Filling a junction crack and repainting over it is the repair most homeowners try first, and it's also the one that fails fastest. If the underlying movement hasn't stopped or been accommodated, the crack reopens along exactly the same line because nothing has changed about the forces causing it.
A proper repair treats the crack as a symptom of a specific mechanism and addresses that mechanism first. Only once the movement is understood, and either stopped or given somewhere to go, does the cosmetic work happen.
The correct sequence of works
The first step is always monitoring, not repair, unless the crack is already clearly stable and long-standing. Tell-tales or simple measured checks over several months establish whether the movement is ongoing, seasonal, or has already settled down.
Once the pattern is clear, the works fall into one of two paths. If it's seasonal movement with no proper movement joint, the fix is to cut back the rigid tie between old and new and form a correctly detailed movement joint, with a flexible sealant that can expand and contract without cracking. If it's genuine ongoing settlement, the foundation itself needs investigating and, in some cases, underpinning.
What a cosmetic patch fails to address
Filling the crack with a rigid filler and skimming over it does nothing about the joint's inability to move. The two structures will keep expanding and contracting at slightly different rates through every seasonal cycle, and a rigid filler simply cracks again at the weakest point — usually the same line.
This is why repeatedly filled junction cracks are so common on older extensions: each redecoration cycle repeats the same mistake without ever addressing why the crack exists.
Interfaces with other trades
If underpinning is required, that work has to be designed and specified before any finishing trade goes near the wall, because it changes what's structurally sound to build on top of. A structural engineer's input is needed here, not a general assumption.
Once the joint or foundation work is complete, the render, pointing and internal decoration can follow in the normal order — but not before, or you're finishing over a problem that hasn't been resolved.
How you know the repair has worked
A correctly formed movement joint will flex with the seasons rather than crack, so the test is simply watching it through a full year and seeing the joint move without a new crack appearing beside it.
Where underpinning was carried out, the test is that movement stops altogether — no reopening, no new hairline cracks radiating from the same area — confirmed by continued monitoring rather than a single inspection.
When specialist structural input is genuinely needed
Most junction cracks are resolved with a correctly formed movement joint and don't need a structural engineer, but that changes if monitoring shows the crack is still widening after a full seasonal cycle, or if it's accompanied by other signs such as doors sticking or floors sloping nearby.
A structural engineer should also be brought in before any underpinning is even discussed, since underpinning is a significant and costly intervention that should only follow a proper assessment of the foundation, not an assumption based on the crack alone.
Common questions
Do I need underpinning for a junction crack?+
Not usually. Most junction cracks are resolved by forming a proper movement joint rather than underpinning, which is a significant structural intervention reserved for cases where genuine ongoing settlement is confirmed.
Can the crack be repaired without opening anything up?+
A cosmetic-only repair can be done, but it will only last if the underlying cause has already stopped acting — for example, if the extension is old and long settled. Where the movement is still active, some opening up around the joint is unavoidable.
Will repointing alone fix it?+
Repointing addresses the mortar but not the joint's ability to move. If there's no flexible movement joint behind the new pointing, the crack will return through the fresh mortar in time.
Get a written quote for a proper junction crack repair
We monitor first, diagnose the mechanism, then quote the actual repair — movement joint, underpinning, or both — in writing.