Choose a terrace drain as part of the roof’s complete drainage design—not by picking a universal diameter. The correct outlet depends on the effective catchment area, local design rainfall, runoff coefficient, roof falls and low points, allowable water depth, pipe arrangement, waterproofing membrane, finish build-up, and a certified capacity rating at the relevant water head. Without those project details, no responsible designer can specify a drain size or model.
Why a concrete roof does not have one standard drain size
A concrete roof terrace is a layered system. A structural slab may support a sloped screed, waterproofing membrane, drainage layer, insulation, pedestals, paving or decking, and a drain connected to internal or external pipework. Each layer affects where water travels and how much resistance the outlet experiences.
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Drain diameter alone does not establish performance. Two outlets with the same nominal connection can have different grate areas, lip elevations, flow paths and tested capacities. A published flow rate is meaningful only when its water head, test arrangement and connected drainage system match the proposed installation.
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A free scan shows the junk files, broken settings and background clutter dragging Windows down - then fixes them in one click.Free scan · Windows 10 & 11The project’s adopted code and the outlet manufacturer’s current certification control the final selection. UK warranty guidance, German roof-terrace details under DIN 18531, Austrian drainage guidance and manuals explaining BS EN 12056-3 use different assumptions. Their figures are useful examples, not interchangeable rules.
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Collect the design information before comparing products
Roof location and design rainfall
Ask the drainage designer which rainfall criterion applies to the project’s location and code. Rainfall intensity varies by country, region, return period and design method. HL Hutterer & Lechner guidance describes the calculation basis as rainfall rate, effective area and runoff coefficient, and notes that national values differ.
Its Austrian guidance gives “300 litres/(s – ha)” as a minimum design rainfall for roof and property areas in that context. It is not a value to reuse for a roof in another country. The designer must obtain the current local criterion rather than copying an example from a product guide.
Effective catchment area
Measure every surface that can drain toward the outlet, including adjacent roof zones, upstands and areas behind the parapet that share the same drainage path. The relevant figure is the effective contributing area, not simply the concrete slab’s plan dimensions.
Where different roof sections drain to separate outlets, calculate each catchment separately. A roof plan should show which low points belong to which outlet and whether a blockage could redirect water into another zone.
Runoff coefficient and surface build-up
The calculation includes a runoff coefficient selected for the actual surfaces and assembly. Smooth, impervious finishes and planted or porous areas do not necessarily contribute runoff in the same way. The designer should document the coefficient used instead of assuming that every square metre behaves identically.
Allowable water head
Outlet capacity is tied to the depth of water above the outlet’s controlling lip or opening. Confirm the maximum water depth the waterproofing, thresholds, parapets, doors and structure can tolerate, and the head used in the manufacturer’s test data. LABC’s 2024 technical manual discusses a typical design head of 30–35 mm for its calculation example; that range is not a universal allowance for every terrace.
Drainage arrangement and outlet connection
Record whether the outlet discharges to an internal rainwater stack, an external hopper, a dedicated pipe or a pipe shared with another outlet. Confirm outlet orientation, connection diameter and the pipe system’s configuration. HL notes that performance can depend on whether the system is open or closed, so a flow number from one arrangement cannot automatically be applied to another.
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Identify the exact membrane—such as a sheet or liquid-applied system—and the manufacturer’s approved drain connection detail. Then list every layer above it: protection board, drainage mat, insulation, screed, pedestal supports, tiles, pavers or decking. The outlet must remain compatible with the membrane and reachable after those layers are installed.
Calculate the required discharge
The designer generally establishes peak discharge from the design rainfall, effective catchment area and runoff coefficient:
Required flow = design rainfall intensity × effective catchment area × runoff coefficient
All quantities must use consistent units. If rainfall is expressed in litres per second per hectare, area must be converted to hectares before multiplying. The resulting flow is then compared with the outlet’s certified capacity at the intended water head and test configuration.
Do not substitute an annual rainfall total, an average storm value or a manufacturer’s example for the adopted design rainfall. The calculation is a peak-flow exercise, and the selected rainfall criterion should be recorded on the drainage drawings so future substitutions can be checked against the same basis.
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HR Wallingford’s manual explaining BS EN 12056-3 describes outlet-capacity calculations using design rainfall, design water depth, outlet-lip elevation and outlet-flow data. Those inputs show why a diameter-only rule is inadequate: changing the lip elevation or permitted head changes the capacity requirement even when the roof area is unchanged.
Set the falls and low points before choosing the grate
Design a continuous route to the outlet
Mark falls on the roof plan and sections. Water must be able to travel from every contributing area to the outlet without a ridge, upstand, pedestal line or parapet creating a trapped pocket. Show how drainage continues beneath raised finishes such as paving on supports or a deck.
