Foundation Crack Size: When Is It Serious? A Homeowner’s Guide (UK)

How big is too big? If you’re staring at a line creeping across your wall or foundation, you want a straight answer. Here’s the short version: width matters, but so do location, pattern, and whether it’s growing. I’ll give you the numbers, show you how to measure properly, flag the warning signs, and help you decide when it’s time to get a structural engineer involved-without panic and without guesswork.

• TL;DR: Cracks under 2 mm are usually cosmetic; 2-5 mm deserve monitoring and minor repair; 5-15 mm often need structural repair; over 15 mm is serious. Movement, water, and horizontal cracks escalate urgency.
• Measure right: use a ruler/feeler gauge, note the widest point, and re-measure monthly. A £1 coin is ~2.8 mm thick-handy for a quick sense check.
• Red flags: widening over time, stepped cracks through brick, horizontal cracks in basement walls, door/window sticking, damp, sagging floors.
• UK benchmark: BRE Digest 251 categories guide crack width severity for homes; NHBC standards and IStructE guidance back up when to repair or investigate.
• Don’t just fill it: fix drainage, roots, or subsidence causes first. Cosmetic patching alone won’t last.

What crack size is serious? The quick answer with context

Width is the easiest starting point because it’s objective. In UK practice, surveyors often reference BRE Digest 251 (Assessment of damage in low-rise buildings). It groups damage by crack width and effect on use:

These bands are widely used in the UK and align with practical thresholds you’ll hear from structural engineers. But width is not the only factor. Consider:

• Orientation: Horizontal cracks in basement or retaining walls are more worrying than vertical hairlines-they can signal soil pressure issues.
• Pattern: Diagonal/stepped cracks through brickwork at window or door corners could mean differential settlement or subsidence.
• Movement: A 2 mm crack that grows to 3 mm in a month is a bigger worry than a stable 4 mm that’s been there for years.
• Location: Cracks concentrated in one corner, near trees, or along a drain run point to a cause you can fix.
• Water: Damp, staining, or efflorescence around a crack adds risk. Water both causes and worsens movement.

Heuristic you can use today: if a crack is wider than a credit card (~0.76 mm) but thinner than a £1 coin (~2.8 mm), log and monitor. At 2-5 mm, get an inspection and plan repair. Over 5 mm, don’t delay-book a structural engineer. If horizontal and over 2 mm (or any bowing), treat as urgent.

Why trust these figures? They’re consistent with BRE Digest 251, NHBC Standards guidance on subsidence and building near trees (Chapter 4.2), and the Institution of Structural Engineers’ advice on subsidence in low-rise buildings. For reinforced concrete, Eurocode 2 sets lower crack limits for durability (often 0.2-0.3 mm), but that’s about steel corrosion risk, not settlement-so don’t mix the two. We’re talking masonry and foundation movement in homes.

How to assess your foundation crack step by step

You don’t need fancy kit. You do need a calm, repeatable method.

1. Map every crack: Walk the inside and outside of the affected area. Mark each crack end with pencil, date it, and number them. Note orientation (vertical, diagonal, horizontal), location (near windows, corners, beams), and whether it goes through mortar and brick/block or just plaster/paint.

2. Measure width properly: Use a steel ruler with millimetres or a feeler gauge. Note the widest point. Quick hack: compare to a known thickness-printer paper (~0.1 mm), credit card (~0.76 mm), 10p coin (~1.85 mm), £1 coin (~2.8 mm). Record width to the nearest 0.5 mm if you’re eyeballing.

3. Check for displacement: Run a finger across the crack. Is one side proud of the other? Even 1-2 mm of step suggests differential movement, not just shrinkage.

4. Look for moisture: Any damp staining, salt crust (efflorescence), or musty smell? If yes, check gutters, downpipes, drains, and ground levels outside. Poor drainage is a top driver of movement in UK homes.

5. Scan the structure: Do doors rub or not latch? Are window heads cracked at the corners? Any sloping floors or racked door frames? These are movement clues, not just cosmetic issues.

6. Consider trees and soil: Big thirsty trees within 10-20 m on clay soils can cause seasonal shrinkage. In the South West and around Bristol, Lias clays and Mercia mudstone can move with dry/wet cycles. On sands/gravels, tree influence is less, but washout from leaking drains is common.

7. Monitor over time: Take dated photos with a ruler in shot. Re-measure monthly for three months. Better, fit a crack tell-tale gauge (they cost a few quid online) and log readings. Movement trending up is your trigger.

8. Decide action level: Use the table above plus the red flags below. If in doubt, get a structural engineer to look. A short visit costs less than bad repair decisions.

