Cracks in walls: which ones are dangerous and which are not

Since the earthquake of 10 August, a great many people in Colombia have been looking hard at the walls of their own house for the first time. And the first thing a search turns up is a figure: that below one millimetre nothing is wrong, and above it you should worry.

That figure has nothing behind it. It is not in the Colombian building code, nor in the technical guide that assessors work from, nor in the international literature. And above all: width is not the first thing you look at. Two cracks of the same millimetre can be a matter of no importance or a reason not to sleep in the room, depending on where they are, what shape they have and which way they open.

What follows is what the sources do say, and how to read a crack with them in hand.

Hairline crack or crack: the line falls at 0.2 mm

The technical guide for inspecting buildings after an earthquake, drawn up by the Colombian earthquake engineering association for IDIGER, the risk management institute of Bogotá, uses two different words consistently: fisura below 0.2 mm, and grieta above it. English normally makes one word do for both, so this article keeps the distinction by calling anything under that line a hairline crack and anything over it a crack proper.

The thresholds change with the material

This is the part almost nobody tells you, and the part that actually helps: the same width means different things depending on where it is. Concrete is more demanding than masonry, and bahareque, adobe and rammed earth (the earth-built systems still common in older Colombian houses) are more forgiving than either.

What caused it: the pattern tells you

Shape before width. Five causes produce five distinct drawings, and telling them apart is what separates a repair that settles the matter from one you have to do again in two years.

Shrinkage: the commonest and the least serious

Mortar and concrete shrink as they dry. If something restrains them (and something always does), they crack. These cracks are vertical, fairly regular, and they appear at the weak points: halfway up an opening, or at intervals along a long wall.

They show up in the first few weeks and go on opening for a year or two, until they settle down. That is the reassuring fact and it is also the diagnostic test: shrinkage settles down by itself; settlement does not.

Where the surface carries a fine map pattern, like crocodile skin, ACI 224.1R puts it down to the surface layer having carried more water than the body beneath. On site that means: water was added to the mix to make it easier to work. It is a defect of execution, not of structure.

Settlement: the stepped crack, and which way it opens

When part of the foundation drops, the wall tears diagonally along the mortar joints, in steps. It usually starts at a corner and affects two walls at once. And it has one telltale sign: it crosses the plinth and runs down to the footing, instead of stopping high up.

The most useful detail in the whole article, and the one almost nobody looks at, is which end of the crack is wider.

• —Wider at the top and tapering downwards: the ground is sinking. Shallow foundations, clay drying out, a large tree nearby drawing moisture from the soil.
• —Wider at the bottom and tapering upwards: the ground is rising. Clay taking up water again (the classic case after an old tree is felled), or roots pushing from underneath.

The wall rotates like a very deep beam about the part that is still firm, and the crack opens where the material is left in tension. That is why the wider end always points to the side that moved.

Thermal: horizontal, on the top floor

The roof slab takes direct sun, warms up far more on top than underneath, arches, and pushes outwards. The wall below loses. The result is a horizontal crack right at the junction between the wall and the slab, with an unmistakable sign: from outside it shows above the level of the slab, and from inside below it.

Two things identify it beyond argument. The first is that it only happens on the top floor: lower down, the weight of everything above stops the edge of the slab from lifting. The second is that it opens and closes with the temperature of the day. No other cause does that.

The real remedy is not to cover it over: it is to insulate or shade the slab from above. Repaired with rigid mortar, this crack comes back.

Shear: the diagonal X

This is the earthquake crack. The structure moves sideways and the wall, being stiff, resists until it tears diagonally. And because the movement goes back and forth, both diagonals appear, crossed.

There is a distinction here that tells you what failed first: if the crack steps along the joints, the mortar was weaker than the brick; if it runs almost straight, splitting the units, the brick was weaker than the mortar.

In a partition wall that only divides, this is non-structural damage: a nuisance, repairable, and no reason to fear the building will come down. It stops being that when the wall starts to move out of its own plane, or when the cracking carries on into the tie-columns and tie-beams that confine it. That last one is the jump: the crack has gone from the infill to the skeleton.

Corrosion: the line parallel to the bar

Steel inside concrete is protected while the medium around it stays alkaline. When it stops being alkaline, the steel rusts; rust takes up more volume than the metal did and bursts the concrete from within. The resulting crack is longitudinal, parallel to the bar, and it follows an unvarying sequence: first a fine line, then the concrete separates along a plane parallel to the face, and finally the cover falls away and the bar is left in view.

There is a field test anyone can carry out that catches the middle stage, before anything falls: tap the surface with a small hammer. If it sounds hollow, a separation has already formed underneath.

The signs that really are alarming

None of these depends on measuring anything. If any one of them appears, the conversation stops being about appearances.

• —It shows on both faces of the wall. A crack that goes right through is not in the render.
• —It runs down to the footing, crossing the plinth, instead of stopping high up.
• —The two lips are offset. Run a finger across it, or lay a straightedge on it: if one side stands proud of the other, that is shear and not tension.
• —The wall is bowed or out of plumb. You see it by sighting along it, or with a spirit level.
• —The reinforcing bar is visible, or the concrete is crushed. On the official scale, that is already heavy damage.
• —It sounds hollow when tapped.
• —It is in a column, a beam, or the joint between the two. Connections are the most critical points of the structure.
• —Doors and windows have stopped closing, or the floor is out of level.
• —It has grown since you last looked. A crack that advances is a live crack, and that changes everything.

