Ask which joints on a project are the difficult ones and the honest answer is never the flat internal floor. It is the podium deck over occupied space, the basement, the terrace and the planter — every location where the joint is also a hole in the waterproofing.
These are also the joints that get value-engineered first, because they are out of sight when the decision is made. They are the only ones whose failure floods something.
The problem stated plainly
A building's waterproofing is a continuous membrane. A movement joint is a full-depth break through the structure. The two are in direct conflict: the membrane has to stay continuous across a gap whose width is changing, over a line that will also shear and offset vertically.
So the joint needs its own membrane, and it has to do three things that a flat sheet cannot:
- Bridge the gap with slack in it. A membrane pulled tight across a joint at installation will be pulled tighter every time the joint opens, and it will tear or debond at the edge. It is installed as a loop — deliberate extra material dipped into the gap — so that opening takes up slack rather than load.
- Bond to the structural membrane on both sides. Not butt up against it. The two have to become one continuous barrier, which means a lapped and bonded joint of adequate width on each side.
- Survive the same movement cycles as the metal above it, for the life of the building, in a location nobody will ever inspect again.
A joint over occupied space with no membrane installed inside it is not a risk. It is a leak with a commissioning date.
What we actually install, and how it is sized
Our floor systems carry an FPO membrane as standard. It is not an optional extra on a horizontal joint and we do not quote it as one — a floor always has to be protected against seepage, so on our floor and floor-to-wall systems the membrane is part of the build whether or not the specification remembers to ask for it.
The membrane is bought as roll goods, one metre wide, and cut on site to the width the joint's gap calls for. That width is a function of the gap, not of which product line the joint belongs to — a point that surprises people who expect a table keyed to the system number:
| Structural gap | Typical membrane strip | Why |
|---|---|---|
| 100 mm | roughly 250–300 mm | Enough to loop into the gap and still leave a bonded lap either side |
| 150 mm | roughly 400 mm | A wider gap needs both a deeper loop and the same lap width on each side |
The exact width is a per-job call rather than a formula. The principle is not: lap width does not shrink as the gap grows, so a wider joint needs a proportionally wider strip, and cutting it to the gap dimension alone leaves nothing to bond with.
The four places it actually fails
1. The termination, every time
Not the middle of the run. The lap where the joint membrane meets the structural membrane, and the upstand where a horizontal joint turns up a wall or a kerb. Those are the two details worth drawing at a large scale and worth inspecting before anything is covered, because after screed and finish go down they are unreachable.
2. The falls run the wrong way
A podium deck is laid to falls. If those falls send water along the joint line, or worse, towards a low point that sits on the joint, then the joint is permanently standing in water rather than occasionally wetted. Moving the low point two metres off the joint line costs nothing on a drawing and is close to impossible afterwards.
3. Nobody owns the lap
This is the real one. The deck waterproofing is the waterproofing contractor's package. The joint and its membrane are ours. The two meet at a lap that appears in the middle of neither scope, and on a compressed programme each party assumes the other has it.
The fix is contractual rather than technical: name in the specification who bonds the joint membrane to the deck membrane, and require it to be inspected and photographed before covering. We supply and install as one package precisely so this interface has one owner — but only up to our side of the lap, and the specification still has to say where that line is.
4. Basements are a different problem from decks
A podium deck sheds water. A basement joint below the water table holds it back under hydrostatic pressure, continuously, from the outside. That is a genuinely harder case: pressure finds any unbonded path, and a detail that performs on a deck may not on a retaining wall. Where a joint runs below the water table, the waterproofing design belongs to the waterproofing consultant and the joint has to be selected to suit it, not the other way round.
Sequence, which is where the programme bites
The joint waterproofing goes in early — it sits at structural level, under the screed and the finish. The joint's visible cover goes in late, at fit-out. Months separate them, often with different crews.
Three things follow:
- The membrane is installed and then buried by other trades. Anything wrong with it is found by a wet ceiling, not by an inspection. Photograph every metre before it is covered.
- Anchors go through it if the sequence is wrong. The frames of the visible system are anchored into the slab. Set out badly, those anchors are drilled straight through the membrane that was installed to protect the slab — which is how a joint leaks in a building where everything was, individually, installed correctly.
- The gap changes between the two visits. The membrane goes in against one measured gap and the cover is manufactured against another, months later. Both need surveying; the second one is the survey most projects skip.
What to put on the drawing
- Which joints are over occupied space, in a basement, or on a terrace or planter. These three are the ones that need a large-scale detail.
- The falls, with the low point deliberately off the joint line.
- The lap detail at each side, with a stated minimum bonded width.
- Who owns the lap, and that it is inspected before covering.
- Whether the joint is below the water table, and therefore whether it is a drained or a tanked detail.
Where our systems sit
Floor and floor-to-wall systems — MegAtec 300, 340 and 370 — carry the FPO membrane as standard. FlexAtec 1200 is the sealed system for runs that have to stay watertight without a metal cover, which is often the right answer on a terrace or a planter. MegAtec 360 takes the upstand where a deck joint turns up a wall, and that upstand is the detail most worth getting right.
On our Bengaluru work the difficult joints were never the internal floors — they were the podium decks and the transitions into the structural membrane. The same was true in Kolkata, where the joint that fails usually fails wet.