Who this guide is for
- Owners adding an extension where the question of tying or separating has arisen
- Readers of a drawing that distinguishes movement joints from construction joints
- Self-builders whose building is long, or has wings meeting at an angle
- Anyone seeing cracking a short distance from a joint that was supposed to take the movement
- Buyers assessing a building made of parts built at different times
What a movement joint is, and what it is not
Materials change size with temperature and with moisture, and different materials do it by different amounts and sometimes in opposite directions. A movement joint is a deliberate break that lets that happen without the material being forced to crack in a place nobody chose.
The published entry separates it from two things it is routinely confused with. It is not a construction joint, which is where work stopped and restarted and is intended to act as continuous. It is not a crack, which is where movement occurred without provision. And it is not a sealant, which closes the joint against weather; the joint is the gap, and the sealant is one component within it.
Why a building is separated rather than jointed
A joint in a wall lets a length of construction change dimension. A separation goes further: it divides the building into parts that are structurally independent of one another, so that differential movement, differential settlement, or two quite different structural arrangements meeting are not forced to reconcile.
That is why the stability entry treats it as a stability question rather than a detailing one. Where a building is separated into parts, each side of the joint stands alone, and the design question it asks is whether each side has a complete arrangement of its own.
- Two parts built in different materials with unrelated movement behaviour
- Two parts whose foundations are expected to settle differently
- A long run where accumulated dimensional change has to be released
- New construction meeting existing construction of unknown behaviour
What each side then owes
Independence is not free. Each separated part needs its own continuous path for vertical load to the ground and its own arrangement for resisting horizontal action, because neither can borrow from the other across a gap that is there precisely so that nothing passes.
It also needs its own foundations, and where the two sit side by side, what each does to the ground the other is relying on becomes a question in itself. The published foundation guidance treats matching founding depth and differential settlement between old and new as part of the same conversation.
A joint only exists while nothing crosses it rigidly
The component entry states the condition plainly: a joint has to pass through everything that needs to move, and one interrupted by a rigid finish is not a joint. The interface record adds what happens then. A rigid finish laid across a movement joint has eliminated it, the movement still occurs, and the crack appears in the finish, usually a short distance from where the joint was and usually where nobody would have chosen.
The stability entry names the structural version: elements that unintentionally bridge a joint, such as a continuous screed or a rigidly fixed lining, tie together two structures that were meant to move independently.
The ordinary operations that undo it
What is striking about this failure is how mundane its causes are. A screed run through, a render or tiling applied over, a mortar bond carried across, a lining fixed rigidly to both sides, a tie or a bracket spanning the gap, a service run clamped at both ends. None of those looks like structural work while it is being done.
The masonry entry describes the equivalent for a wall: bridge a movement joint with a tie or bond a return across it, and the movement still occurs and appears instead as a crack at the weakest position available.
- Screeds, toppings and paving carried across the line
- Renders, tiling and jointing applied over the gap
- Linings, trims and skirtings fixed to both sides
- Services clamped at both sides of the gap
The gap still has to keep the weather out
A joint through an external wall is a hole through it, and the component entry is direct that closing it against weather while letting it move is exactly the difficulty. Joint sealants have a finite life and are among the components most likely to need replacing during a building's life, which makes reachability part of the design rather than an afterthought.
Where the joint passes through a water-resisting element, what maintains that resistance is a question in its own right, and it is one of the ones the published interface record puts to professionals.
Telling movement joints from construction joints on a drawing
In cast construction the two have opposite intentions and are easily confused. Where a movement joint is treated as a construction joint and continuity is carried across it, the structure cannot make the movement it needed and cracks somewhere it was not intended to. Where a construction joint is treated as a movement joint and continuity is left out, the frame has a break where it needed to be whole.
That is why one of the published questions for professionals is simply which of the joints in this structure are construction joints and which are movement joints. It is a reasonable thing to ask of a drawing set, and the answer should be unambiguous on the drawing rather than in somebody's head.
Positions follow the movement, not the plan
A joint's position follows from where movement concentrates, which follows from the materials and the geometry rather than from convenience. The entry rejects the idea that a joint can be moved to a tidier position, and the pier entry adds a related point: a change in section along a wall is also a change in stiffness, and the joint arrangement has to account for it.
Where a run is long, where openings already weaken it, or where different unit families meet, the positions at which movement is permitted are a design decision for the specific construction.
Working with a separated structure
- 1Establish whether the building is one structure or several standing beside one another.
- 2Ask what the separation was provided for, and what each side is expected to do.
- 3Ask whether each side has its own complete arrangement for resisting horizontal action.
- 4Ask whether each side has its own foundations and how they relate to one another.
- 5Ask which joints on the drawings are movement joints and which are construction joints.
- 6Ask whether each joint passes through every layer that needs to move.
- 7Check whether any screed, render, tiling or paving has been carried across a joint.
- 8Check whether any lining, trim or skirting is fixed to both sides of a joint.
- 9Check whether any service is clamped on both sides of the gap.
- 10Ask what closes each joint against weather, and what its expected life is.
- 11Ask whether the joint can be reached for the sealing to be renewed.
- 12Where cracking appears near a joint, record its position relative to the joint line.
- 13Keep the drawing showing the joint positions with the building record.
Common mistakes to avoid
- Treating a movement joint as an unsightly compromise, when it puts the discontinuity where it was chosen.
- Moving a joint to a tidier position, when its position follows from where movement concentrates.
- Confusing a construction joint with a movement joint, which produces the opposite error in each direction.
- Applying a rigid finish over a joint, which eliminates it and moves the crack somewhere unchosen.
- Separating a building structurally and then tying the two parts with a continuous screed or a fixed lining.
- Giving one side of a separation a stability arrangement and assuming the other borrows it.
- Treating the sealant as the joint, when the joint is the gap and the sealant is one component in it.
- Detailing a joint in a position that cannot be reached when the sealing needs renewing.
When to involve a professional
- Movement joint provision is a design calculation based on the materials, geometry and exposure of the specific construction.
- No joint spacing, width, sealant type or movement allowance is stated here, and none can be inferred from any description given.
- Whether two parts of a building should be tied or separated is a determination for a structural engineer for that building.
- Where a joint appears to have been bridged, what to do about it is a design question rather than a repair to improvise.
- Where the joint passes through a water-resisting element, its arrangement belongs with the professionals responsible for that element.
Frequently asked questions
Questions readers ask about this topic
Why would a building be split into separate structures?
So that parts which would otherwise force each other to move can behave independently. That may be because they are built in different materials, because their foundations are expected to settle differently, or because a long run accumulates dimensional change that has to be released somewhere chosen.
What does each side of a separation need of its own?
A complete load path to the ground, a complete arrangement for resisting horizontal action, and its own foundations, because nothing passes across a gap that exists precisely so that nothing does. The stability entry asks exactly that as a design question.
Why is there cracking just beside a movement joint?
That pattern is characteristic of a joint having been bridged. Where a rigid finish is applied across it the joint has been eliminated, and the movement appears instead in the finish a short distance away, in a position nobody would have chosen. What it means in a specific building is for a professional to establish.
Is a construction joint a weakness?
No. It is a planned interruption of the work with continuity carried across it, and it is intended to act as continuous. Confusing it with a movement joint produces errors in both directions: continuity carried where movement was needed, or continuity omitted where the frame needed to be whole.
Do movement joints need maintenance?
The component closing them does. Joint sealants have a finite life and are among the components most likely to need replacing during a building's life, which is why whether the joint can be reached for that renewal is part of the design rather than a later problem.
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