Almost every brick house in South London has a crack somewhere, and the vast majority are nothing to lose sleep over. A small number, though, are early warnings of subsidence, failed lintels or wall tie corrosion, and catching those early is far cheaper than dealing with them late. Here is how we assess cracks on the Victorian and Edwardian terraces around Balham, Tooting and Clapham, and what we would tell a friend who asked.
Structural engineers grade cracks by width, and the Building Research Establishment scale is a useful yardstick. Hairline cracks under 1mm are cosmetic. Anything up to 5mm is usually repairable with repointing or localised brick replacement. Cracks between 5mm and 15mm need investigation, and anything wider than 15mm, or where the wall is visibly out of line, needs a structural engineer before anyone picks up a trowel.
Measure the widest point with a ruler or a crack width gauge, write the figure and the date on a bit of masking tape next to it, and check again in three months. A crack that stays the same width is old movement that has settled. One that grows, especially between late summer and autumn, is live and needs a professional opinion.
London clay shrinks in dry summers and swells in wet winters, so a little seasonal movement is normal for a house built in 1890 with shallow footings. Fine vertical or stepped cracks in mortar joints, cracks around a door or window frame that have been filled and repainted before, and cracks in render rather than the brick itself are all typically low risk.
Thermal cracking is common on long garden walls and on the south facing elevation of a terrace, where the bricks heat up and cool down every day. Cracks that follow a straight line where an extension meets the original house are usually a differential settlement joint rather than a fault, particularly if the extension is more than a couple of years old.
Diagonal stepped cracks that run from a corner of a window or door towards the ground, and are wider at the top than the bottom, are the classic sign of subsidence. Around Balham and Streatham the usual culprit is a mature tree or a leaking Victorian drain drying out or softening the clay beneath one corner of the house. If you can also see doors sticking, sloping floors or gaps opening between skirting and floor, treat the crack as urgent.
Horizontal cracks every five or six courses of brick, sometimes with bulging, point to corroded wall ties, which is common in cavity walls built between the 1920s and 1980s. A crack directly above a window or door with the bricks sagging suggests a failed lintel, often a rotten timber one hidden behind the brick arch. None of these get better on their own, and all of them are considerably cheaper to fix in year one than year five.
For cosmetic and minor cracks, the fix is raking out and repointing the affected joints in a lime or sand and cement mortar to match the original, sometimes cutting out and replacing a handful of spalled bricks. On a typical terrace that is a day or two of work, and around South London you should expect somewhere between £300 and £900 depending on access and the number of bricks involved.
Live structural cracks usually need helical bar stitching, where stainless steel bars are bonded into slots cut across the crack every few courses. That commonly runs from £800 to £2,500 for a single elevation. Wall tie replacement is priced per tie and often lands between £1,500 and £4,000 for a whole house. Where subsidence is confirmed, drain repairs, tree management or underpinning may follow, and that is where an insurance claim rather than a private job usually makes sense.
If you suspect subsidence, notify your buildings insurer before commissioning any repair. Most policies cover subsidence with an excess of around £1,000, and the insurer will normally appoint a loss adjuster and engineer to investigate, monitor and then reinstate. We do a lot of this reinstatement work, so we can tell you honestly whether a crack is a claim or a Saturday job with a pointing trowel.
Yes, very. Most Victorian and Edwardian terraces have shallow footings on clay and show some seasonal hairline cracking, which is usually cosmetic. It is the width, direction and whether a crack is growing that decide if it matters.
Mark and measure it, then check after three to six months, ideally spanning a summer into autumn when clay movement peaks. If it has grown by more than a millimetre or two, or new cracks have appeared, get a professional to look at it.
A surveyor will flag any crack over about 5mm and any stepped diagonal cracking, and buyers often ask for a structural engineer's report. Having a repair done properly, with paperwork and a guarantee where applicable, is far better than filling and painting over it.
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