At specification stage the question usually arrives as a preference: the architect wants something quiet underfoot, the contractor wants something quick to fit, the client wants something that will not need attention. Each of those is reasonable, and none of them is how the choice should be made. The choice between a metal cover joint and a sealed joint is decided by the gap, the movement, the seismic case, the traffic and what sits below. The preferences come after.
This piece compares the two families we manufacture in India: MegAtec, the aluminium cover systems, and FlexAtec 1200, the seal joint. Every figure is read off the current Vexcolt datasheets. Where a datasheet does not say something, neither does this article.
What each one actually is
A metal cover joint is a pair of aluminium frames anchored into the slab on either side of the gap, with a plate that bridges the gap and slides as the structure moves. The walking surface is metal. The MegAtec 340 datasheet lists its material as 6063 T5 and T6 aluminium and its seal as “no seal used”.
A sealed joint is a flexible seal held between the two slab edges. It closes and opens with the gap rather than sliding over it. The walking surface is the seal itself. The FlexAtec 1200 datasheet lists its metal type as “none” and its seal as black, with a ribbed surface.
The difference in mechanism is the root of every difference that follows. A plate that slides can be made to span a wide gap and to move a long way. A seal that stretches can only move as far as the material allows, and only across a gap it can physically bridge.
Question 1: how wide is the gap?
This settles more cases than anything else, and it does so in seconds.
- FlexAtec 1200 is rated for gaps of 25 to 75 mm (25, 40, 50, 65 and 75 mm on the datasheet). We make it in India at 25 and 50 mm.
- MegAtec covers 25 to 300 mm on floors (the 340), 25 to 200 mm on walls and ceilings (the 360) and 25 to 300 mm on roofs (the 610).
So a structural gap over 75 mm is a metal joint, whatever else is true. Seismic separation gaps in tall buildings are often wider than that, so for them the question is usually settled here.
Question 2: how far does it have to move?
This is where the two families separate at gaps they share. Read straight off the two datasheets:
| Gap | FlexAtec 1200 (sealed) | MegAtec 340 (metal) |
|---|---|---|
| 25 mm | ± 7 [15 total] | ± 12 [25 total] |
| 50 mm | ± 15 [30 total] | ± 25 [50 total] |
| 75 mm | ± 25 [50 total] | ± 37 [75 total] |
Put plainly: at the same gap, the seal is rated for about 60% of the metal joint’s movement. The 340 moves half its gap each way. The 1200 moves a little under a third.
That matters most on long bays and exposed slabs, where the calculated movement is largest. If your calculation says a 50 mm joint must take 40 mm of total movement, the seal at 50 mm (30 total) is not enough and the metal joint at 50 mm (50 total) is. You can widen the gap and stay with a seal, but only up to its 75 mm limit. Past that there is no seal option at all.
If you have not yet calculated the movement, the thermal movement calculator does it from bay length and temperature range. If you are not sure how to read a ± figure against your number, this piece on movement capacity tables walks through one row.
Question 3: is there a seismic case?
The FlexAtec 1200 datasheet rates it to ASTM E-1399 Class I (thermal) and Class II (wind). It carries no Class III (seismic) rating. The MegAtec 340 and 360 datasheets add Class III: seismic.
So where the joint is a seismic separation, or the structural engineer has asked for seismic movement to be accommodated, the seal is out regardless of gap. What the zone actually changes is set out in our note on seismic zones.
Question 4: what crosses it?
Both datasheets carry a traffic-frequency table and a load rating per gap. Read them against the traffic the joint will actually see, not the traffic the room is named for. A “corridor” in a hospital carries beds and trolleys. A “lobby” in a mall carries cleaning machines every night. A podium carries cars.
Two things are worth knowing before you open the tables.
- Load rating falls as the gap widens on a metal cover. On the 340 the rated loads at 75 mm are well below those at 25 mm, because the plate spans further. So check the row for your gap, not the headline.
- A seal has no plate to rattle. A cover joint under trolleys that is not fixed properly will announce itself. That is a fixing fault, not a reason to choose a seal, but it is part of why some facility managers ask for one.
Question 5: what is underneath?
The MegAtec 340 datasheet offers a moisture barrier as a system option. The FlexAtec 1200 datasheet lists waterproofing as “not applicable”.
Read that literally. A seal in the walking surface is not the waterproofing layer. Over an occupied basement, a car park or a podium with rooms below, the membrane has to be detailed across the joint separately, whichever cover goes on top. How that membrane must be looped and lapped at the joint is in where the joint meets the waterproofing.
Then the preferences: height, look and finish
Once the five questions are answered, the remaining differences are real but secondary.
- Height. The 1200 is 55 or 64 mm high; the 340 is offered at 25, 38 and 50 mm. The height sets the blockout depth in the screed, so it has to be agreed before the floor build-up is fixed. What the joint needs from the slab covers that.
- Surface. The 1200 has a ribbed black seal; the 340 a hairline-serrated aluminium plate. Both datasheets state ADA compliance across the entire movement cycle.
- Appearance. A seal reads as a dark line. A metal cover reads as a strip of aluminium, which is either a detail the architect wants or one they will want hidden. That is a design call, and a legitimate one, but it is the last question, not the first.
The short version
| Choose a sealed joint (FlexAtec 1200) when… | Choose a metal cover (MegAtec) when… |
|---|---|
| The gap is 25–75 mm (25 or 50 mm for India-made) | The gap is over 75 mm, or up to 300 mm |
| Calculated movement fits within the seal’s rating at that gap | Movement exceeds the seal’s rating at the gap available |
| Thermal and wind movement only | There is a seismic case (ASTM Class III) |
| The traffic table and load rating cover the actual use | A moisture barrier is wanted as a system option |
If you want this done for a whole building rather than one joint, the system selector narrows it by location, gap and traffic, and the product pages for the FlexAtec 1200 and the MegAtec expansion joint covers carry the full datasheets.