Take one row from our own catalogue, the Megatec 300 floor joint at a 100 mm gap: 100 | ±25 | [50]. Three numbers. We have seen every possible misreading of them on tender schedules — “suitable for gaps up to 50 mm”, “movement 100 mm”, “opens by 50 mm” — and each one produces a joint that is either far too big for the job or, more often, quietly too small.
This post is the slow read of that block, using the numbers from our published tables, and then the four things the table does not tell you that decide whether the row you picked will survive.
The three numbers, and what each one is
Gap width, B. The structural gap the joint is built to sit in — the distance between the two slab edges at the moment of installation. It is a size, not a capacity. Every other number in the row is stated for this gap; move to a different gap and you are on a different row.
Movement, ±. How far the joint can travel each way from that installed gap. ±25 on a 100 mm row means the joint keeps working while the gap closes to 75 mm and while it opens to 125 mm. Not a 50 mm gap; not 50 mm of opening. Twenty-five in, twenty-five out.
[Total]. The two halves added together — the full span of travel the system was cycled through in test. It is there for comparing against a calculated range (a bay that swings 20 mm between January and June has a range of 20, or ±10). It is not a margin, and it is not a second, larger allowance.
The ± only means what it says if the joint is installed at the middle of its travel. Fit it on a June afternoon with the gap at its narrowest and you have already spent part of the closing half.
That last point is the single most common way a correctly chosen row fails in service, and it is an installation matter rather than a table-reading one — it is covered in calculating the gap width, under setting the joint to the temperature on the day. Here, just hold on to the fact that the table assumes a mid-travel start.
The movement is a fraction of the gap — and the fraction is the system
Read down any one of our tables and the ± column tracks the gap column at a fixed ratio. Read across systems and the ratio changes. Both readings matter, because a specification that names only a gap width has not specified any movement at all until the system is known.
| System (India-made range) | ± at 50 mm gap | ± at 100 mm gap | Movement as a share of gap |
|---|---|---|---|
| Megatec 300 floor, A01 | ±12 [24] | ±25 [50] | about a quarter |
| Megatec 340 floor, A01 | ±25 [50] | ±50 [100] | half |
| Megatec 370 floor, A01 | ±25 [50] | ±50 [100] | half |
| Megatec 360 wall to wall, A07 | ±25 [50] | ±50 [100] | half |
| Megatec 360 corner and ceiling, A09 | ±12 [25] | ±25 [50] | a quarter |
| Megatec 610 roof, A01 | ±25 [50] | ±50 [100] | half |
| SeismAtec 450 floor, A01 — thermal | ±25 [50] | ±25 [50] | fixed, whatever the gap |
| SeismAtec 450 floor, A01 — seismic | ±25 [50] | ±50 [100] | half |
| FlexAtec 1200 seal, A01 | ±15 [30] | — (range ends at 75) | about three-tenths |
Figures from the current Vexcolt Systems tables. Two consequences fall straight out of it.
Two “100 mm joints” from the same catalogue can differ by a factor of two in what they will actually do. A Megatec 300 at 100 mm gives ±25; a Megatec 340 or 370 at the same gap gives ±50. Neither is wrong — the 300 is a male-female plate system set on the mother slab, the 340 carries a centring bar that keeps its plate self-aligning across a much larger swing — but a schedule that says “100 mm aluminium expansion joint” has left the most important number to whoever prices it lowest.
The seismic table has two movement columns, and they are different promises. The SeismAtec 450’s thermal rating is ±25 mm at every gap from 50 to 500; its seismic rating is half the gap. That is not an inconsistency. Thermal movement is small, daily and cyclic — thousands of cycles a year — and the pan system is rated for that duty at a fixed figure. Seismic movement is rare and large, and the system is rated to survive it, once, by a different mechanism. Specifying a 300 mm seismic joint because the structural separation is 300 does not buy you ±150 of thermal travel; it buys ±25 thermal and ±150 seismic. See what the seismic zone changes for where the separation figure comes from.
Same series, different joint: the corner row
The row that catches out the most careful specifiers is not a different system but a different joint within one. The Megatec 360 wall system rates its wall-to-wall joint (A07) at half the gap — ±50 on a 100 mm gap. Its corner and ceiling-to-wall joint (A09), on the same page and the same gap, is rated at a quarter: ±25.
