Roof Plumbing: Stormwater and Roof Drainage
What the exam expects
Roof plumbing questions test your understanding of collecting and disposing of rainwater from roofs, gutters, downpipes, rainheads, overflow measures and stormwater drains. The relevant references are AS/NZS 3500.3 and the Plumbing Code of Australia, plus manufacturer and council requirements for specific products. You need to read roof plans, calculate catchment areas and select gutter and downpipe sizes.
Concepts to master
Understand rainfall intensity and how it is applied to design: catchment area multiplied by intensity gives design flow, which then sizes gutters, outlets and downpipes. Know the effect of roof pitch, gutter fall, gutter profile and the number and spacing of outlets. Learn overflow provisions: weirs, overflow slots and downpipe spread, and why overflow must be visible and not discharge where it causes damage. Understand box gutter design, rainhead sizing, sumps, and the difference between charged and free-discharging systems. Know flashing and penetration requirements, expansion joints, and the need for leaf guards and debris control. Also know stormwater disposal options: connection to council system, charged systems, detention and soakage, and the approval that each requires.
Traps the exam sets
Expect questions that confuse gutter cross-section with gutter fall, or that assume more downpipes always solve a capacity problem. Watch for overflow arrangements that are hidden or that discharge onto a neighbour's property, and for box gutters sized without allowance for the design rainfall intensity. A frequent trap is mixing up catchment area with roof plan area on a pitched roof.
Worked example
A rule of thumb: effective catchment area equals plan area plus half the area of any vertical roof surface facing the wind. A 60 m² plan roof with a 20 m² vertical gable end facing the prevailing wind gives 60 + 10 = 70 m². Multiply by design rainfall intensity to get litres per second, then check the selected gutter and outlet capacity against that figure.