Reading the cracks in a house

Before you panic, and before you ignore it

Every house cracks. Materials shrink as they dry, they expand and contract with temperature, and a building settles slightly in its first years. The great majority of what people find alarming is cosmetic.

The reason it is worth learning to read them is that the small number which are not cosmetic get considerably cheaper to fix the earlier they are found. A foundation problem caught in year one is an investigation and a localised repair. The same problem found in year eight is a much bigger job.

The six kinds of crack, and what each looks like

1. Plaster shrinkage

Fine, often map like or randomly branching, confined to the plaster and not continuing into the wall behind. They appear in the first months as the render dries. Common where plaster was applied too thick in one coat, mixed too rich in cement, or not cured. Cosmetic. Fill and redecorate when the movement has stopped.

2. Thermal movement

Straight, usually vertical, often at a regular spacing along a long wall or at a change of material. They open slightly in cool weather and close in hot. Long unbroken walls without a movement joint are the usual cause, and parapet walls and boundary walls in full sun are where they show up first. Cosmetic unless very wide, though on a long wall a movement joint may be worth forming.

3. Shrinkage at a junction of materials

A clean vertical line exactly where two different materials meet, such as where a concrete column meets stone walling, or where blockwork abuts a beam. The two materials move at different rates and the plaster over the junction is what tears. Cosmetic, and best solved with a bead or a deliberate joint rather than by filling repeatedly.

4. Settlement

Diagonal, usually wider at the top than at the bottom, often starting at the corner of a window or door and running up and outwards towards the nearest corner of the building. Caused by part of the foundation moving downwards more than the rest, from poorly compacted fill, a soft pocket in the ground, or a foundation that was too narrow for the load.

The important characteristic of settlement is that it usually stops. The ground compresses, finds equilibrium and the movement ends. A settlement crack that has been stable for two years is generally a repair job, not a structural one.

5. Heave

Diagonal as well, but the geometry is inverted. Cracks tend to be wider at the bottom, and the pattern is one of the edges of the building lifting relative to the centre. The signature is seasonal movement: the crack opens through the dry months and closes through the wet ones, and doors that bind at one time of year swing freely at another.

This is the expansive soil problem, and unlike settlement it does not stop. It repeats every year for as long as the building stands, unless the cause is dealt with.

6. Structural overload or corrosion

Cracks in the concrete itself rather than in the walling. Horizontal cracks along the underside of a beam near midspan, cracks running along the line of the reinforcement, or rust staining and spalling where the concrete cover has broken away and exposed steel. These are a different category and need looking at promptly rather than monitoring.

How much width is too much

Engineers in this region generally classify damage in low rise buildings using the BRE categories. They are a useful yardstick, with one condition attached: width alone never tells the whole story. A 3mm crack that has not moved in three years is less concerning than a 1mm crack that has doubled since January.

How to monitor a crack properly

Monitoring is not watching. It is recording, in a way that lets someone compare a measurement made today with one made eight months ago. It costs nothing and it is what turns a worry into evidence.

1. 1Photograph the crack with a tape or a coin against it for scale, and photograph the whole wall as well so the position is clear later
2. 2Draw a fine pencil line across the crack at each end and write the date beside it. If the crack extends past your line later, you know it has grown and roughly when
3. 3Measure the width at the same marked point each time, ideally with a crack gauge card, which costs very little
4. 4Record the date, the width and whether it has rained recently. On expansive soil the rainfall context is half the information
5. 5Repeat at the end of the dry season and at the end of the rains, so you capture the two extremes
6. 6For anything you are genuinely worried about, fix a tell tale, the small glass or plastic gauge that is bonded across the crack and shows movement directly

A year of that record is worth more to an engineer than a site visit on the day. It shows whether the movement is seasonal, progressive or finished, and those three lead to completely different conclusions.

The causes we actually find in this region

In order of how often they turn out to be the answer on the jobs we are called to look at around Laikipia and the Mount Kenya region.

1. 1Expansive soil under a foundation that was too shallow for it. Almost always a strip foundation at 600mm to 900mm sitting inside the active zone of black cotton soil.
2. 2Water reaching the soil beside the wall. No gutters, no apron, a downpipe discharging at the base of the wall, or a soakpit too close to the building.
3. 3Poorly compacted fill under a ground bearing slab or under part of the foundation, particularly on plots that were levelled with imported material.
4. 4A large thirsty tree close to the building, or one that was recently removed, drying or rewetting the soil at one corner.
5. 5Extensions built without a movement joint at the junction with the original house, where new foundations settle at a different rate from the old ones.
6. 6Weak concrete or missing reinforcement in a lintel or beam, usually visible as cracking directly over an opening.

Repairs, and repairs that are not repairs

The instinct is to fill a crack and paint over it. On a dead crack that is exactly right. On a live one it achieves nothing except concealing the evidence, and the crack reopens, usually along a slightly different line so that the patch looks worse than the original.

• Do not fill anything until you know whether it is still moving. Monitor first
• Cosmetic cracks in plaster: rake out, fill with a flexible filler, redecorate
• Cracks at a junction of two materials: form a proper joint with a bead rather than filling repeatedly
• Structural cracks in walling: after the cause is dealt with, stitching with helical bars bedded into the mortar joints is the usual method, and it is done by a specialist
• Cracks caused by ground movement: nothing done to the wall is a repair until the ground movement has been addressed. Anything else is decoration
• Corrosion damage in concrete: the cover is broken out, the steel is cleaned and treated, and the section is made good with a repair mortar. Painting over rust staining accelerates the problem by trapping moisture