The reason is geometry. A flat wall joint can slide its centre plate symmetrically over both frames; a corner has a fixed leg on one side and all of the travel has to be taken on the other. Half the freedom, half the movement.
In practice this means a wall line that turns a corner or meets a ceiling needs its gap sized for the corner row, not the wall row. If the calculated movement is ±25 and the wall run is on a 50 mm gap (A07-050 gives exactly that), the corner on the same line needs a 100 mm gap to reach the same ±25 — or it becomes the point at which the run fails. Transitions are the part of the schedule most often left off, and this is the number that makes them matter; there is more on them in how to specify an expansion joint.
The load columns are on the same row for a reason
Next to the movement block sits the load rating, with the wheel icons — pallet truck, forklift, car, lorry. Those figures are stated per gap, and they fall as the gap widens, because a wider gap means a longer unsupported span of plate under the same wheel.
The Megatec 340 row makes this stark. At a 25 mm gap the pallet-truck rating is 10 kN; at 50 it is 5 kN; at 75 it is 2 kN; from 100 mm upward the table gives no wheeled-traffic rating at all. The Megatec 300 tells the same story more gently — its heaviest-vehicle rating goes from 600 kN at 25 mm to 200 kN at 250 mm.
So the instinct to “go a size up for margin” on movement can quietly delete the traffic rating the corridor needs. A hospital service corridor that takes pallet trucks and a warehouse aisle that takes forklifts are load selections first and movement selections second. Read the whole row, not the one column you came for.
Four things the table does not say
- Which direction the movement is in. The ± is opening and closing across the gap — the axis the ASTM E-1399 cycling rig tests. Vertical differential movement between the two slabs, and shear along the joint line, are separate questions; if the structure has either, they go in the enquiry as their own figures, and the answer will not come from this column.
- Where in the cycle the joint was installed. Covered above. The table is mid-travel; the site is whatever temperature it was that day.
- What the class means. “ASTM E-1399 Class I thermal, Class II wind, Class III seismic” describes the kind of movement the system is rated for, not a larger or smaller number. A Class III system is not automatically better at thermal movement — the 450’s fixed ±25 thermal figure is the proof.
- Anything about the slab edge. The row assumes the frames are anchored into sound concrete per the method statement. A rating is a promise about the joint, not about the rebate it goes into — which is why most remedial jobs are substrate jobs.
A worked selection
Take the example from the gap-width post: a 45 m bay in Delhi with an effective range of 45 K moves about 20 mm, or ±10 about its mid position. Say the surveyed gap is 50 mm, the location is a hospital service corridor with pallet-truck traffic, and the finish is vinyl over screed.
- Megatec 300-A01-050 — ±12 [24]. Covers ±10 with 2 mm to spare each way. That is not margin; that is a coincidence waiting for a hot-day installation.
- Megatec 340-A01-050 — ±25 [50], pallet-truck rating 5 kN at this gap. Two and a half times the calculated movement, and rated for the traffic. This is the row.
- Megatec 370-A01-050 — also ±25 [50], a 6 mm-high plate fitted from finish-floor level. The right answer where the finish is thin and the rebate depth is not there; the same movement, a different build-up.
Then the specification line writes itself, and it names the movement rather than hoping the gap implies it:
Movement joint, surveyed gap 50 mm, rated movement not less than ±20 mm (calculated ±10 mm), pallet-truck rated at the stated gap, tested to ASTM E-1399 Class I — reference row Megatec 340-A01-050 or equivalent meeting every figure above.
“Or equivalent” is harmless in that sentence because every figure it has to equal is on the line. It is only dangerous when the line says “100 mm aluminium expansion joint” and nothing else.
How to read any manufacturer’s table, including ours
- Find the row for the surveyed gap, not the drawn one.
- Read ± as travel each way from that gap. Ignore [Total] until you are comparing it against a calculated range.
- Check the ratio ± ÷ gap. If it is a quarter, you are on a system that needs a wide gap for modest movement; if it is a half, the reverse. Neither is better — but the schedule has to say which.
- If the joint turns a corner or meets a ceiling, find the corner row and size the gap to that.
- If the table has two movement columns, decide which duty governs — and check the thermal figure on its own.
- Read the load columns at the same gap. A rating that is not stated at your gap is not a rating.
- Insist on a margin over the calculated movement that survives a hot-day installation, then write the required ± into the clause.