it will hold the wall together.

04

Apply the specified filler

Choose a filler approved for the actual substrate, interior or exterior exposure and defect. Press it into the prepared crack with a putty knife. Follow the permitted layer thickness and any multi-layer method instead of applying one thick wet blob. A product for surface repairs is not a structural repair material.

05

Allow the full specified cure

Use the manufacturer’s drying and overcoating conditions for the layer depth, temperature and humidity. There is no universal number of hours for all fillers. A skin that feels dry can conceal wet material underneath; do not rush sanding or paint to hide it.

06

Finish and spot-prime as instructed

Once cured, smooth excess material using a method safe for the coating and with the specified dust controls. Clear residue and spot-prime where the chosen paint system requires it. Fresh filler may absorb paint differently and leave a visible patch if preparation is skipped.

07

Repaint, then monitor

Match the finish and sheen; repainting to a natural corner may look more even than a small touch-up. Keep a dated photograph of the repair. If the same crack opens again, stop repeating cosmetic treatment and investigate movement, moisture or adhesion before another patch.

For an outdoor finish, read our peeling exterior paint guide. Record follow-up checks in your home maintenance plan.

Before you start

Expert tips

  • A thin line is not automatically harmless, and a diagonal line alone does not prove subsidence. Use the combination of change, depth, displacement and other symptoms to decide when to seek professional assessment.
  • For a moving material junction, ask which complete joint treatment is approved for that location. Not every flexible filler can accommodate every joint, coating or expected movement.
  • Hot sun, seasonal moisture and rain exposure affect South African homes differently. Match the repair to the actual building, exposure and product rather than a blanket local crack-width rule.
  • Check nearby gutters, downpipes and ground-level drainage where dampness recurs. Filling the visible line does not establish that the water route has been resolved.
  • This is research-led, AI-assisted general guidance, not a field-tested repair or an engineer-reviewed assessment of your building. References were checked on 25 October 2026; confirm the latest technical sheet for the product you buy.
People also ask

Common questions

Can I simply paint over a hairline crack?+

First check for moisture, movement and loose coating. Ordinary paint is not a reliable substitute for a compatible repair system. A coating specifically rated for hairline crack bridging is different from generic wall paint, and neither establishes that the underlying wall is stable.

Does a diagonal crack always mean my house is subsiding?+

No. Shape alone cannot establish the cause. A new, widening or displaced crack, especially with sticking openings or changing floor levels, warrants assessment by an appropriately qualified structural engineer. Keep people clear if there is a safety concern.

Should I use flexible or hard crack filler?+

Match the product to the substrate, exposure and expected movement. A movement-tolerant filler may suit a limited surface defect or an approved joint; rigid filler may fail where movement continues. Neither replaces structural investigation or a designed movement-joint detail.

How do I know whether the plaster or the bricks are cracked?+

Visible cracking through mortar joints or on the opposite face is a reason to investigate, not proof of a particular cause. Do not pry into masonry or make major exploratory openings yourself. Ask a competent tradesperson or engineer where the depth or cause is uncertain.

Is crack repair waterproofing?+

No. Filling a visible crack does not necessarily stop the water route. Exterior drainage, flashing, joints and the coating system may still need attention. Resolve the source and use an appropriately specified weatherproofing system where needed.

Primary references

Sources & further reading

We link to official building-code, government and public-health material where a technical or safety claim needs authoritative context. Local rules and adopted code editions can change.

  1. Dulux South Africa / AkzoNobelPre-Paint Interior Crackfiller technical data sheet — May 2025Manufacturer guidance on substrate preparation, curing and painting; excludes exterior use and structural masonry defects. Confirm the latest issue for the actual product.
  2. Dulux South Africa / AkzoNobelPre-Paint Multipurpose Crackfiller technical data sheet — April 2025Interior/exterior product guidance, layer and overcoating conditions, safety precautions and structural-repair exclusions.
  3. Plascon South AfricaPolycell Polyfilla Ready Mixed Crack FillerManufacturer listing distinguishes limited normal thermal movement from structural cracks. Product description checked through the search index; direct page retrieval was unavailable. Obtain the current technical sheet before use.
  4. RDA ConsultingFive warning signs your building needs a structural assessmentSouth African engineering practice: changing cracks, uneven floors, sticking openings and dampness warrant investigation. This guide does not adopt a width-only diagnosis or the publisher’s numerical thresholds.
  5. Royal Institution of Chartered SurveyorsSubsidence consumer guideProfessional-body guidance on expanding cracks, sticking openings, assessment and monitoring. UK context; not South African legal, insurance or site-specific engineering advice.
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Know your limit

Stop if the work involves structural uncertainty, hazardous materials, heights, live services, gas, serious water damage, or regulated work. Local rules differ—use an appropriately licensed professional where required.