Measuring cracks
What a Crack's Width Actually Tells You
Measure the crack, note where it tapers and which way it runs. Send those details through and a specialist will read them without any charge or obligation to you.
Why width alone does not tell the whole story
A ruler held against plaster gives you one number. That number matters, but it sits inside a larger picture. A two-millimetre gap running from a window corner to the ceiling behaves differently from the same width appearing as a horizontal line above a door frame.
Specialists grade movement by combining width with taper and direction. Taper means the crack is wider at one end than the other. Direction tells them whether the wall is dropping, leaning or bowing outward. Without all three, the measurement is incomplete.
Knowing this changes what you do when you notice something wrong. Instead of staring at the widest point and worrying, you can start building a record that actually helps someone else understand what is happening in your home.
How to take a useful measurement
You need a steel rule, not a tape measure. Tape bends and stretches; steel stays flat. Place it across the narrowest part of the crack and note the reading in millimetres. Then move along the length and measure again at the widest point. Write both numbers down alongside the date.
If the crack runs vertically, measure at the top, middle and bottom. If it runs diagonally, follow the line and take readings every thirty centimetres or so. The goal is to show whether the gap stays even or gets progressively wider toward one end.
Take the measurements at eye level where you can. Do not stand on furniture to reach high plasterwork. A specialist can work with partial data far better than they can work with an injury. Photograph each spot with the rule in frame so the numbers are legible later.
Reading the taper
Taper is the difference between the wide end and the narrow end of a crack. A gap that measures five millimetres near the ceiling and closes to nothing at the skirting board has a pronounced taper. That shape suggests rotation: the wall above is moving while the base stays put.
An even width from top to bottom points to something else entirely, often shrinkage in the plaster itself rather than movement in the structure behind it. Shrinkage cracks tend to be fine, straight and consistent.
When you record taper, write it simply. Something like "5mm at top, hairline at bottom" is exactly what a structural surveyor needs. They will combine your taper reading with the direction to form an early view of whether the ground beneath your home is shifting or the masonry is just settling.
Direction and what it suggests
Cracks follow the path of least resistance through brickwork and mortar. Vertical lines often relate to thermal movement or drying shrinkage. Diagonal cracks stepping through mortar joints suggest differential settlement, where one part of the foundation sinks faster than another.
Horizontal cracking in a cavity wall can mean the ties holding the inner and outer leaves together have corroded or snapped. This is common in older Kent houses built before stainless steel ties became standard.
Note the direction relative to fixed points: windows, doors, corners. A diagonal crack radiating from the top corner of a window is a classic sign of localised subsidence. Record it as "diagonal, running up and left from the top right corner of the kitchen window". Precision here saves time later when a specialist visits your home.
Turning your notes into a proper record
Once you have width, taper and direction, add context. Note which room the crack is in, whether it appears on the outside brickwork too, and if doors nearby stick or swing open on their own. These small details help a specialist build a picture before they arrive.
Keep the record in one place. A notebook page or a phone note works. Update it every few weeks, always noting the date. Seasonal change matters in Kent because clay soils swell in winter and shrink in summer. A crack that opens in August and closes in January tells a specific story about the ground underneath.
This record becomes the starting point for any structural movement survey. Handing over dated measurements means the specialist begins with facts rather than memory. It also means monitoring equipment, if needed, goes in the right place from the first visit.
I measured the crack but I don't know what it means
Width, taper and direction are the three things a specialist grades. Seeing a gap widen is unsettling, but raw numbers without context leave you guessing. Here is what to look for around your home.
- 1
A gap wider at one end
The crack tapers from thick to thin. This usually means part of the wall has rotated or dropped while the rest stayed still.
- 2
Diagonal lines through bricks
Cracks stepping through mortar joints rather than splitting bricks often point to uneven settlement in the ground below.
- 3
Doors catching on frames
A door that suddenly sticks or won't latch suggests the frame has distorted because the surrounding wall has moved.
- 4
Hairline cracks in plaster only
Fine, straight lines confined to plaster rarely indicate structural trouble. They often result from drying or temperature changes.
Questions about measuring cracks
Do I need a special tool to measure a crack?
No. A standard steel rule marked in millimetres is enough. Avoid fabric tape measures because they stretch. Hold the rule flat against the wall and read the widest point of the gap.
How often should I check if it has grown?
Every four to six weeks is usually enough. Checking daily creates anxiety without adding useful data. Always write down the date alongside the measurement so changes can be tracked over months.
Does a wide crack always mean subsidence?
No. Wide cracks can come from thermal movement, failed wall ties or old repairs failing. Width matters, but direction and taper tell a specialist much more about the actual cause.
Should I fill the crack before measuring it?
No. Filling it hides the evidence a specialist needs to read. Leave it open, measure it, photograph it and let the surveyor decide whether filling or structural repair is the right step.
Can I measure cracks on the outside brickwork?
Yes, and you should. Exterior cracks often mirror interior ones. Measure them the same way with a steel rule, but avoid using ladders. Ground-level observations are enough for an initial record.
Send your measurements for review
Write down what you found and send it through. A specialist will read your notes and tell you whether the crack warrants a site visit. There is no fee for this and no commitment to book any work.
- No cost to ask
- No obligation to proceed
- Read by a specialist