Where this starts
A pop is a small circle of filler that has lifted, cracked or fallen away over a fastener, sometimes with the head just visible. The instinct is that the nail worked its way out. It usually did not.
WHAT ACTUALLY HAPPENS is the reverse. A timber stud or joist installed wetter than the finished building will be loses moisture as the heating goes on, and it shrinks ACROSS its width — several times more than it shrinks along its length. The fastener stays where it is relative to the timber, but the face of the timber retreats from the back of the board. The board then has a small gap behind it at that point, and the first thing that pushes on the board — someone leaning on it, a door slamming, a floor deflecting — moves the board back against the head, which cracks the filler.
THE TIMING IS THE GIVEAWAY. Pops appearing across a whole new ceiling or wall in the first autumn and winter, and then stopping, are drying shrinkage. Pops that keep arriving year after year, or arrive in a line, or arrive along with a crack that is not round, are something else.
THE PATTERN IS THE SECOND GIVEAWAY. Scattered pops over a large area are a material and moisture story. Pops concentrated in one line, or along a joint, or clustered where a floor above bounces, are a movement story — and movement is the one worth understanding before filling anything.
One practical point about the repair, since it decides whether it comes back: filling the crater over the same fastener leaves the gap behind the board intact. The board has to be pulled tight to the timber again, with a new fastener placed close to the old one, before anything is filled.
What it could be, and what tells them apart
These are candidates to check against your own building, in the order they most often turn out to be the answer. None of them is a diagnosis of your situation.
Timber drying and shrinking after the boards went on
Most commonScattered pops appearing across a new or recently boarded area during the first heating season, with no pattern following a joint or a line, and slowing after the first year.
How to check: When the building was boarded and whether the heating had run before. Pops in the first winter after boarding are drying, not defects.
Fasteners that never pulled the board tight
Most commonPops appearing early and in clusters rather than evenly, often where a boarder was working fast, sometimes with the board flexing slightly when pressed beside the pop.
How to check: Press beside the pop. A board that moves at all was never held tight against the timber there.
Deflection in the floor or ceiling above
CommonPops in a line or a band rather than scattered, often across a ceiling under a room that gets walked on, and worsening in use rather than in a season.
How to check: Whether the line of pops follows a joist and whether the floor above is walked across at that point.
Fasteners too short, too long, or the wrong type
CommonHeads visible rather than merely cracked, or the crater deep enough to expose the fastener, and often the same fastener used throughout a job regardless of board thickness.
How to check: The depth of the head below the board face. A head sitting proud, or driven so hard it broke the paper, was never going to hold.
Truss or roof movement transmitted into the ceiling
Less commonPops and cracking concentrated at the junction between ceiling and partition walls, appearing and closing seasonally rather than accumulating.
How to check: Whether the pops sit near where a ceiling meets an internal wall, and whether they open and close with the seasons.
Something behind the board being pushed against it
Less commonA local bulge rather than a flat area with pops, sometimes warm or cold to the touch, occasionally following the line of a pipe or a duct.
How to check: Whether the surface around the pop is flat. A bulge means something behind the board is bearing on it.
What to check, in order
The order matters — each step is here because of what it rules out for the ones after it.
- 1
It separates drying shrinkage from everything else, and it is the single question that decides whether waiting is the right answer.
- 2
Scattered is material and moisture, a line is a joist or a structural member, and a cluster is usually workmanship in one part of one day.
- 3
Movement means the board is not held tight to the timber there, which is both the cause and the reason a fill-only repair fails.
- 4
A round pop is a fastener problem. A crack running away from it is the board or the structure moving, and that is a different investigation.
- 5
A ceiling under a bouncy floor gets pushed at every footstep, and no refixing survives that until the deflection is addressed.
The calculations behind this
Several of the causes above are a quantity that was specified or assumed. These compute them, with the formula and the standard behind each.
When to stop and get somebody in
- Where pops arrive with cracks running out of them, particularly diagonal cracks from the corners of openings, since that is movement rather than fastening.
- Where a ceiling shows pops in lines along every joist together with visible sag, which is a question about the ceiling's support rather than its finish.
- Where pops keep returning in the same places after correct refixing, meaning something is still moving.
- Where the boards are on a ceiling below a roof space and the pops coincide with damp staining, which brings a moisture source into it.
This page lists candidates to check, not a diagnosis — nobody has seen your building. If the symptom involves anything structural, read this first.