For a balcony or roof-terrace surface draining to an outlet, the sourced minimum is a fall of 1:80 toward the drainage system in Victorian Building and Plumbing Commission (BPC) guidance. That equals 12.5 mm per metre of horizontal run, or 1.25%. It is a Victorian balcony/terrace requirement, not a universal rule for every country, building type or meaning of “terrace.” Confirm the locally adopted code and have the construction, waterproofing and drainage layout checked by a qualified designer or licensed installer before building.
The sourced answer: a 1:80 fall toward the outlet
The BPC’s 2026 practice-note guidance for the Victorian balcony context specifies a minimum fall of 1:80 in the substrate and associated waterproofing toward the drainage system. In practical terms, the surface must drop 12.5 mm for every metre it travels horizontally toward the drain.
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| Horizontal run to the outlet | Minimum vertical fall at 1:80 | Equivalent percentage |
|---|---|---|
| 1 m | 12.5 mm | 1.25% |
| 2 m | 25 mm | 1.25% |
| 3 m | 37.5 mm | 1.25% |
| 4 m | 50 mm | 1.25% |
Calculate the fall by dividing the longest horizontal distance that water must travel by 80. A 2.4 m path, for example, needs 30 mm of vertical difference at this minimum. The measurement is from the high point of the draining surface to the relevant outlet, not simply from one edge of the terrace to another.
This figure should be treated as the minimum cited for that Victorian application. A different state, territory, country, building classification, surface assembly or drainage arrangement may adopt another requirement. The BPC guidance does not make 1:80 a worldwide terrace standard.
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“Terrace” can describe two different drainage problems
Balcony or roof-terrace floor
A balcony or roof terrace is a constructed surface above or beside an occupied building. Its fall directs rainwater across the finish and waterproofing build-up to a floor waste, outlet or channel. The BPC’s 1:80 figure belongs to this context. The substrate, waterproofing layer, finish, outlet body and surrounding thresholds have to work as one system so water does not pond or enter the structure.
Hillside grading terrace or drainage swale
A grading terrace on a hillside is earthwork. Its swale carries runoff along a contoured or longitudinal channel rather than across a balcony slab. Los Angeles County’s current grading code specifies a 5% to 12% longitudinal grade for drainage swales on hillside grading terraces. That range is not a balcony-floor slope and should not be substituted for 1:80 on a roof or balcony.
| Application | Published numerical guidance | What the number describes |
|---|---|---|
| Victorian balcony or roof-terrace surface | Minimum 1:80 (12.5 mm/m; 1.25%) | Fall of substrate and associated waterproofing toward the drainage system |
| Los Angeles County hillside grading terrace swale | 5%–12% | Longitudinal grade of an earthwork drainage swale |
How to set out the required fall
Use the following sequence when a designer or installer is establishing levels. The exact construction method depends on the terrace system, but the calculation must remain tied to the actual water path.
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- Measure the horizontal run. Measure in plan from each high point to the outlet, following the intended drainage direction. Do not use the diagonal thickness of a slab or a visual estimate of the finished surface.
- Apply the 1:80 calculation where Victorian guidance applies. Multiply the run in metres by 12.5 mm. For example, 3.2 m requires 40 mm of fall.
- Coordinate all build-up levels. Transfer the high and low levels through the structural substrate, screed or topping, waterproofing, adhesive and finish. The finished surface can only drain correctly if each layer preserves the designed direction and the outlet flange is set at the right elevation.
- Check thresholds and edges. Door sills, parapets and perimeter upstands must remain compatible with the fall and waterproofing detail. A low doorway or an interrupted edge can become the first path for water during a blocked-outlet event.
- Verify the completed work. A level survey or equivalent site check should confirm the designed difference in elevation and reveal local hollows. A terrace that averages 1:80 but contains depressions can still retain water.
The slope is a geometric requirement; it does not guarantee that every drop of rain will leave through the outlet. Surface finish, joints, debris, outlet position and hydraulic capacity determine what happens once water reaches the drain.
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Drain capacity must be checked separately from surface fall
BPC guidance treats the fall of the terrace and the carrying capacity of the drainage assembly as separate design checks. The outlet has to accept the design flow, and the pipe has to carry it away without backing up to the waterproofing or threshold.
Confirm the grate’s rated flow
Compare the calculated outlet flow with the grate or strainer manufacturer’s specified maximum design flow. BPC cautions that grate designs may require a larger effective open area than their visible dimensions suggest. A narrow slot, fine mesh or partially blocked grate can pass substantially less water than an open outlet of the same nominal size.
Check the complete hydraulic path
BPC identifies the following factors as affecting carrying capacity:
- pipe diameter and length;
- pipe material and its internal roughness;
- available hydraulic head;
- pipe gradient after the outlet;
- the number and type of bends and fittings.
Therefore, outlet size cannot be selected from the terrace slope alone. Two terraces with the same 1:80 surface fall can require different outlet arrangements if one has a longer or rougher pipe run, less available head or more restrictive fittings.
