Most cracking in buildings runs vertically, diagonally, or steps through the mortar joints. Horizontal cracking is less common and it has a shorter list of causes - which is useful, because it means working out which one you have is a relatively contained question.
It also spans the widest range of severity of any crack direction. One cause is essentially cosmetic. Another means a wall is being pushed over.
The causes, from most to least common
- Wall tie corrosion - regular, repeating horizontal cracking in bed joints up a cavity wall, worst on the weather-exposed elevation. Corroding steel expands and jacks the joints open.
- Cracking at the damp-proof course - the DPC is a deliberate discontinuity in a wall, and slip or movement along it produces a single horizontal crack at a consistent height above ground.
- Cavity trays and lintel bearings - horizontal cracking localised above or beside an opening, following a tray or the top of a lintel.
- Floor restraint positions - where a floor structure ties into a wall, differential movement can crack the joint at that level.
- Thermal and moisture movement at a change of material - render meeting masonry, or an addition meeting the original wall.
- Lateral pressure - a retaining wall, or a basement or cellar wall, being pushed inwards by the ground and water behind it. This is the serious one.
- Frost damage and saturation at a specific course, particularly in exposed stonework or below a defective sill.
The one to take seriously
A horizontal crack in a retaining structure - a garden retaining wall, a basement wall, a cellar wall holding back ground - is a different proposition from a horizontal crack in a cavity wall. It indicates the wall is bending under lateral pressure, typically because water cannot escape from behind it, and it can be a precursor to failure.
In a retaining wall the crack usually appears in the lower third, where the bending is greatest, and it is often accompanied by the wall leaning, bulging, or moving forward at the top. If that is what you are looking at, it warrants attention rather than monitoring, and it is worth keeping people away from the downhill side in the meantime.
What is normal
A single hairline horizontal crack at a change of material, at a DPC, or above one window is common and usually cosmetic. Under the BRE Digest 251 damage classification, hairline to a few millimetres in an ordinary wall sits in the negligible to slight range - repairable during redecoration and with no effect on stability.
The things that change the assessment are regularity, extent, bulging, and location in a structure that is retaining ground.
What you can check yourself this week
- Establish whether the crack repeats. Regular horizontal cracks at consistent vertical spacing point at wall ties; a one-off points elsewhere.
- Sight along the wall from a corner at a shallow angle to see whether it is bulging or leaning. This is the single most useful check and it costs nothing.
- Work out what the wall is doing. Is it a normal external wall, or is it holding back ground on one side? That changes everything.
- Measure the crack's height above ground and see whether it matches the DPC line, a lintel, or a floor level.
- Check the other elevations. Damage on the weather-facing side only is consistent with ties; damage everywhere suggests something else.
- Photograph with a scale and a date, and repeat in three months.
When to call an engineer
Get it inspected if the crack is in a retaining or basement wall, if the wall is bulging or leaning when sighted along, if the cracking repeats regularly up an elevation, if it is wider than about 5mm, or if it is measurably growing. Get it inspected sooner if it is in a retaining structure with anything below it that people use.
If it is a single hairline crack at a DPC or a material change and nothing else has moved, it is very likely doing what walls do.
How we would investigate it
Classification of the cracking, sighting and measurement for bulge and lean, identification of what the wall actually is and what it is carrying or retaining, and where the pattern suggests ties, locating and inspecting them properly rather than inferring. Where the answer is not resolvable in one visit, monitoring across the seasons establishes whether it is progressing.
We do not carry out repairs and take no commission from anyone who does - which matters in this particular defect, because horizontal cracking is the signature of the one problem most aggressively marketed to homeowners. A structural survey and report is a fixed £580 + VAT, with the report inside 10 working days of the inspection, guaranteed.
Frequently asked questions
Are horizontal cracks worse than vertical ones?
Not automatically, but they have a narrower set of causes and one of those causes is serious. A horizontal crack in an ordinary cavity wall is usually ties, a DPC or a lintel. A horizontal crack in a wall retaining ground means the wall is bending under lateral pressure, and that is a different order of problem.
My crack is at the same height all the way round the house. What is it?
A crack at a consistent height above ground, continuous around a building, most often relates to the damp-proof course - a deliberate discontinuity in the wall that movement can slip along. It is usually not serious, but it is worth confirming rather than assuming, particularly if it is widening.
How wide does a horizontal crack have to be before it matters?
Width is the starting point rather than the test. Under the BRE Digest 251 classification, up to about 5mm in an ordinary wall is generally negligible to slight. What changes the assessment is regularity, bulging, extent across elevations, whether it is growing, and above all whether the wall is retaining ground.
Can I just fill it?
You can fill anything. Filling a crack removes the evidence without addressing the cause, and if the movement continues the filler cracks and you have lost the record of how fast. Establish the cause first; making good is the last step, not the first.