Cracks in the Walls and Ceiling of a Delhi House: Which Ones Matter, What Causes Each Pattern, and How They Are Repaired

Most cracks in the walls and ceilings of a Delhi house are not structural. They come from the materials moving: brick, mortar, plaster and concrete shrink as they dry, expand and contract with the day's heat and the season, and move differently from one another where they meet. The Bureau of Indian Standards' handbook on the subject, SP 25:1984, Handbook on Causes and Prevention of Cracks in Buildings, which BIS still lists as active, divides cracks into structural ones, 'due to incorrect design, faulty construction or overloading', which 'may endanger the safety of a building', and non-structural ones, 'mostly due to internally induced stresses in building materials', which generally do not. It adds a warning that applies to every builder floor with a damp wall: non-structural cracks can, 'because of penetration of moisture through cracks or weathering action, result in corrosion of reinforcement and thus may render the structure unsafe'. This guide sets out how to read a crack, the causes and the patterns each produces, the signs that make a crack an emergency, and how cracks in masonry and concrete are repaired, drawing on SP 25, the Central Public Works Department's Handbook on Repairs and Rehabilitation of RCC Buildings and IS 456.

How Do You Tell a Structural Crack From a Harmless One?

By width, movement, position and company. On width, SP 25 gives the common classification: 'thin, less than 1 mm in width; medium, 1 to 2 mm in width; and wide, more than 2 mm in width', with hair cracks of about 0.01 mm at one end of the scale and 'gaping cracks 5 mm or more' at the other, and it notes that many thin cracks are 'less damaging to the structure' than a few wide ones. For reinforced concrete, IS 456 clause 35.3.2 sets the design intent: the surface width of cracks 'should not, in general, exceed 0.3 mm' where cracking is not harmful, 0.2 mm where the member is exposed to weather, moisture or soil, and 0.1 mm in aggressive conditions; the CPWD handbook puts the codes' thresholds for a 'macro crack' at 0.1 to 0.3 mm and says any crack wider than that 'is likely to cause durability problems'. So a 1 mm crack is thin in a plastered brick wall and already outside the design intent in a beam soffit. On movement, SP 25's advice is to find out whether a crack is active or static before anything else, by fixing tell-tales across it, a strip of glass or a patch of plaster that breaks if the crack moves, and watching for weeks. On position and company, the handbook's diagnostic checklist in section 9.3 is what an engineer runs through: when the crack was first seen and whether it has widened; whether it opens and closes with the day's temperature; how deep it goes, probed with a fine wire, and whether it passes through the wall; where it starts and ends, and whether it relates to an opening or passes below the damp-proof course into the foundation; whether it is horizontal, vertical or diagonal, straight through the bricks or stepped along the joints, uniform or tapering, and which end is wider; whether there is a difference in level across it, felt with a fingertip or a straight edge, which marks a shear crack; whether its edges are sharp or rounded, rounded edges meaning repeated opening and closing; and whether there is dampness, leakage, subsidence or a tree nearby. The cracks that need an engineer are the ones in the structure itself and the ones in company with other signs: a crack in a column or along a beam, a diagonal crack that keeps widening, a crack with a step across it, a crack with rust stains, and any crack in a wall that is leaning or bulging. SP 25 puts a number on the last: walls 'not more than 25 mm out of plumb, or which do not bulge more than 10 mm in a normal storey height' generally need no structural repair. Our guide to the warning signs of an unsafe building covers the company a serious crack keeps.

What Causes Cracks?

SP 25 lists seven causes: 'moisture changes, thermal variations, elastic deformation, creep, chemical reaction, foundation movement and settlement of soil, and vegetation'. Moisture movement is the drying shrinkage of mortar, plaster, concrete and concrete blocks, and the irreversible expansion of fresh bricks as they take up moisture after leaving the kiln. Thermal movement is the daily and seasonal expansion and contraction of every material, worst in an uninsulated roof slab under the Delhi sun; the handbook notes that cracks from thermal movement have rounded edges and open and close with the weather. Elastic deformation and creep are the shortening of columns under load and over time, which squeezes the brick panels built between them. Chemical reaction covers sulphate attack on mortar, carbonation of concrete and, most seriously, the rusting of reinforcement, which expands and cracks the cover concrete along the line of the bar. Foundation movement is the settlement or heave of the soil under part of a building, including the compaction of soil under a new extension and the swelling and shrinking of clayey soils. Vegetation is tree roots drying the soil under a footing or lifting it, a common cause of cracks in compound walls. Delhi adds its own pattern to the list: an RCC frame with brick infill, a roof slab that bakes in May and cools to 7°C on a January night, builder floors raised fast with bricks straight from the kiln, and the leaks our guide to wall dampness describes, which turn a harmless crack into a corroding one.

What Do the Common Patterns Mean?

