Who this guide is for
- Owners whose newly built partition has cracked along its head
- Anyone who has filled the same crack more than once
- Self-builders detailing partitions against a new floor
- Owners fitting out a converted commercial building
- Anyone whose internal doors have needed easing at the top since the work
Floors move, and that is not the problem
Deflection under load is ordinary behaviour, and it continues over time rather than settling on the day the floor is finished. The library treats it as something to be designed around, which is why the arrangement at the top of a partition is described as a designed movement condition rather than a way of closing a gap.
The problem is not that the floor moves. It is that something was built into the space the movement needs.
A partition built tight becomes a prop
The deflection head entry states the mechanism in one line: a partition built tight to the soffit becomes a prop. It then takes load it was never arranged for, which is a different proposition from a wall designed to carry something.
The arrangement that prevents it is described as a one-sided condition. The structure above moves and the partition beneath must not be made to follow it, which is what separates a deflection head from a movement joint between two elements that each move.
The linings show it before anything else does
What the building shows is specific enough to be useful. The entries describe cracking in a long straight horizontal run along the head line that is filled and comes back, doors within the partition binding at the top and needing easing more than once, and studs found bowed when the lining is opened.
The most diagnostic of those is the repetition. It is the crack returning after a second making-good that separates a propped partition from a finish that simply moved.
- A long straight horizontal line along the head, rather than a diagonal from a corner
- The same line reappearing after it has been filled
- Doors in the partition binding at the top and needing easing repeatedly
- Studs found bowed when a lining is opened
The head has to do two opposite things
This is why the detail is easy to lose. The same head has to hold the wall sideways, which is a positive connection, and let the deck descend, which is the absence of one, and it has to manage both along the entire length of the wall rather than at chosen points.
It also has to stay closed while it moves where the partition separates. An open head is a direct path over the partition, so the detail has to be both free to move and closed to sound, and those two requirements pull against each other.
Where the arrangement is lost
The gap the detail depends on is invisible once the partition is lined, and the entries name the routine ways it disappears: a screw through both legs of the track, a service bracket, or a board fixed hard to the soffit. Fixing the studs vertically reinstates the restraint the detail exists to remove, and it then does nothing.
There is a quieter version too. Where the soffit is profiled or ribbed, the head is closed where the deck comes down to it and open in every trough between, so light, air or sound pass over the partition at each rib while the wall below reads as complete.
The difference between the drawn soffit and the built one
A partition is set out from a drawn soffit. What it actually meets is a deck that goes on coming down over the life of the building and whose underside was irregular before it started. The difference between the two is taken up in exactly the same band, by the same people, usually on the same day.
That is the practical reason this detail is so often compromised: it is the place where a setting-out tolerance, a movement allowance and a closure requirement all have to be satisfied at once by whoever is fixing the track.
What to establish before filling it again
Before the third round of filler, the questions worth putting are how much movement the structure above was expected to undergo and who established it, whether this partition is free of the deck or acting as a prop, and whether anything crosses the head — services, framing, door frames, ceiling components — that bridges the gap.
Those are questions for the people responsible for the design and the installation. A crack that returns is information, and it is more useful before it is covered up again than afterwards.
A crack along a partition head
- 1Record where the crack runs and whether it is a straight horizontal line along the head.
- 2Record whether it has been filled before, and how many times it has come back.
- 3Note whether doors in the partition bind at the top and have needed easing repeatedly.
- 4Note whether the same line appears on both faces of the wall.
- 5Photograph it before any further making-good.
- 6Ask how much movement the structure above this partition was expected to undergo.
- 7Ask who established that quantity, and whether it is recorded.
- 8Ask whether this partition is free of the deck above or acting as a prop to it.
- 9Ask how the studs are located at the head without being fixed vertically.
- 10Ask what crosses the head — services, framing, door frames, ceiling components.
- 11Where the partition separates in fire or acoustic terms, ask how the head does that while still moving.
- 12Where the soffit is profiled, ask how the head is closed at the ribs as well as between them.
Common mistakes to avoid
- Treating cracking along the top of a partition as a decorating problem.
- Filling the same line repeatedly without asking why it returns.
- Building a partition tight to the soffit for stability, which is what turns it into a prop.
- Screwing the studs through both legs of the head track, which reinstates the restraint the detail removes.
- Allowing a service bracket, a door frame or a board to bridge the head gap.
- Assuming a head that looks closed from the room is closed at every rib of a profiled soffit.
- Reading a deflection head as a movement joint, when only one side of this junction is moving.
When to involve a professional
- How much movement a structure above a partition is expected to undergo is a design quantity established for the building rather than a property of the partition.
- Deflection head details in fire-separating construction form part of a tested assembly, and both the movement allowance and the head arrangement are determined by the design team.
- Whether cracking in a particular building indicates this condition or something else requires investigation by qualified professionals.
- Nothing here states a gap, a movement allowance, a track dimension, a fixing arrangement or a rating.
Frequently asked questions
Questions readers ask about this topic
Why does the crack come back after filling?
Because filling addresses the finish and not the condition producing it. Where a partition is restraining or being loaded by the structure above, the movement continues, and the entries describe the crack returning after a second making-good as the distinguishing observation.
Is a deflection head the same as a movement joint?
No. A movement joint is a discontinuity between two parts that both move and must be free to move relative to each other. A deflection head is a one-sided condition in which the structure above moves and the partition beneath must not be made to follow it.
Can the studs be fixed at the head for stability?
The entries treat that as the classic way the detail is defeated. Fixing them vertically reinstates the restraint the deflection head exists to remove, and the arrangement then does nothing, while the head still has to be located laterally by some other means.
Why do the doors in the wall bind?
A partition acting as a prop is being loaded from above, and the entries list binding doors alongside bowed studs and cracking at the head line as what the condition produces. Doors needing easing more than once is part of the pattern rather than a separate fault.
What if the soffit above is ribbed?
The entries describe a particular failure there: the head is closed where the deck comes down to it and open in every trough between, so light, air or sound pass over the partition at each rib while the wall below reads as complete from the room.
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