Drainage outlet (roof)
A roof drainage outlet is the primary sump or drain that takes water off a flat or membrane roof into the downpipe network under AS/NZS 3500.3.
Ask Chalkline about this →A drainage outlet is the primary sump or drain that takes water off a flat or membrane roof into the downpipe network. It is the everyday drainage path; a scupper is the emergency overflow when the outlet blocks. Outlets are sized to AS/NZS 3500.3 from the catchment area they serve and the design rainfall intensity, and the membrane is dressed into them with a clamping ring or puddle flange.
What it does
- Receives water from the roof field, which falls at minimum 1:80 toward each outlet under AS/NZS 3500.3.
- Forms a watertight transition between the membrane and the downpipe stub.
- Has a leaf-guard dome in domestic installs to keep debris out.
- Pairs with a scupper overflow at 150 mm above membrane on parapeted roofs, so a blocked outlet doesn’t pond water beyond the structure’s design load.
Sizing
Per AS/NZS 3500.3, outlet sizing is a function of:
| Factor | Effect |
|---|---|
| Catchment area | Larger area = bigger outlet or multiple outlets |
| Rainfall intensity | Site-specific 1-in-20 or 1-in-100 year storm intensity |
| Roof slope | Steeper = water gets to outlet faster |
| Outlet type | Domed / flat / spitter, different discharge coefficients |
Two outlets per catchment is standard practice: one for primary flow, one as redundancy. For domestic flat-roof areas under ~50 m² a single 100 mm outlet plus a scupper is the typical answer.
Install
- Pre-pour or pre-deck blocking to support the outlet body in the membrane plane.
- Plumber connects the outlet stub to the downpipe before membrane.
- Waterproofer dresses the membrane into the outlet, clamped or heat-welded depending on system.
- Inspection access: leaf-guard dome must be removable for maintenance.
Common failure modes
- Membrane termination at outlet: blow-back from wind-driven rain if the clamping detail is wrong. Source of slow leaks 2-5 years after handover.
- Blocked leaf guard: causes ponding, which loads the structure and tests the membrane.
- Undersized outlet: cheap to fit a larger one at install; expensive to retrofit. Always spec to AS/NZS 3500.3.
- Missing overflow: outlet plus scupper is the requirement; the outlet alone is not redundant.
For a builder
- Coordinate plumber + waterproofer scope at the outlet. Each subbie does half the detail; both must understand the membrane termination.
- Spec at least two outlets per catchment + scupper on parapeted designs.
- Include outlet maintenance in the owner’s manual. Annual leaf-guard clean-out is the difference between 25-year and 8-year membrane life.
Category: Roofing / drainage.
Related
See also
Last updated: 2026-05-29. Verified: 2026-05-29. Quarterly review for currency.