Concrete slabs are rarely flat enough to serve as the finished drainage plane by themselves. A sloped screed or another shaped layer is commonly used above the slab, with the waterproofing following the designed fall. Structural deflection, construction tolerance and settlement must be considered when setting levels.
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LABC Warranty’s 2024 Technical Manual says that, after allowing for deflection and tolerance, a minimum 1:80 fall should be achieved in the waterproof membrane under its stated BS 6229 design conditions. A 1:80 fall is a context-specific design requirement, not permission to use that ratio on every roof.
Schlüter’s C.5 roof-terrace assembly, shown for a concrete slab, sloped screed, waterproofing, drainage layers and tile finish, depicts a 1.5–2% slope under DIN 18531. That detail demonstrates how an assembly can be designed; it does not replace the structural and waterproofing design for another project.
Check thresholds and local obstructions
Door thresholds, rooflights, plant plinths, parapets and raised supports can become unintended dams. Confirm finished levels at each obstruction and ensure the waterproofing remains protected while preserving the designed flow route. If a zone cannot drain by gravity to the primary outlet, it needs a separately designed outlet or a revised roof layout.
Match the outlet to the waterproofing membrane
Verify the connection method
Check whether the drain uses a clamping flange, bonded flange, welded connection or another proprietary detail. The flange material, seal, fasteners and membrane interface must be approved for the actual membrane, primer and protection layers.
LABC advises confirming compatibility from manufacturer information or independent certification, together with capacity and attachment method. Calling a product a “roof drain” does not prove that it can be bonded or clamped to the membrane on your project.
Coordinate movement and protection
The outlet should be installed without puncturing or stressing the membrane and should accommodate the movement expected between the structural slab, screed and finish. Follow the membrane manufacturer’s detailing requirements for corners, laps, sealants and protection boards; do not improvise a flange detail on site.
Allow for the finish build-up
Set the drain body, grate or adjustable extension to the final finished level. Tiles, pavers and decking must not bridge the opening or force water to flow uphill. Where a drainage layer is required beneath a raised finish, select a drain body and grate arrangement that can receive that layer’s water and remain cleanable.
Compare certified capacity, not marketing diameter
After calculating the required discharge, obtain the manufacturer’s certified capacity table for the exact outlet, grate, extension, pipe connection and drainage arrangement. Compare capacity at the project’s anticipated water head. If the manufacturer reports several heads, use the value that matches the design condition rather than the largest number in the brochure.
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|---|---|---|
| Required discharge | Calculated from local design rainfall, effective area and runoff coefficient | Establishes the flow the drainage system must handle |
| Water head | Depth above the outlet’s controlling lip or opening | Capacity changes with available head |
| Test basis | Open or closed system, connected pipework and stated test arrangement | Flow figures are not comparable when conditions differ |
| Outlet geometry | Grate, lip elevation, extension and outlet orientation | Small geometry changes can alter inlet performance |
| Certification | Current manufacturer data or independent certification applicable to the region | Provides evidence for the specified capacity |
LABC’s guidance states: “Rain water outlet capacity should be taken from properly certificated information provided by manufacturer, and the resulting number and layout of outlets should allow for obstruction and drag due to any additional surface finishes, such as walkways.” Treat surface finishes as part of the hydraulic design, not as a cosmetic decision made after the drain is sized.
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Choose the number and arrangement of primary outlets
One outlet serving one catchment
A single outlet may serve a defined catchment where the calculated flow, certified capacity, pipe route and maintenance access are all acceptable. The design must also address what happens if that outlet blocks; a lone outlet should not be treated as self-protecting.
Multiple outlets on separate routes
Separate outlets and independent discharge routes can reduce the consequence of one blockage, but only if the roof levels direct water to each outlet and the pipework remains independent. Confirm how the catchment is divided and where each route discharges.
Combined outlets on one downpipe
When outlets share a downpipe, size the combined flow and assess the system using the correct test basis. The pipe and junction arrangement can govern performance, so do not add isolated outlet capacities unless the manufacturer’s data permits that calculation.
Provide an inspectable overflow or emergency route
Where required by local rules and the roof design, provide a visible, independent route that warns occupants of a blocked or overwhelmed primary drain. It should discharge safely away from the building and be inspectable from a normal vantage point.
LABC’s manual calls for a conspicuous overflow for roofs draining to one internal outlet, or for combined outlets on one downpipe. In that guidance, the overflow capacity should be at least that of the primary outlet or combined outlets, with visible discharge directed away from the building. Other jurisdictions may set different thresholds, dimensions or arrangements; the drainage designer must confirm the applicable rule.
Locate the overflow high enough to remain inactive during normal storms but low enough to limit water depth to the design allowance. Coordinate its outlet through the parapet or wall without compromising the waterproofing, insulation or fire details.
Keep the finished drain accessible
A drain that cannot be inspected is difficult to maintain and easy to obstruct. LABC says outlets should be accessible without disrupting pedestrian finishes, and that deck supports must not restrict drainage.
