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Roof Drainage by the Numbers

Design storms, catchments, gutters, outlets, box gutters, overflow and thermal movement, worked through one warehouse roof

Size roof drainage from the design storm: catchment flows, eaves and box gutters, outlets, rainheads, overflow, materials and thermal movement, then install and maintain it.

8 chapters · About 80 minutes · Free to read · Intermediate building services and sheet metal practice

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About this book

Roof drainage looks like simple sheet metal: a channel along the eaves, a pipe down the wall. It is also a small hydraulic system that must carry a short, intense storm off a large area without letting any of it into the building, and keep doing so when leaves block an outlet or the gutter has expanded on a hot afternoon. When it fails, the damage is inside the building: wet stock, ceilings, switchboards and insulation, usually found the morning after a storm.

This book works roof drainage through by numbers. It starts with the design storm and how intensity, duration and probability are chosen, turns a roof catchment into a flow, checks eaves gutters, outlets and box gutters against that flow, plans overflow for the rarer storm, deals with materials and thermal movement in long metal gutters, and finishes with setting out, testing, inspection and maintenance. Each chapter ends with an exercise and its worked answer.

It assumes basic arithmetic and some familiarity with sheet metal work. The hydraulics are explained from the start.

The running case

The book follows a fictional light industrial warehouse, W-40, 40 m long and 24 m wide, with a gable roof pitched at 5° and its ridge along the 40 m length. Each roof slope covers 40 m by 12 m in plan. At the rear, the roof drains to an eaves gutter. At the front, facing the street, a parapet rises 1.0 m above the roof and the roof drains to a box gutter behind it, discharging through the parapet into rainheads at each end.

The design rainfall intensities used throughout are 180 mm/h for a 5-minute storm with a 5% annual exceedance probability and 250 mm/h for a 5-minute storm with a 1% annual exceedance probability. This is an illustration. They are round teaching numbers, not values for any real location. All dimensions, flows, prices and results are invented for teaching and do not describe a KEVOS® client, supplier or project. Money is in Australian dollars, excluding GST.

How to use the book

Chapters 1 and 2 turn rain into flow. Chapters 3 to 6 size the parts that carry it and the overflow that protects the building when they cannot. Chapter 7 covers the metal itself, and Chapter 8 the work on site and the years after. The geometry of making gutter corners, stopends and other profiled sheet metal is beyond this book’s scope.

Source limits and professional responsibility

The supplied source, a 1922 American treatise on architectural sheet metal work, informed only the range of subjects. The methods here are standard hydraulics and current Australian practice, with every calculation shown. The hydraulic estimates are teaching tools. Real roof drainage must be designed and installed to the National Construction Code and AS/NZS 3500.3 or another approved method, by people licensed for the work where the law requires it, using the design rainfall for the actual site from the Bureau of Meteorology. Working on roofs is high-risk work and must be planned under the work health and safety law that applies.

Who this book is for

Roof plumbers, sheet metal fabricators, building designers, project engineers and facility managers who specify, make, install or look after gutters, downpipes and box gutters on commercial and industrial buildings.

What you will learn

Read a design rainfall intensity and turn a roof's catchment into a flow in litres a second.
Check an eaves gutter, an outlet and a box gutter against that flow, and see which part limits the system.
Plan overflow so water leaves the building when outlets block, sized for the rarer storm.
Allow for thermal movement and material compatibility in long metal gutters.
Set out, test, inspect and maintain roof drainage so it keeps working.

Inside this book

Chapters

Figure 2Rear outlet options checked against the 100 mm effective gutter depth

	Water depth needed at 5% AEP	Water depth needed at 1% AEP	Verdict
Four downpipes, 90 mm outlets	about 126 mm	about 243 mm	fails at 5% AEP
Four downpipes, 100 mm outlets	about 83 mm	about 159 mm	thin margin at 5%; spills well before 1%
Four downpipes, 150 mm outlets in sumps	about 37 mm	about 46 mm	works; sumps add cost and detailing
Six downpipes, 100 mm outlets	about 37 mm	about 71 mm	works with margin at both

This is an illustration using the weir and orifice estimates with a coefficient of 0.6. The depth needed must stay below the gutter's 100 mm effective depth, with margin, for the gutter to carry the storm without spilling.

Outlets and downpipesCheck the outlet, the usual bottleneck in roof drainage: weir and orifice flow into a downpipe, the depth each outlet needs, nozzles, sumps, strainers, offsets and the connection below ground.

Apply This in Practice

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References and source notes

The manuscript and worked examples are original KEVOS® writing. References inform particular concepts; their authors do not endorse this book. Web references were consulted on 26 October 2026.

William Neubecker, The Sheet Metal Publication Company, New York. The Universal Sheet Metal Pattern Cutter, Volume II: Architectural Sheet Metal Work (1922). Supplied source, read as a subject map of the gutters, leader heads, conductors and roof work a sheet metal shop made a century ago. Its imperial patterns, prose and engravings are not reproduced; this book's methods, case, calculations and figures are its own and follow current Australian practice.
Australian Building Codes Board. ABCB Housing Provisions, Part 7.4 Gutters and downpipes (NCC 2022) (2022). Source for the 5% and 1% annual exceedance probabilities, the 5-minute duration, the 1:500 minimum eaves gutter fall, the 12 m of gutter per downpipe limit and the referral of box gutters to AS/NZS 3500.3, cited in Chapters 1, 3, 5 and 6. These provisions apply to housing; the warehouse case uses them as reference points only. Check the edition in force.
Bureau of Meteorology. Design Rainfall Data System (2016) (2016). The source of design rainfall intensities for a given location, duration and probability in Australia, described in Chapter 1. The intensities in this book are illustrations, not values for any real place.
Standards Australia and Standards New Zealand. AS/NZS 3500.3, Plumbing and drainage, Part 3: Stormwater drainage, and AS/NZS 2179.1, metal rainwater goods (current editions). Named by number and scope only. AS/NZS 3500.3 governs the design of box gutters, sumps and rainheads and much commercial roof drainage. No values or methods are quoted from either standard.
The Engineering ToolBox. Manning's Roughness Coefficients for Open Channel Flow; Open Channel Weirs, Volume Flow Measurements (current web pages). Source for a Manning's n of 0.012 for smooth steel and for the rectangular weir formula used in Chapters 3 to 6. The weir page is at engineeringtoolbox.com/weirs-flow-rate-d_592.html. Discharge coefficients used here are stated teaching assumptions.
The Engineering ToolBox. Linear Thermal Expansion Coefficients of Materials (current web page). Source for the expansion coefficients of steel, aluminium, copper, zinc and PVC used in Chapter 7.
Safe Work Australia. Model Code of Practice: Managing the risk of falls at workplaces (2018). Background for the roof access and inspection safety points in Chapter 8. Check whether the code has legal effect in your jurisdiction.

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