Sanitary Plumbing and Drainage Design

What You Need to Know

Sanitary drainage takes wastewater from toilets, basins, and sinks to the sewer. AS/NZS 3500.2 sets the rules for pipe sizing, grades, traps, and vents. Every drain must be sized by fixture units, laid to the right fall, and vented so trap seals hold. Get the grades or venting wrong, and you get blockages, odours, or failed inspections.

• Every fixture has a fixture unit (FU) rating based on its flow rate and use pattern. These FU values set the pipe size for every drain, stack, and branch (AS/NZS 3500.2, Table 6.1)
• DN 100 drains need a minimum grade of 1.65%, DN 150 drains need 1.00%, and DN 65 branches need at least 2.50% (AS/NZS 3500.2, Table 3.2)
• Unvented branch drains are limited: DN 65 handles 5 FU (no WC), DN 80 handles 12 FU (one WC max), and DN 100 handles 30 FU (two WC max). Beyond that, you need a vent (AS/NZS 3500.2, Table 3.6)
• Trap seals must be 50 mm to 100 mm deep. Venting keeps air pressure within ±375 Pa so seals do not get pulled out by siphonage (NCC 2025, C1P5 and C1P6)
• Drain velocity must stay between 0.8 m/s and 2.0 m/s under normal flow. Too slow and solids settle. Too fast and water outruns the solids (NCC 2025, C2V1)
• Pipes must not reduce in size in the direction of flow. A DN 100 branch flowing into a DN 80 main is a code breach (AS/NZS 3500.2, Cl 3.3.6)

Take a commercial office floor with 4 WCs, 4 basins, and 2 floor wastes. WCs carry the highest FU rating, basins carry the lowest. A typical floor like this totals roughly 20–30 FU depending on the fixture types. A DN 100 drain at 1.65% grade handles up to 165 FU on a vented system (rising to 515 FU at steeper grades per Table 3.1), so one DN 100 main drain per floor is common for small to mid-size tenancies.

The grade is where things get tight. A DN 100 drain at 1.65% grade drops 16.5 mm per metre of run. Over a 10 m horizontal run in the ceiling void, that is 165 mm of fall. Add the pipe diameter itself (about 110 mm OD for DN 100 PVC) and you need at least 275 mm of vertical space just for the drain. Structure and other services compete for the same ceiling void, so coordinate early.

Stack sizing matters in multi-storey buildings. A DN 100 stack handles 4.0 to 7.3 L/s depending on vent configuration (NCC 2025, C1V5). For a 10-storey office with standard amenities per floor, a single DN 100 stack is typically enough. High-rise residential buildings with laundries and kitchens on every floor can push loads higher, so check the cumulative FU count against Table 3.1.