The crack in the wall: the two-minute read that says paint job or structural engineer

Almost every crack is cosmetic and almost every household still pays the structural anxiety in full, because nobody hands them the reading key. The honest frame first: a house is a rigid thing on soft ground with seasons that move it, and a wall that never showed one hairline in its life is usually a wall too young to speak about. The useful question is never 'is there a crack' - it is 'what shape, which direction is it growing, and what does the rest of the house agree', and this page is the two-minute version of asking exactly those three.

The two families, named by their shape

• The plaster's own cracks - fine, webbed, shallow: hairlines that follow no line you could draw with a ruler, craze the width of a fingernail's edge, the map that paint sits on and light rakes along. Causes are the render's own life - cement that shrank as it cured, a skim too thick, the harmattan opening the fine crazing and the rains closing it. These are cosmetic with a capital and their only structural sin is being a channel for water into a skin that was thin to begin with; they get filled, taped where two materials meet, and painted.
• The wall's cracks - straight-ish, continuous, deeper than the skin: the diagonal at a door's corner, the stair-step that walks the block courses, the horizontal that runs along the mortar line, the vertical that separates one wall from another exactly at the joint where two builds met. These are geometry speaking: movement of the thing behind the plaster, and their reading needs the page's second half.

The two-minute read, in the order that produces data

First the light: a torch flat against the wall at dusk turns every shadow-line honest - a crack in the paint only will vanish under a fresh skim's shadow while the real one keeps its black edge. Then the tape test, which is the whole diagnostic in one strip: a piece of cheap masking tape across the crack's middle, dated in pencil, photographed, checked monthly - a crack that is finished has a tape that stays straight across it, and a crack that is still moving shreds, bows or opens past the tape's edge, which is the exact sentence the engineer wants from you, with your dates on it. Then the width test, the one that sounds trivial and decides consultations: a coin in the gap - a crack that takes a thin coin by its rim is telling the house's story, not the paint's. Then the company the crack keeps, which is the paragraph that turns a single crack into a diagnosis: the door in the same room that started sticking the year the crack appeared (the doors page's 'the house is moving' answer belongs here, not there), the window that no longer slides the way its siblings do, the floor with the new level to it, the external plinth where the ground and the wall's foot now disagree - a crack with a door's testimony is a set of symptoms, and sets get the professional.

The four signs the ground is in the story

1. Stepped and diagonal together on the same wall, the pattern of a footing that dipped on one side: the block courses tell where the movement is, and the diagonal corner crack that crosses courses tells which way the house leaned.
2. The ground-floor version is worse than the upstairs one, as a rule - the movement lives where the foundations live, and the crack that starts at the floor line and climbs is the same sentence in a taller font.
3. Seasonal opening that has begun to leave a permanent gap: the house that closes its crack in the rains and opens it in the dry weeks has a breathing story; the year the gap stops closing is the year the ground's repair of its own movement ran out - and the tape with its dates is what proves which of the two you are watching.
4. The water accomplice: the crack that appeared along the path of a leak, a blocked gutter's overflow line, or the soakaway field the plumbing page warned about (saturated ground beside a footing is the most preventable cause on this list - the gutters guide and the soakaway guide are this crack's two alibis, and both are fixable this month).

Who you call, with what you hand them

The plasterer's job is the first family, the fill-and-tape-and-paint close of cosmetic crazing, and the honest quote begins with 'it's only the render' with a light in his hand. The structural engineer's job is the second family, and the house should meet them prepared: the dated photographs, the tape's history, the width by the coin, the list of doors and floors that agree - the professional's visit is halved and the recommendations get cheaper in direct proportion. The questions a Nigerian house should expect them to ask about - the soil's own character (the swelling clays of some estates are a named actor), the nearby digging (the neighbour's new foundation, the drain trench, the road works), the water's management (the same soakaway and gutters list) - are the case's facts, not the client's confessions; a movement story is read from all of them at once. The serious-quickly signs that skip both trades and go to the exits and the pro this season: a crack wide enough for the coin's full edge with a bulge at the wall's face, a window or door frame suddenly racked where its siblings are square, the ceiling that sagged in the same rooms (the plaster page's sagging sequence runs this floor), or the new crack with the new tilt - that group is the house's last calm sentence before shouting, and the tape test is for everything before it.

The desk's standing rule

Light, tape, coin, company - the two-minute exam every crack deserves before money moves. Hairlines take a paint day; the deep shapes with witnesses take the engineer, and the one the household can fix from the ladder of prevention is the water: the gutters clear and the soakaway's thirst honoured before the first hairline darkened into a story.