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Compare drain options on the right criteria
When reviewing a point outlet, channel drain or grate assembly, compare the design information that affects the whole system rather than choosing by appearance or nominal outlet diameter.
| Item to compare | Why it matters |
|---|---|
| Design flow capacity | Shows how much calculated runoff the assembly is intended to discharge. |
| Manufacturer-rated grate flow | Checks that the grate’s effective opening can pass the outlet flow. |
| Outlet dimensions and compatibility | Confirms that the grate, outlet body, waterproofing flange and connecting pipe fit together. |
| Hydraulic head | Determines the driving pressure available to move water through the outlet and pipe. |
| Pipe length and material | Influences resistance and therefore carrying capacity. |
| Number and type of bends | Each fitting changes resistance and can reduce the useful capacity of the run. |
| Overflow route and threshold layout | Provides a controlled path if the primary outlet is obstructed or rainfall exceeds the primary design flow. |
The cited materials do not establish one universally correct outlet diameter. Local rainfall data, the contributing area, overflow requirements and the complete pipe layout are needed before an outlet can be sized responsibly.
Design an overflow, not just a primary drain
A terrace should have a planned response to blocked outlets and exceptional rainfall. BPC guidance discusses overflow provisions and identifies an overflow event with a 1% annual exceedance probability in the Victorian regulatory context. That probability is part of the cited framework; confirm the current local standard and how it applies to the building.
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Do not treat an overflow as permission to reduce the primary outlet or to omit the designed surface fall. It is a secondary protection measure for a different failure or rainfall condition.
Coordinate the fall with waterproofing requirements
Waterproofing is part of the drainage design because the membrane must shed water toward the outlet and remain continuous at penetrations, corners and upstands. For applicable Australian roof terraces and balconies, NCC 2022 provision H2D8 references membranes complying with AS 4654.1 and design and installation in accordance with AS 4654.2.
Those references are Australian and edition-specific. Check the current National Construction Code edition, the building class and any state or territory variation before relying on them. The waterproofing specification should identify the membrane system, substrate preparation, outlet detailing, perimeter terminations and interfaces with doors or balustrades. A drain body that is hydraulically adequate can still fail if its waterproofing flange or junction is incompatible with the membrane system.
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Applying the hillside-swale percentage to a balcony
A 5%–12% swale grade from Los Angeles County’s grading code is for a hillside earthwork channel. Using it as a balcony-floor target would impose a much steeper condition than the Victorian 1:80 guidance and could create unacceptable threshold, accessibility or build-up problems. Identify the drainage type and governing jurisdiction first.
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Assuming a large grate fixes inadequate fall
A higher-capacity grate cannot make water flow uphill or remove ponding caused by low spots. Correct the substrate and waterproofing levels, or redesign the drainage layout, before relying on outlet capacity.
Selecting an outlet from diameter alone
Nominal diameter does not describe the complete hydraulic performance. Check the grate’s rated flow, pipe length and material, gradient, hydraulic head and fittings together.
Checking only the finished tiles
The BPC minimum concerns the substrate and associated waterproofing fall as well as the surface. A tiled appearance can hide a membrane or screed that does not drain correctly. Verify the levels of the underlying build-up and the outlet interface.
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A primary drain can be obstructed by leaves or debris. Without a planned overflow route, water may rise to a doorway or penetrate the building before anyone notices. Coordinate overflow levels with the rest of the envelope.
What to do if an existing terrace ponds
- Map the ponding. Identify where water remains, how deep it is and whether the area is localised or follows the entire route to the outlet.
- Check for obstruction. Remove safe-to-access debris from the grate and confirm that the outlet is not covered by a finish, sealant or temporary protection.
- Measure the actual fall. Compare high and low points with the required 1:80 calculation where that Victorian guidance applies. Do not infer compliance from visual appearance.
- Inspect the membrane and outlet junction. Look for cracking, separation, damaged collars or signs that the outlet body and waterproofing were not designed as a system.
- Have the hydraulic route assessed. A slow-draining grate may reflect undersized or obstructed downstream pipework, excessive bends or insufficient hydraulic head rather than a surface-slope problem.
- Use a qualified waterproofing or drainage professional for alteration. Cutting, chasing or raising a terrace finish can damage the membrane and create a larger leak. Any remedy should preserve the waterproofing warranty and comply with the applicable code.
Practical design checklist
- Have you established whether the terrace is a balcony/roof surface or a hillside grading swale?
- Have you confirmed the governing jurisdiction, current code edition and building classification?
- For the Victorian balcony context, does the substrate and associated waterproofing fall at least 1:80 toward the drainage system?
- Have you calculated the horizontal run and translated it into a millimetre fall?
- Does the grate’s manufacturer-rated maximum design flow meet the calculated outlet flow?
- Have pipe diameter, length, material, gradient, hydraulic head and bends been checked as one hydraulic system?
- Are the outlet body, grate, membrane, finish and connecting pipe dimensionally compatible?
- Is there a controlled overflow route, including the cited Victorian 1% annual-exceedance-probability rainfall design where applicable?
- Are thresholds, parapets, upstands and penetrations coordinated with the finished levels?
- Has the completed terrace been measured for local hollows rather than judged only by its average slope?
The dependable answer is therefore conditional but clear: use a minimum 1:80 fall for the Victorian balcony or roof-terrace application described by the BPC, equal to 12.5 mm per metre, then verify outlet capacity, overflow, waterproofing and local code requirements as separate parts of the design. A terrace drain works only when all of those elements are coordinated.
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