Red flags that override “wait and see”:

• Horizontal crack in a basement or retaining wall, any width, especially with bowing.
• Any crack over 5 mm, or any crack that widens quickly (more than ~0.5 mm/month).
• Stepped cracks through bricks around openings combined with sticking doors/windows.
• New cracks after a suspected leak, flood, or drain issue.
• Cracks with water ingress or noticeable damp.
• Bulging wall, sagging floor, or chimney lean.

Common causes in UK homes:

• Seasonal shrink/swell of clay (dry summers, wet winters). The hot 2022-2023 summers left a lot of legacy movement.
• Leaking drains softening or washing soil away beneath footings.
• Inadequate foundations for soil type (older Victorian/Edwardian terraces can vary house-to-house).
• Groundwater changes after extensions, loft conversions adding load, or nearby excavations.
• Trees or hedges planted too close to shallow footings.

Sources that underpin this approach: BRE Digest 251 (damage categories), IStructE guidance on subsidence and movement in low-rise buildings (investigation and monitoring), NHBC Standards Chapter 4.2 (trees and foundations), and CIRIA guidance on clay shrink/swell behaviour. You don’t need to read them now, but know that the thresholds above aren’t guesswork.

Examples: what different cracks usually mean (and what to do)

Example 1: Hairline vertical crack in plaster, 0.3 mm, near a new radiator. Likely cause: shrinkage/thermal movement. Action: fill, redecorate, and ignore unless it reopens.

Example 2: 1-2 mm diagonal crack from the corner of a window in a brick wall, step pattern following mortar joints outside, no sticking doors, no damp. Likely cause: minor settlement or thermal movement. Action: monitor for three months; if unchanged, repoint and stitch if needed.

Example 3: 3-4 mm stepped crack in a corner of the house, wider at the top, doors upstairs now rub slightly, big tree about 8 m away on clay. Likely cause: differential settlement from seasonal shrinkage or root influence. Action: get a structural engineer. They may recommend drain survey and soil assessment; repairs might include crack stitching and possibly root management. Avoid drastic tree removal without advice-it can cause heave.

Example 4: 6-8 mm vertical crack running through brick and blocks in a garage foundation wall, one side sits 2 mm higher, gutters overflowing for months. Likely cause: softened ground/washout near footing due to poor drainage. Action: urgent drainage fix, structural inspection, crack stitching or local underpinning may be required.

Example 5: Horizontal crack mid-height in a basement wall, 2-3 mm, with slight inward bow and damp staining. Likely cause: lateral pressure from saturated soil or inadequate retaining design. Action: urgent assessment by a structural engineer; this can be a stability issue. Remedial options include wall anchors, braces, improved drainage, or rebuilding sections.

Example 6: Stair-step cracks around a new rear extension junction, 2 mm, appeared within months of completion. Likely cause: differential settlement between old and new foundations. Action: talk to your builder and engineer; often this stabilises. Stitching and repointing once movement stops.

Example 7: Multiple hairline crazing cracks across a new concrete slab, all under 1 mm, random map pattern. Likely cause: plastic shrinkage cracking from curing. Action: usually cosmetic; seal and monitor. If you see curling or differential slab levels, get advice.

Example 8: 15-20 mm gap at corner, you can see daylight, skirtings pulled away, floor slopes slightly. Likely cause: significant subsidence. Action: stop DIY, get a structural engineer immediately. Insurance may classify as subsidence and bring in their panel engineers; expect investigation, monitoring, and designed repair such as underpinning or piling.

Cost reality check (UK 2025 ballparks):

• Structural engineer inspection: £250-£600 for a site visit and brief report; complex cases £600-£1,200.
• Drain CCTV survey: £120-£300 for a typical house run.
• Crack stitching (helical bars, resin) and repointing: £80-£150 per metre depending on access and finish.
• Local underpinning or mini-piling: £1,500-£2,500 per linear metre; small jobs often £8k-£20k; larger or tricky sites £25k+.
• Root management (arborist advice and careful pruning): a few hundred pounds; complete removal plus stump grinding varies widely-get advice first.

These are wide ranges because access, finishes, and cause matter more than crack width alone.

Cheat-sheets: crack width chart, quick tests, and decision tools

Fast field tests you can do today:

• Credit card test: If it won’t fit, your crack is under ~0.76 mm. Likely cosmetic unless there are other symptoms.
• £1 coin test: If the crack is about the thickness of a £1 coin (~2.8 mm), you’re in the 2-5 mm zone where monitoring and planned repair make sense. Past that, act.
• Finger glide: If you feel a step across the crack, think movement, not just shrinkage.
• Water wipe: Wipe with tissue-any damp pickup points to a moisture problem to fix before cosmetic work.

Decision guide (rule-of-thumb):

• <2 mm, stable, vertical/hairline, no other issues: Fill/paint, monitor every few months.
• 2-5 mm, diagonal or stepped, minor door rub: Book an engineer/survey; check drains; plan repair.
• >5 mm, any orientation; or horizontal crack; or noticeable displacement: Treat as serious; urgent engineer visit.
• Any size + bowing, damp ingress, or rapid change: Urgent investigation.

