You’ve noticed vertical cracks running down the corners of your internal walls — perhaps worse upstairs than down, and wider near the ceiling than the floor. It’s easy to jump straight to the word “subsidence,” but in my experience these particular cracks usually have a far more straightforward explanation. Let me walk you through what I found on a real job, and what it means for your home.
Key Takeaways
- Vertical cracks at internal corners of cavity-wall homes built around 2000 are usually caused by thermal and moisture movement of the aircrete inner leaf — not subsidence.
- The inner leaf is the load-bearing skin, so while this cracking is normally cosmetic, you should have an engineer confirm that before assuming the worst is over.
- Red flags that change the picture: matching cracks in the outer brickwork, diagonal or tapering cracks, sticking doors or windows, cracks wider than 5 mm, or cracks that keep growing.
- The standard repair is crack-stitching with stainless helical bars and forming a proper movement joint near the corner — no underpinning required for this mechanism.
- Retrofitted cavity insulation and mortar voids in the bed joints can make the movement worse by amplifying internal temperature swings over the years.
The House I Was Called To
The property was a detached house built around 2000 — traditional cavity wall construction: 102 mm clay brick outer leaf, roughly 50 mm cavity, and a 100 mm aircrete (aerated concrete) blockwork inner leaf. Cavity insulation had been retrofitted at some point after the original build. The homeowner’s concern was exactly what you might be experiencing: vertical cracks at the internal corners only, noticeably worse at first-floor level than ground floor, and wider near the ceiling, tapering down towards the skirting.
Critically, there was no cracking visible in the external brickwork at all. That single observation already told me a great deal. If this were foundation movement — subsidence or heave — you would almost certainly expect to see corresponding distress in the outer leaf, diagonal cracking, and likely some misalignment at window or door openings. None of that was present here. The problem was confined entirely to the inner leaf, and it had a mechanical explanation.
Why Aircrete Blocks Crack at Corners
Aircrete blockwork moves — it shrinks as it dries after laying, and it expands and contracts with temperature changes throughout its life. Clay brickwork also moves, but in the opposite direction: clay bricks tend to expand over time, particularly on a warm south-facing elevation. These two materials sit side by side in a cavity wall, connected by wall ties, and they respond to heat and moisture differently.
In this house, the inner leaf had never been provided with movement joints. Industry guidance — including NHBC Technical Standards guidance 6.1/28 — recommends movement joints in concrete and aircrete blockwork at roughly 6 m centres, and at no more than approximately half that distance from a corner. For clay brick outer leaves the spacing is wider, around 10–12 m. These are guidance figures, not Building Regulation requirements, but they exist for good reason.
Without those joints, movement has nowhere to go. The corner is a restraint point — the wall is held in two directions — so that’s where the stress concentrates and the block eventually cracks. The crack opens wider at the top because the cumulative movement is greatest there; the lower storeys are more restrained by the floor structure and the weight above them.
Two additional factors made things worse in this particular house. First, the cavity insulation had been absent for years before it was retrofitted, meaning the inner leaf had experienced large swings in internal temperature — accelerating the movement cycle. Second, there was evidence that the mortar in the bed joints had voids, possibly because the blocks were laid in wet weather and the mortar hadn’t cured uniformly. When the blocks subsequently dried and shrank, the weakened joints offered less resistance.
Is It Structural? The Honest Answer
Here is where I want to be direct with you, because this is the question that matters most. Thermal and moisture movement cracking of the aircrete inner leaf is, in the vast majority of cases, a cosmetic defect — it does not represent a structural-safety failure of the building.
However — and this is important — the inner leaf of a cavity wall is the load-bearing skin. It carries the floor joists, the roof loads, and the weight of everything above. So while the mechanism I’ve described is benign, you cannot simply assume that without an engineer looking at it. The cracks I examined on this job were consistent with movement cracking and nothing more. But I was there in person to rule out the other possibilities.
The red flags that would change my assessment entirely are: cracking that also appears in the outer brickwork; diagonal cracks rather than clean vertical ones; cracks that taper in an irregular way suggesting differential settlement; doors or windows that have started sticking or binding; any individual crack wider than approximately 5 mm; or cracks that you can observe widening over weeks or months. If any of those apply to your situation, the picture is different and you need professional input promptly.
For the house I visited, none of those red flags were present. The vertical cracks at the corners of your internal walls, in isolation, are telling you about movement — not about the foundations.
The Repair: What Actually Works
There is no need for underpinning with this mechanism. Underpinning addresses foundation movement; it would do nothing for thermal and moisture movement in the blockwork above. The correct repair has two components.
First, crack-stitching. Stainless steel helical bars are bedded in resin or grout into raked-out bed joints at approximately 450 mm vertical centres, spanning across the crack. This reinstates the tensile continuity of the blockwork across the damaged zone without being rigid — the bars have some flexibility. The joints are then filled flush and made good.
Second, and equally important, a proper movement joint needs to be formed near the corner. A compressible filler material is inserted into a formed vertical joint, sealed with a flexible sealant or fitted with an expansion bead. This gives the blockwork somewhere to accommodate future movement, so the same cracking doesn’t recur. Without this second step, you’re repairing the symptom without addressing the cause.
Once both elements are in place, the wall can be made good and redecorated in the normal way. This is competent building contractor work once an engineer has specified it correctly.
What About the Cavity Insulation?
The retrofitted cavity insulation in this house is worth a brief mention. Cavity insulation does reduce the temperature differential across the inner leaf, which in principle reduces the amplitude of thermal movement going forward. So its presence now is not a bad thing. But if the insulation was installed poorly — with gaps, or if the original installation disturbed the mortar in the bed joints — it can create localised cold bridges or moisture pathways that introduce their own problems. It’s worth having the installation checked if you have any doubts, particularly if you’re seeing damp patches as well as cracking.
The key point is that the years before insulation was installed are likely when most of the cumulative movement occurred. The cracking you’re seeing now is often the legacy of those earlier cycles, not necessarily evidence that the movement is still actively progressing at the same rate.
When to Call a Structural Engineer
Call a structural engineer if the cracks are wider than 5 mm, if you can see matching distress in the external brickwork, if doors or windows are sticking, or if the cracks appear to be actively widening. You should also get professional input before instructing any repair contractor — crack-stitching and movement joint formation need to be specified correctly for the repair to hold. If you’re buying or selling a property with vertical cracks in internal walls and the vendor or surveyor can’t give a clear explanation, a structural engineer’s report will give you and your solicitor the clarity you need.
Need expert eyes on your project?
I am a Chartered Structural Engineer (CEng, MIStructE) based in Huddersfield. The Beam Doctor offers homeowners and builders:
- Structural Engineer’s Report from £350 — a signed diagnosis of cracking, movement or defects with a remedial specification for lenders, insurers or your builder. Request a report →
- Steel beam (RSJ) calculations from £295 — Eurocode-compliant, Building Control-ready, in 5 working days. See all services →
- On-site attendance from £225 — a Chartered Engineer at your property, tiered by duration. See site visit fees →