This is a short, practical piece for whoever is actually going to stand on the slab with a tape: a site engineer, a PMC inspector, a facility manager getting a remedial quote. It sets out what to record, why each item matters, and the one mistake that turns a correct measurement into a wrong order.

The whole thing takes about ten minutes per run and needs a tape, a thermometer and a phone camera.

Measuring an expansion joint gap: three points and a temperature Top: a plan view of one continuous joint run with three measurement stations A, B and C reading 96, 104 and 112 millimetres, showing that a built gap tapers. Bottom: the same joint drawn in section twice, once on a 12 degree January morning reading 108 millimetres and once on a 43 degree May afternoon reading 94 millimetres, a difference of 14 millimetres. 1 · Three points on one continuous run slab slab A 96 mm B 104 mm C 112 mm One 48 m run. Measured 22 May, 15:40, 41 °C. Range 96–112 mm. That is not measurement noise — it is a taper, and it changes what can be installed. 2 · The same joint, measured twice 14 January, 07:30 12 °C — gap reads 108 mm 22 May, 15:40 43 °C — gap reads 94 mm Same joint. 14 mm apart. Neither number is usable without the temperature written beside it.
Two things a single reading cannot tell you. Above: the built gap tapers along the run, so one station is not the joint. Below: the same joint, six months apart — the number moved 14 mm and nothing was wrong with either measurement.

Why one reading is not a measurement

A movement joint is not a fixed-width slot. It is the current position of two structures that are moving relative to each other all year, and the width you measure is wherever those two structures happen to be standing on the day you turn up.

The size of that effect is easy to get wrong by assuming it is small. Take a 45 m concrete bay, a coefficient of thermal expansion of 10 × 10−6 per °C, and the swing between a 12 °C winter morning and a 43 °C May afternoon:

45,000 mm × 10 × 10−6 × 31 °C ≈ 14 mm

Fourteen millimetres, on a nominal 50 mm gap, is more than a quarter of the gap — and it is shared between two bays meeting at the joint, so the joint itself can see more. A gap read on a hot afternoon is near its most closed. A gap read on a cold morning is near its most open. Neither is wrong. Both are useless without the temperature written next to them. The arithmetic behind that figure is worked through properly in calculating the gap width, with Indian numbers.

Record the ambient temperature at the joint, and the time. Not the weather app — that is the air temperature at the nearest observation station, which on a podium slab in direct sun can be twenty degrees adrift. A cheap thermometer in the shade next to the joint, left for two minutes, is enough.

Why three points, not one

The second thing a single reading hides is that the built gap is almost never the drawn gap, and almost never constant. Shuttering moves. Slab edges are cast against timber that bows. On nearly every building we survey, a run nominally drawn at 100 mm arrives somewhere between about 85 and 125.

So measure three points on one continuous run — both ends and the middle — and write down all three. What you are looking for is not an average. You are looking for whether the run is parallel or tapered:

  • Three readings within a few millimetres — a parallel run. Straightforward.
  • A steady drift from one end to the other — a taper. This matters, because a joint system is manufactured to a gap, and a run that goes 96 to 112 either needs to be split into lengths or needs a system whose frame tolerates the range. That is a decision someone has to make before anything is cut.
  • One reading well outside the other two — usually a local defect: a broken slab edge, a nib cast proud, debris. Photograph it; do not average it away.

One run, three points, three numbers. If the building has several runs, each gets its own set — they will differ.

The measurement that gets confused: structural gap or finish gap

This is the one that causes real ordering errors, and it is entirely avoidable.

There are two different gaps at every floor joint, and they are not the same number:

  • The structural gap — concrete slab edge to concrete slab edge, at mother-slab level.
  • The finish gap — the opening left in the tile, stone or screed above it, at finished-floor level.

They are usually different, because the finish is rarely stopped exactly over the slab edge. And they matter to different systems: some joint systems are set on the mother slab and built up to, others fit from finished-floor level into the finish itself. Our Megatec 300 and SeismAtec 450 sit on the structural slab; the Megatec 310 and 370 are fitted from finish level. Give us the wrong one of the two numbers and the frame will not sit where it has to sit.

So the measurement is only complete with three more things:

  1. Which level you measured — structural or finish.
  2. The finish thickness — tile plus bed, or screed depth.
  3. The depth from finished floor down to the structural slab at the joint. This is the number that decides whether a system physically fits, and it is the one most often missing.

The five-minute checklist

Everything above, in the order you would actually do it:

  1. Pick one continuous run and note where it is — level, block, grid line.
  2. Measure the gap at three points: both ends and the middle. Write all three.
  3. Measure the temperature in shade at the joint, and note the time of day.
  4. Say which level you measured at, and give the finish thickness and the depth down to the structural slab.
  5. Photograph the run end to end, and close up at anything odd — a step, a taper, debris, a previous repair.
  6. Note what is underneath — occupied space, basement, car park, open air. That decides whether the joint needs to be watertight and to what standard.
Also worth recording, if it is a remedial jobWhatever is in the gap now — grit, cement droppings, a filler board, sealant, mortar — and whether the existing cover rattles, sits proud or leaks. A gap that has been filled solid tells you the movement has been going somewhere else, possibly for years. The signs are in five things to look for before it becomes a leak.

The mistake the measurement is supposed to prevent

Here is the trap, and it survives even a perfect survey.

The gap tells you what will fit. It does not tell you what will work.

A 100 mm gap does not mean the building needs a 100 mm system. It means 100 mm is what somebody left. Whether that is right depends on the movement the structure will actually see — bay length, exposure, seismic zone — which is a calculation, not a tape measurement.

And the reverse error is more common and more expensive: a 100 mm system does not provide 100 mm of movement. Movement capacity is a fraction of the gap, the fraction differs between systems, and the corner version of a system usually offers less than the straight one. That is the single most misread thing on a datasheet, and it is set out in how to read a movement capacity table.

Which is why we ask for the survey and the drawings. The gap tells us what can be installed on Monday. The bay length and the zone tell us what should have been installed.

Two things that make the precision less fussy than it sounds

Measure carefully, but do not agonise over a millimetre, for two reasons.

Systems are banded, not continuous. A range of joints is manufactured at set gap sizes, and several sizes share components — in some families the frames are identical across a band of gaps and only the cover plate changes. So a reading of 96 or 104 will very often land in the same manufactured size. What changes the answer is which band you are in, not the last millimetre.

The joint has to accommodate your reading plus the movement either side of it. That is the point of recording the temperature: it lets the gap you measured be converted back to the gap at mean temperature, which is the number a system is actually selected against. A reading taken at 41 °C is not the design gap — it is the design gap minus most of a summer's closure.

What to send us

If you want a system recommendation rather than a price for something you have already chosen, this is the whole list:

  • The three gap readings for each run, the temperature and the time
  • Structural or finish level, finish thickness, depth to slab
  • Photographs along the run
  • What is underneath, and whether it is trafficked by people, trolleys or vehicles
  • If you have them: the bay length either side, and the seismic zone

That is enough for us to size the joint rather than guess at it, and to say plainly if what you need is not a joint at all. If you are at design stage and there is no gap to measure yet, the drawings are enough — send those instead and we will return a joint schedule with the sizes on it, which is described on the joint schedule page.

The honest versionMost sites cannot give all of this, and that is fine — three gap readings and a temperature already put you ahead of nearly every enquiry we receive. Send what you have. What we cannot do is size a joint from one number with nothing attached to it, and it is better to say so than to quote against it and be wrong on site.
Site surveyGap measurementSpecificationThermal movementRemedialInstallation