Nine patterns account for most of the cracks we are asked to look at in Delhi houses, and SP 25 describes the cause of each. A horizontal crack at the top of the walls on the top floor, where the roof slab sits on them, with a matching line outside at slab level: the roof slab expanding and contracting with the sun because it has no insulation or protective cover, and dragging the wall with it; the remedy is to insulate or cover the terrace, which our terrace waterproofing guide describes, and to cut a groove in the plaster at the junction so that the movement happens in the groove rather than in a crack. Diagonal cracks rising from the corners of door and window openings: drying shrinkage of the in-situ concrete lintel, appearing 'in the first dry spell after the completion of the building'; they stabilise and are cosmetic. A vertical crack where a brick wall meets an RCC column, a few months after construction: differential strain between concrete and masonry from 'elastic deformation, shrinkage and creep in RCC column'; the handbook calls these generally thin, to be filled at the next repainting, or cut back as a 10 cm strip and replastered. Horizontal cracks across the brick panels of an RCC frame, years after construction and often with the panel bowing: the columns have shortened under load and creep and the panel, 'built too tightly between the beams of the frame', has been squeezed, worse where bricks were laid fresh from the kiln; the fix is to open a joint at the top of the panel, about 10 mm, and fill it with a compressible filler. A vertical crack at the junction of an old house and a new extension or added floor: the soil compacting under the new load; fill it with weak mortar once the movement has stopped, or form a vertical groove in the plaster at the junction, and see our guide to adding a floor for what should have been checked first. Vertical cracks at regular intervals of a few metres along a long boundary wall: shrinkage and thermal movement in a wall built without expansion joints, non-structural, and prevented by joints 5 to 8 mm wide every 5 to 8 m, as our boundary wall guide explains. Map or crazing cracks across a plastered surface: surface shrinkage of a rich or quickly dried plaster, cosmetic. Cracks along the line of a bar in a beam, slab or chhajja, with rust staining and the cover flaking: corrosion of the reinforcement in porous, damp concrete, which is structural if left, and which our column jacketing guide follows to its end. And horizontal cracks in the mortar joints two or three years after construction, with the mortar itself crumbling: sulphate attack from the bricks or the soil, for which the handbook offers no cure except raking the joints and replastering with sulphate-resisting cement, and rebuilding when the wall becomes unserviceable. Diagonal cracks with outward-leaning walls and heaving floors point to shrinkable clay under shallow footings and need a foundation investigation, not a plasterer.

When Is a Crack an Emergency?

When it is in the structure, when it is moving fast, or when it follows an event. The condition assessment guide published by the National Institute of Disaster Management lists the 'typical visible distress detrimental to the safety of buildings': cracks in RC beams, columns and slabs and in load-bearing walls, spalling of concrete, sagging of beams or slabs, and tilting of columns or frames out of plumb. Any of those, a crack that widens visibly within days, a diagonal crack that crosses a whole wall with a step across it, a crack that appears after an earthquake or after excavation on the plot next door for a basement, or cracks in a building that is already leaning, call for the same response: take the load off the area if you can, prop it, and have a structural engineer inspect it before anyone patches it. The engineer's tool is the structural audit, which our structural audit guide explains, with the rebound hammer, ultrasonic pulse velocity and core tests that tell whether the concrete behind the crack is sound. Plastering over a structural crack hides the only evidence of what is happening and delays the repair that would have been cheap.

How Are Cracks Repaired?

By the cause first and the crack second, and only once the crack has stopped moving. SP 25's rule is that 'no useful purpose is served in repairing cracks when these are still developing', and that cracks due to thermal movement 'generally recur when repaired with mortar' and should be filled with a mastic instead. The CPWD handbook draws the same line: inactive cracks in masonry can be stitched or grouted, while 'the active cracks required for accommodating thermal movements shall be repaired by suitably locating the expansion joints and filling them with flexible materials like poly-sulphides, bituminous fillers'. For masonry walls SP 25 section 9.6 is practical. Cracks up to 1.5 mm in walls of ordinary absorbent brick 'generally need no repairing'; where they do, the joints are raked out and refilled with a 1 cement, 1 lime, 6 sand mortar. Cracks wider than 1.5 mm in a wall built in weak mortar are enlarged, raked to a depth of about 25 mm, refilled with a 1 cement, 2 lime, 9 sand mortar and replastered as a 10 cm wide strip; in a wall built in strong mortar, the bricks adjoining the crack are cut out and replaced. For a crack that has broken the bond across a wall, the CPWD schedule provides for cutting the masonry and letting in precast RCC runners across the crack, 600 mm long by the wall's thickness, with two 6 mm bars, packed in stiff mortar, or for stitching: holes drilled on both sides of the crack, a groove chased between them, U-shaped steel bars set across the crack and the holes grouted with epoxy or a non-shrink grout. For concrete the method is injection grouting, and the handbook's specification shows what a proper job involves: the crack opened to a V-groove about 12 mm by 12 mm, holes about 25 mm in diameter and 40 mm deep drilled along it at 300 mm or the member's thickness apart, 12 mm nipples fixed in the holes and the surface between them sealed with epoxy or polymer-modified mortar, and then a cementitious or epoxy grout pumped through the nipples under pressure; the epoxy grout it specifies has a compressive strength of at least 60 MPa at 7 days. For corrosion cracks the sequence is to remove all loose and damaged concrete, clean the bars of rust, apply a protective coating, and rebuild the section in polymer-modified mortar or concrete; where the bar has lost section, the member needs strengthening, which is the jacketing described in the guide above. None of these repairs lasts if the cause survives it: a leaking terrace, a missing roof cover, an open joint at a column, a tree against the footing. Fix the water first.

What Does a Crack Repair Cost, and Who Should Do It?

It depends entirely on the diagnosis, so we price crack repairs after an inspection rather than from a list. A raked and refilled shrinkage crack in a bedroom wall is an hour's work for a mason and a repaint; a stitched masonry crack, an injected beam or a corroded chhajja rebuilt in polymer mortar is a specified repair on an engineer's note, with its own materials and curing. For a building we look after under an annual maintenance contract, from ₹2 per sqft per month, periodic inspections pick up new cracks and movement in old ones before the monsoon finds them, and our civil repair service carries out the masonry and concrete work. Structural repairs follow a structural engineer's drawing, never a plasterer's judgement. We are trusted by DAV Pitampura and DPS Dwarka for campus civil repairs and maintenance. To have a crack looked at before it is painted over, call +91-7838355055 or book a free site visit.