Crack in the wall? How to tell a harmless crack from one that needs attention
Most cracks in plastered walls are shrinkage or temperature cracks: fine, shallow and harmless to the structure, but a path for rainwater on an outside wall. A few cracks are a sign of movement in the building and need an engineer before anyone fills them. The shape, width and location of a crack tell you which kind you have.
Quick answer
How do I fix a crack in the wall?
First read the crack. Fine, shallow cracks in plaster are cut open into a small groove, cleaned, filled with a flexible crack filler and painted; on an outside wall they also get a waterproof exterior coat. Cracks that are wide, diagonal from door or window corners, through the wall, or growing need an engineer to find the cause before filling. Rigid cement fillers usually crack again.
Published
Signs you will notice
- Hairline cracks in a web or map pattern on plaster
- Cracks running diagonally from the corners of doors and windows
- A straight crack where a wall meets a column or beam
- Damp or paint peeling along a crack on an outside wall
- Doors or windows that have started to stick
What the crack is telling you
Read the shape, width and location first. They decide the repair.
Crack type 1
Fine shrinkage or map cracks in plaster
Very thin cracks in a web pattern, usually on large plastered surfaces and outside walls exposed to sun.
How to recognise it: Hairline width, shallow, and they do not continue into the brick or block behind the plaster.
How it is usually handled: Open the bigger lines slightly, fill with a flexible crack filler, then apply a flexible exterior coat that bridges hairline cracks.
Crack type 2
Crack at a wall-column or wall-beam joint
A straight vertical or horizontal crack where masonry meets concrete, often on every floor.
How to recognise it: It follows the edge of the column or beam exactly. Different materials expand differently in heat.
How it is usually handled: Cut open, fill with a flexible sealant or filler, and embed a mesh strip across the joint in the plaster or coat over it.
Crack type 3
Cracks letting rain in on an outside wall
Damp patches inside the room line up with cracks on the outside face, and get worse in rain.
How to recognise it: Look at the outside face directly opposite the damp patch for cracks, peeling paint or algae.
How it is usually handled: Pressure-wash the outside face, fill the cracks with a flexible crack filler, then apply an elastomeric exterior waterproofing system.
Crack type 4
Diagonal, wide or growing cracks
Cracks wider than a few millimetres, stepping diagonally from door or window corners, visible on both faces of the wall, or getting longer.
How to recognise it: Mark the ends and width with a pencil and date. If they grow over weeks, or doors start sticking, the building is moving.
How it is usually handled: Do not fill yet. A structural engineer should find the cause, such as settlement or overload, before any repair.
Methods follow proven, manufacturer-approved waterproofing systems. The right one for your building depends on what is found on site.
How a lasting crack repair is done
Smearing filler over a crack closes it for a season. These steps make it last.
- 1Watch the crack for a few weeks with pencil marks at its ends and a date. A crack that grows is not filled yet.
- 2Cut the crack open into a small V-groove so the filler bonds to sound material, not dust.
- 3Remove loose plaster and dust, and dampen or prime as the filler requires.
- 4Fill with a flexible, polymer-based crack filler for cracks that move with heat; use a repair mortar only for wider, stable cracks.
- 5On outside walls, cover the repaired area with a flexible waterproof exterior coat, carried well beyond the crack.
- 6Where plaster has separated from the wall, embed a mesh or fabric strip before the final coat.
What a photo can and cannot tell you
Photo guidance points to the likely source. It is not a confirmed diagnosis.
A clear photo can show
- The shape, direction and pattern of the crack
- Whether it starts at a door or window corner or follows a column
- Damp or peeling paint along it
A photo cannot settle
- How deep the crack goes and whether it passes through the wall
- Whether it is still growing
- The cause of structural movement
Simple checks before you call anyone
- 1Measure the widest point of the crack. A coin or a feeler card helps compare over time.
- 2Pencil-mark both ends with the date and check again after a few weeks.
- 3Check whether the crack shows on both faces of the wall.
- 4Note whether damp or peeling paint follows the crack after rain.
When you need someone on site
- The crack is wide, diagonal or visible on both faces of the wall
- It grows over weeks, or doors and windows start sticking
- Cracks appear in beams, columns or the slab
- Plaster sounds hollow over a large area
A site diagnosis helps check the likely source. For hidden or recurring leaks, thermal imaging and moisture checks can help narrow down the water path before deciding whether anything needs opening up. In Vadodara? See leak detection and waterproofing help in Vadodara for site services, coverage and contact options. The app confirms availability at your building's pincode before you book.
Common mistakes that waste money
- Filling with plain cement or wall putty. Both are rigid and the crack reopens.
- Painting over a crack without opening and filling it.
- Filling a growing structural crack, which hides the warning sign.
- Repairing the inside face only when rain enters from the outside face.
Before you accept a contractor's quote
- Does it say how the crack will be opened and cleaned before filling?
- Does it name a flexible filler rather than plain cement or putty?
- On outside walls, is a waterproof exterior coat included over the repair?
- For wide or diagonal cracks, has anyone checked the cause first?
Already have quotes? Membrain can compare them on scope, not just price. Planning the work instead? A written specification makes quotes comparable.
How this guide is built
Diagnostic basis
Membrain separates visible signs from possible water-entry mechanisms, then asks for evidence that can distinguish the remaining causes.
Repair-source governance
Public guides stay brand-neutral. When Membrain creates a repair plan, methods and products come from current approved manufacturer documents in its technical library.
Evidence limits
A photo cannot confirm concealed pipes, waterproofing layers or moisture depth. When remote evidence cannot separate the causes, a site inspection is the correct next step.
Deewar mein crack kyun aata hai?
Zyada tar crack plaster ke sukhne ya dhoop-garmi se aate hain. Ye patle hote hain aur building ke liye khatarnaak nahi, par bahar ki deewar par baarish ka paani inse andar aata hai.
Crack ko V-groove mein khol ke flexible filler se bharo, cement ya putty se nahi. Agar crack chauda hai, darwaze ke kone se tircha ja raha hai ya badh raha hai, to pehle structural engineer ko dikhao.
Frequently asked questions
Which cracks in a wall are dangerous?
Cracks that are wide, diagonal from door or window corners, visible on both faces of the wall, wider at one end, or growing over time can mean the building is moving. Have a structural engineer look at those before filling them.
What is the best filler for wall cracks?
For fine and moving cracks, a flexible, polymer-based crack filler in an opened groove lasts longer than cement or putty. For wider, stable cracks a polymer-modified repair mortar is used. No filler is right for a crack that is still growing.
Why do cracks appear above door frames?
The corners of openings concentrate stress, so shrinkage and small movements show there first. A fine crack is common. A wide or widening one, especially with sticking doors, needs an engineer.
Can hairline cracks cause leakage?
On an outside wall, yes. Driving rain enters through hairline cracks and shows as damp inside. Filling the cracks and applying a flexible exterior waterproof coat stops that path.
Should I fill cracks before or after the monsoon?
After, in dry weather, once the wall has dried. Filler and coatings bond poorly to wet surfaces.
Not sure which one it is?
Share a photo and answer a few questions. Membrain shows the likely sources and a repair plan built from proven waterproofing methods, in minutes. Free to start.
Remote guidance, not a site inspection.