Why the Drain That's "Too Big to Block" Will Block

The intuition about drain sizing goes like this: bigger pipe means more capacity means fewer blockages. It's wrong about half the time. A drain that's oversized for its actual load at the installed grade won't achieve self-cleansing velocity — and when velocity drops below the minimum, solids drop out of suspension and settle on the pipe floor. The drain blocks slowly, reliably, and in a way that's hard to diagnose because the pipe looks fine on the outside.

What self-cleansing velocity means and why AS/NZS 3500.2 grades aren't arbitrary

A sanitary drain needs to flow fast enough to keep solids suspended and moving. The minimum is around 0.75 m/s for foul drainage — roughly the speed of a brisk walk. Below this, grit, fat, and solids start settling. Above about 3 m/s at very steep grades, the opposite problem occurs: liquid races ahead of solids, leaving them stranded on the pipe floor.

The grade requirements in AS/NZS 3500.2 aren't arbitrary. They're set to maintain self-cleansing velocity at the design DFU load for each pipe size. A 100mm drain at 1:100 grade with a heavy load achieves the required velocity. The same 100mm drain at 1:100 with two hand basins attached does not — there's simply not enough flow volume to move the pipe diameter's worth of water fast enough.

The real-world scenario: ensuite added to an existing slab

A new ensuite is added to an existing slab house. The plumber runs 100mm drain because the 100mm is already under the slab and it's easier to connect than to reduce. The ensuite has a basin and a shower — 4 DFU total. At 1:80 grade, a 100mm drain at 4 DFU is well below self-cleansing velocity. Within 12–18 months the customer is calling with a slow drain. It gets jetted, clears, and six months later it's back. The pipe isn't blocked — it's never been able to clear itself.

The too-steep grade: the opposite failure mode

Grades steeper than about 1:20 cause liquid to outrun solids. Wastewater flows fast, solids stay behind. This is most common in short runs where the height drop to the stack is significant relative to the horizontal distance — a bathroom that's close to the stack with a lot of floor level to fall.

The design principle: size to the load, not the available space

Size the drain to the DFU count, then check that the grade you can achieve maintains velocity. If the grade is too flat for a smaller pipe, the solution is a steeper grade — not a bigger pipe that achieves even lower velocity at the same grade. The AS/NZS 3500.2 sizing tables exist for this reason: they pair pipe diameter with DFU count at grades that produce the right velocity.