How to Detail a Waterstop at a Cavity Sliding Door

Cavity sliding doors are the hardest wet-area doorway to waterproof. There's no fixed jamb to terminate the membrane against on the pocket side, the door slot is a direct water path into the wall framing, and the waterstop has to do its job while a door leaf passes back and forth over the top of it. In our inspection work, failed cavity-door thresholds show up constantly — swollen pocket framing, lifted floor boards outside the bathroom, and skirting stains that appear metres away from the actual leak. Here's how we detail it, with an interactive 3D model of the complete assembly below.

1. Why the Doorway Needs a Waterstop at All

AS 3740 requires the wet-area floor membrane to be turned up or terminated against a waterstop at door openings, so that water on the bathroom floor cannot migrate under the door and into adjoining rooms. At a hinged door this is straightforward — the angle runs across the opening and dies into the two fixed jambs. At a cavity slider, the "jamb" on one side is a hollow pocket with a slot in it, and anything you stand up in that slot has to clear the sliding leaf.

2. Use an Equal-Leg Angle, Flush With the Floor Finish

The waterstop itself is a small equal-leg metal angle — aluminium or stainless — sized so the top of the upstand finishes flush with the finished floor level. With a typical 2 mm membrane and 12 mm tile-and-adhesive build-up, that's roughly a 14 mm upstand; with a screeded floor it will be taller. Flush matters for three reasons: no trip lip at the doorway, no visible ridge under the door, and the tile on the wet side butts the upstand so the finished joint reads as a neat metal seam in the floor.

3. Position the Angle Behind the Door Leaf

Set the angle on the dry side of the door leaf line, with the horizontal leg pointing back into the wet area, fixed and sealed to the slab. The membrane then runs across the floor, over the leg, and up the inner face of the upstand to its top edge. The door leaf passes in front of the upstand and shadows it from the wet side — the waterstop works even while the door is open.

4. Run the Angle Wall to Wall — Never Stop It Short

The door angle must run the full clear opening and land on the jamb face at both ends. An angle that stops partway across the opening — or short of the pocket — leaves an unprotected flank at exactly the point where water tracks along the upstand looking for a way around. This is the single most dangerous junction in the whole detail.

5. Return the Angle at Both Jambs

At each end, a return angle is fixed with its back directly against the door jamb — no packing, no gap — crossing in front of the door angle's end and extending past it. Where the return crosses the door angle, notch the return over it and seal the cut. On the pocket side, the return also closes the cavity mouth below door level: the sliding leaf passes over the top of it (allow around 10 mm clearance under the door). Seal the back of every return to the jamb, and dress the membrane over the exposed end of each return so there is no raw termination.

6. Fillets: Size Them by What They're Bridging

• At the angle toe — angle thickness only. The fillet at the leg's free edge exists so the membrane can transition over the 3 mm step without bridging. It sits on the slab, under the membrane, and matches the angle thickness. Oversizing it here just builds a ramp into the doorway.
• At wall-to-floor junctions — 12 mm bond-breaker fillet, hard against the wall with no gap, membrane dressed over. This is the standard perimeter treatment and it must be continuous.
• Vertical corners too. The internal vertical corners at the door reveals get the same fillet treatment up the height of the membrane — corners that only get filleted at floor level crack first.
• No breaks in the bead. The fillet line should run continuously: along the wall, around the reveal corner, around the end of each return and along the angle toe, with mitred changes of direction. Every gap in that line is a future leak path.

7. Membrane Dressing Heights

• Floor membrane across the whole wet area, over the toe fillet, over the angle leg, and up the inner face of the upstand to its top.
• 150 mm minimum upturn on the wall faces either side of the opening.
• Up the door reveals, return the membrane to the height of the waterstop angle — it terminates with the angle line, not at 150, because the finished doorway sits at floor level.

8. The Failure Modes We Keep Finding

• Waterstop stopped at the cavity mouth with the pocket slot left open — water runs straight into the wall framing.
• Angle proud of the finished floor — the tiler stops short of it, the gap gets grouted, the grout cracks.
• Membrane terminated at the angle instead of dressed up and over it — capillary path under the leg.
• Returns packed off the jamb "to suit the architrave" — the packing gap becomes the leak.
• No fillet at the toe, so the membrane bridges the step and punctures under tile-bed loading.

9. Check It Before It Disappears

The whole assembly is buried under the floor finish within days. Photograph it, flood test it (dam the doorway, 25 mm of water, 24 hours), and only then release the tiler. On Class 2 work in NSW, this detail sits squarely inside the regulated waterproofing scope — it's exactly the kind of junction a certifier or the Building Commission will ask to see evidence for.