Balcony Waterproofing and Fall Detail on a Coastal Apartment Project
You are detailing a new four-storey apartment building 400 m from the coast. Each apartment has a 2.4 m deep balcony with a concrete slab, a tiled finish and a proprietary membrane. The architect's sketch shows the balcony slab falling to a linear drain at the outer edge, with a 100 mm upstand at the wall line and a flush threshold at the sliding door. The builder has queried the detail because the slab fall shown is 1:200 over 2.4 m, giving only 12 mm of fall, and the tile adhesive and bedding would consume most of it. The site is exposed to salt spray and driving rain, and the client wants a 'clean line' with no visible step at the door. The structural engineer has confirmed the slab thickness is fixed and cannot be increased.
1. Assess the proposed fall and threshold detail and identify the construction and waterproofing risks.
Read the worked answer
A 1:200 fall over 2.4 m gives only 12 mm, which is marginal for a tiled external balcony. Once tile bedding and adhesive are allowed for, the effective fall at the tile surface can be reduced to near zero or even reversed at high points, leading to ponding, efflorescence and membrane deterioration. External balconies generally need a steeper fall, commonly around 1:80 to 1:100, to drain reliably, and the fall must be achieved in the substrate, not in the tile bedding. The detail as sketched does not demonstrate that.
The flush threshold is a second risk. A flush threshold at a sliding door on an exposed coastal balcony relies on the door sill and the membrane upstand to resist wind-driven rain. If the finished tile level is close to the internal floor level, there is little or no step to stop water tracking inward, and the membrane must be turned up behind the door frame and terminated correctly. The 100 mm upstand at the wall line is good, but the junction at the door needs a specific detail.
The coastal exposure adds salt spray, which attacks fixings and metal flashings, and driving rain, which increases the water load on the door and threshold. Materials should be selected for the exposure, and the drain should be sized and positioned so that the lowest point is at the outlet.
2. Prepare a written description of a revised detail that resolves the fall and threshold issues within the fixed slab thickness.
Read the worked answer
The revised detail should achieve the fall in the structural topping or screed rather than in the tile bedding. Within the fixed slab thickness, I would specify a screed or topping laid to fall of 1:80 to the linear drain, with the drain set at the lowest point and the surrounding substrate shaped to a shallow channel so water cannot pond at the edges. The membrane would be applied over the screed, turned up 100 mm at the wall line and dressed behind the door frame at the threshold, with a bond breaker at the internal corner.
At the door, I would specify a raised door sill or a rebated threshold that provides a physical upstand above the finished tile, even if it is kept as low as practicable to preserve the clean line. The membrane would be carried up the door jamb and sealed to the frame, and a secondary drainage path or weep detail would be provided so that any water reaching the threshold can escape outward. All fixings and flashings in the splash zone would be stainless steel or another corrosion-resistant material suitable for a coastal site. The detail should be drawn at 1:5, with the fall, the membrane termination, the drain connection and the door sill all dimensioned, and it should reference the membrane manufacturer's tested system.
3. What would you need from the architect, the structural engineer and the membrane manufacturer before you finalise the detail?
Read the worked answer
From the architect I would need confirmation of the finished floor levels inside and outside, the door type and sill height, the tile thickness, and whether the clean-line threshold is a fixed client requirement or can be adjusted. I would also need confirmation of the drainage strategy and the location and size of the linear drain. From the structural engineer I would need confirmation of the available slab thickness, the topping or screed thickness that can be accommodated, and the allowable imposed load from the screed, tiles and any planter or balustrade. I would need to know whether the slab can be shaped to fall or whether a topping must provide it.
From the membrane manufacturer I would need the tested system for a tiled external balcony, the required fall, the upstand height, the termination detail at a door frame, and the warranty conditions, because deviating from the tested system can void the warranty. I would also check whether the manufacturer requires a specific drain type or a bond breaker at junctions. Only when those inputs are confirmed can the detail be finalised and issued, and I would record the confirmations in the project file.