Checklist before you call an engineer (so your visit is efficient):

• Photos of each crack with a ruler for scale.
• A simple sketch plan marking cracks and widths.
• Notes on timing: when first noticed, and any growth.
• Recent changes: leaks, new trees or pruning, extensions, floods, drought periods.
• Drain history: any known blockages or past repairs.
• Soil and site context: clay/suspected shrink-swell, nearby trees, sloping site.

What not to do:

• Don’t fill and forget a moving crack. It will reopen and can hide a developing problem.
• Don’t remove large trees near clay soils without professional guidance-sudden changes can trigger heave.
• Don’t ignore drainage. Fix gutters, downpipes, and ground falls so water runs away from the house.
• Don’t self-inject structural epoxies into load-bearing elements without a diagnosis. You can lock in stresses.

Materials and repairs that actually work (when cause is addressed):

• Crack stitching: Helical stainless bars grouted across mortar joints; strong, discreet, reliable for stepped masonry cracks.
• Resin or cementitious injection: For concrete cracks where water exclusion or bonding is needed-after cause is fixed.
• Localised underpinning or mini-piles: Where a corner or strip has settled; designed by an engineer.
• Drain repairs and soakwaways: Fix leaks, reroute water, add land drains where appropriate.
• Root management: Selective pruning under arborist guidance; root barriers in some cases.

Insurance angle (UK): If the cause is subsidence (soil movement under foundations), your buildings insurance may cover investigation and repair. Cosmetic cracking without structural cause usually isn’t covered. Evidence helps-photos, measurements, and any drain surveys. Insurers often appoint their own engineer and may monitor for a season before repair.

Mini‑FAQ and what to do next

Is a 1 mm crack serious? Usually no. It sits in the very slight range. Fill and redecorate, but check it doesn’t widen over the next few months.

Is a 3 mm crack serious? It’s in the 2-5 mm bracket-worth investigating, especially if diagonal/stepped or if doors stick. Many of these get fixed with stitching and addressing the cause (drains, trees).

Is a 5 mm crack serious? Yes. It falls into the 5-15 mm category where structural repair is commonly needed. Book a structural engineer.

Do horizontal cracks always mean trouble? In basement/retaining walls, yes-treat as urgent. In above-grade walls, a horizontal bed-joint crack can also be serious if associated with bowing or displacement.

Can hot summers cause new cracks? Yes. Clay soils shrink in drought, then swell in wet periods. The UK’s recent hot, dry summers increased seasonal movement, particularly near trees. Many homes stabilise once moisture returns, but don’t bank on it-monitor.

How long should I monitor before deciding? If there’s no urgent red flag, three months of monthly readings gives a baseline. If growth exceeds ~0.5 mm in that time, escalate.

Can I DIY crack stitching? Some experienced DIYers do, but it’s easy to get wrong on live cracks or without tying into the right courses. If the crack is structural or near openings, get it specified and inspected.

Who should I call first-builder, surveyor, or engineer? For suspected movement, go straight to a chartered structural engineer (MIStructE/CEng). A good builder executes the fix; the engineer diagnoses and designs it.

Will underpinning damage my home’s value? Poorly executed underpinning can put buyers off; professionally designed and documented underpinning can restore confidence. Keep all reports, calculations, and completion certificates.

Next steps for different scenarios:

• Hairline/under 2 mm, no movement: Fill, paint, fix any obvious water issues, and set calendar reminders to recheck.
• 2-5 mm diagonal/stepped: Book a structural engineer, get a drain CCTV survey, and plan stitching once stable.
• Over 5 mm or horizontal/bowing: Urgent engineer visit; expect a designed repair and possibly temporary support if needed.
• After a leak: Fix the leak first, then monitor for at least a month before repairing cracks.
• Near trees on clay: Get arborist advice and a structural review before major pruning or removal.

A final sanity check: Width is your first filter, not the verdict. Combine it with pattern, movement, moisture, and context. If your crack is brushing past the £1 coin thickness, or you’re seeing doors jam and bricks step, it’s time for professional eyes. Most issues are fixable without drama when you act early.

One last tip: keep a one-page log-dates, widths, notes, and photos. If you end up talking to insurance or an engineer, that log is gold. It shows you’re on top of it and speeds up a correct diagnosis.

And if you’re searching this at 11 pm because you spotted a crack by the skirting, breathe. Plenty of cracks look scary and turn out to be manageable. Use the chart, do the checks, and get help if it crosses the thresholds. You’ll sort it.

PS: If you want a phrase to remember, make it this: address the cause first, then fix the crack. That single line prevents most repeat problems with serious foundation cracks.