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- Provide a removable grate or access cover suited to the finish.
- Keep paving joints, pedestal feet and decking boards clear of the inlet.
- Leave a practical route for removing leaves, sediment and construction debris.
- Show access zones on the drawings so later trades do not cover them.
- Confirm that any adjustment ring remains usable after final levels are installed.
During handover, record the drain location, membrane detail, grate type, overflow route and cleaning method. Photograph concealed components before covering them and retain the manufacturer’s installation and capacity documents with the building records.
Compare candidate terrace drains on these criteria
| Decision axis | Questions for the designer or supplier |
|---|---|
| Hydraulic capacity | What certified flow is available at the calculated head and in the proposed test arrangement? |
| Membrane compatibility | Is the flange, clamp, bond or weld approved for the specified waterproofing system? |
| Pipe connection | Does the outlet orientation and connection suit the planned pipework without an improvised transition? |
| Roof build-up | Can the body, extension and grate accommodate the screed, membrane, drainage layer and finish thicknesses? |
| Finish access | Can the grate be removed and cleaned without dismantling the terrace finish? |
| Overflow provision | How will a visible emergency discharge be formed, protected and directed away from the building? |
| Documentation | Are installation details, certification, capacity tables and regional support available for the project? |
The available technical material does not establish a ranking of specific drain models. Select the product that satisfies the documented hydraulic, waterproofing, structural and maintenance requirements rather than the product with the largest advertised connection.
Coordinate installation in a controlled sequence
- Confirm the design basis. Record location, adopted code, design rainfall, effective catchment, runoff coefficient, allowable water head and outlet arrangement.
- Set structural and finished levels. Coordinate slab deflection, screed falls, thresholds, parapets, plinths and the final finish thickness.
- Approve the membrane detail. Obtain the outlet manufacturer’s flange or connection detail and the waterproofing manufacturer’s compatibility confirmation.
- Verify capacity. Check certified flow at the calculated head and with the intended grate, extension and pipe configuration.
- Design overflow protection. Confirm whether an emergency route is required, its level, capacity, visibility and safe discharge point.
- Install without blocking drainage paths. Keep protection layers, drainage mats, pedestals and finish materials clear of the inlet.
- Inspect before concealment. Check levels, membrane continuity, fasteners, seals, grate access and overflow visibility before the finish is completed.
- Document the completed roof. Keep as-built levels, product data, certification, photographs and maintenance instructions.
Common selection and installation failures
Choosing a diameter from roof area alone
Symptom: A drain is specified from a generic diameter chart with no rainfall or head calculation.
Correction: Recalculate peak discharge from the local rainfall criterion, effective catchment and runoff coefficient, then compare it with certified capacity for the actual arrangement.
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Copying a rainfall example from another country
Symptom: An Austrian rainfall figure or another published example appears on drawings for a different location.
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Correction: Replace it with the criterion required by the project’s adopted code and document the geographic and design assumptions.
Using a capacity figure with the wrong test conditions
Symptom: A brochure’s highest flow rate is used even though it was tested at a different head or in a different system configuration.
Correction: Request the complete capacity table and select the value tested under comparable conditions.
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Installing a compatible-looking flange on an incompatible membrane
Symptom: The drain body fits physically, but the membrane manufacturer has not approved the interface.
Correction: Obtain written compatibility information or independent certification for the exact membrane and connection method before installation.
Covering the drain with the terrace finish
Symptom: Pavers, tiles or decking prevent grate removal or restrict the inlet.
Correction: Set the finished level and access zone during design, and keep supports and joints outside the drainage opening.
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Symptom: A secondary route exists but cannot be seen, inspected or safely discharged.
Correction: Design a conspicuous overflow or emergency outlet to the local code, with a visible discharge path away from the building.
Final specification checklist
- Location and adopted drainage standard are recorded.
- Effective catchment area is mapped to each outlet.
- Design rainfall and runoff coefficient are documented.
- Maximum allowable water head is established.
- Falls, low points and drainage paths beneath raised finishes are shown.
- Membrane, flange or bonding detail is approved.
- Certified outlet capacity matches the calculated flow, head and test arrangement.
- Pipe orientation, connection and combined-flow condition are verified.
- Grate and adjustment components suit the complete finish build-up.
- Cleaning access remains available after completion.
- Overflow or emergency discharge is provided where required and exits safely.
- Product certification, installation instructions and as-built records are retained.
What you can specify—and what you cannot—without project data
You can specify the performance requirements: a membrane-compatible terrace outlet, certified capacity at the design head, a connection suited to the pipework, integration with the roof build-up, accessible maintenance and a compliant overflow strategy.
You cannot responsibly specify a drain diameter, outlet count or model from the phrase “concrete roof” alone. The roof’s location, area, rainfall basis, membrane, levels, pipe configuration and allowable water depth must be supplied to the drainage designer before procurement.
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