How to tell whether it is still live

What decides whether a crack is at rest or still moving is not time: it is the cause. Shrinkage runs out; active settlement does not.

To find out, you have to measure twice. What an owner can do alone, with no instruments:

1. 1Photograph it with something of known size beside it (a coin, an open tape) and with the date visible.
2. 2Mark the two ends with a pencil and write down the day.
3. 3Repeat in a month, and again in three. If it has not moved, you know something. If it has, you know more.

A crack width gauge (a transparent card printed with lines of calibrated width) costs very little and makes the arguable measurement far more precise. What is no longer an owner’s job is chiselling into the render to see whether the crack is in the plaster or in the wall itself.

After the earthquake: what the colour means

In Cali, the buildings that were inspected were marked with a coloured sticker. It is worth reading carefully what the city itself says they mean, because a good deal of confusion is circulating.

• —Green. No conditions were found that prevent the building from being lived in. It does not rule out that damage exists or that repairs are needed, and it is not a specialist technical opinion.
• —Yellow. There is localised damage or hazard. The building may be used with restrictions, keeping clear of the areas marked, and a specialist technical assessment is needed.
• —Red. There are conditions of serious risk to life. Nobody should go in on their own initiative.

Two more things worth knowing. The visit is not a structural verdict or an engineering study: it is a rapid visual inspection, lasting between fifteen minutes and an hour. And the sticker can only be changed by the city department, through a fresh assessment; taking it off or moving it yourself does not change the state of the building, it only erases the information.

Who can sign for what

It is worth being direct here, even (especially) writing from an architecture studio.

The technical guide expressly contemplates the rapid visual inspection being carried out by civil engineers, architects, builders or technicians in civil works, preferably with five years of experience in structural design or construction. And even so it recommends that the team be led by a structural engineer, who is the one taking the final decision on classification.

What an assessor at that first stage cannot do is written out in full: under no circumstances should they recommend demolition. And the detailed assessment, the one that does produce a verdict, belongs to specialists in structures, geotechnics or building pathology. The same goes for tall buildings, for those in the worst affected areas, for essential facilities and for those with ground problems.

Put briefly: an architect can look at your crack, tell you what produced it and whether there is cause for concern. The verdict on structural safety is signed by a structural engineer or a building pathology specialist. Anyone offering you the second without being one is selling you something they cannot deliver.

In propiedad horizontal: who pays

This is the question that escalates fastest into an argument at the general meeting, and Ley 675 de 2001, the statute on propiedad horizontal, settles it better than it looks.

Propiedad horizontal is the Colombian regime under which a building is split into privately owned units and property held in common. Its article 3 defines as essential common property the land, the foundations, the structure, the façades and the slabs serving as roof at any level. Read that again: a column, a beam, a structural wall or the façade remain common property even when they sit inside your flat. A crack there belongs to the community of owners. One in an internal partition, in the skim coat or in the tiling is yours.

On the money, three rules that are often misquoted:

• —The contingency fund comes first. The statute allows an additional extraordinary levy to be approved only once the resources of the fund prove insufficient.
• —Earthquake cover over the common property is compulsory, and the indemnity is earmarked in the first place for reconstruction. It covers common property, not the finishes of your flat.
• —The 70% majority that article 46 requires for certain decisions (an extraordinary levy of more than four times the monthly service charge, for instance) is 70% of the building’s total participation coefficients, not 70% of those present. And decisions of that kind cannot be taken at a meeting held remotely.

One qualification that saves a fright: a structural repair does not require the 70% because it is structural. It requires it only if the extraordinary levy exceeds that threshold. If it fits inside the ordinary budget or comes out of the contingency fund, a simple majority is enough.

If the building is new: ten years

Ley 1796 de 2016 obliges the builder or the seller of a new dwelling to cover financial loss for ten years counted from the Certificación Técnica de Ocupación (the technical certificate of occupancy), not from the deed, and not from the handover of the keys.

Two points that change who it reaches. It governs projects of five or more dwelling units intended for third parties. And only those whose permit was filed after the statute came into force, in July 2016: a good part of the stock built before that falls outside.

What not to do

• —Skimming and painting before you know the cause. It is the most expensive of them all, because it erases the evidence and stops nothing. The crack comes back, and by then nobody knows how much it grew.
• —Filling a live crack with rigid mortar. It opens again, almost always right beside the filling.
• —Injecting without a diagnosis. Repairing the symptom while the cause is still at work is paying twice.
• —Taking it for granted that "the house is settling". Sometimes it is true, and sometimes it is the comfortable explanation for not looking.
• —Taking off or changing the coloured sticker. Only the authority can do that, and only after a fresh assessment.

The essentials

Look at the shape first, not the width. A regular vertical crack in a new wall is almost always shrinkage and will settle down by itself. A stepped one that runs down to the footing and opens upwards is settlement, and the ground has to be sorted out before the wall is touched. A horizontal one at the junction with the top-floor slab, opening and closing with the sun, is thermal.

And if it is in a column, in a beam or in the joint between the two; if it shows on both faces; if the lips are offset; if the wall is bowed or the bar is showing: at that point there is nothing left to interpret from a screen. Call somebody.