What Is the 135 Rule in Plumbing?

If you’ve ever heard a plumber mention the “135 rule,” you might have wondered what on earth they meant. It’s a rule of thumb in drainage design, and it comes down to this: the more bends you put in a horizontal drain, the more likely it is to clog, and the harder it is to clear when it does.

Knowing about it also helps explain why plumbers choose particular fittings, and why some jobs cost more than others.

One thing to be upfront about: “the 135 rule” isn’t written into any plumbing code. It’s a practical shorthand plumbers use, based on the drainage principles in Australian Standard AS/NZS 3500.2 (Sanitary Plumbing and Drainage).

What it actually means

The rule is about the total change of direction in a horizontal drain. The classic example is a 90-degree bend followed by a 45-degree bend, which adds up to 135 degrees. Once a pipe has turned that much, a plumber starts asking whether the layout will cause trouble, and whether it needs better fittings or an access point so it can be cleaned later.

Why it matters

Drains rely on gravity to carry water and waste along. A sharp bend creates turbulence and slows the flow, and when water slows down, solids settle out. They tend to pile up at or just past the bend, and over time that buildup turns into a blockage.

The Australian Standard deals with part of this by setting minimum pipe gradients so wastewater has enough speed to carry solids with it. A 100 mm main drain, for example, often needs a fall of 1 in 60 (roughly 16.7 mm per metre). But even with the right slope, a badly designed bend can still cause problems.

How plumbers deal with it

Gentler fittings: Instead of a tight 90-degree elbow, plumbers often use a long-sweep bend, which curves gradually. Two 45-degree fittings can also make a smoother 90-degree turn, and a wye with an eighth-bend works well in some layouts. The right choice depends on the pipe size and the type of system.

Fewer turns between access points: Plumbers try to keep the route as straight as they can. When a drain has to turn several times, they add inspection or cleaning openings so a blockage can be cleared without pulling pipes apart.

Vertical drops instead of extra bends: If a drain has to get around an obstacle, a vertical section can sometimes replace a series of horizontal bends. The rules differ for vertical-to-horizontal and horizontal-to-horizontal connections, so the fittings need to be chosen carefully.

Inspection openings: AS/NZS 3500.2 requires them in certain places, such as the base of a vertical stack, at some changes of direction, and at set intervals along a drain. The exact locations depend on the pipe size and layout, but the point is the same: you can get to the pipe without digging it up.

Where it comes up

• Bathroom renovations: Moving a toilet, basin or shower means rerouting waste pipes. A toilet drain with lots of tight bends is much harder to clear later.
• Kitchen sink moves: The new pipe often has to work around cabinets or beams, so the plumber has to pick fittings carefully and keep the right slope.
• Stormwater: The same ideas apply outdoors, though stormwater is covered separately by AS/NZS 3500.3. Good slope and fittings help it carry leaves and dirt away.
• Apartments and townhouses: Several fixtures often share the same drain, so a poor design can affect more than one household.

What happens when it’s ignored

• Repeated blockages in the same spot. Some people call a plumber every few months without realising the layout is the real culprit.
• Slow drainage, like basins that take ages to empty, showers that pool, or toilets that flush weakly.
• Sewer smells. Bad drainage can cause these, but so can dry traps, damaged seals or venting problems. If the smell keeps coming back, get a licensed plumber to find the source.
• Costly repairs. If the pipes are under a slab or inside a wall, fixing them can mean cutting into concrete or ripping up flooring. Getting the layout right the first time is far cheaper.

The 135 rule and Victorian regulations

In Victoria, all plumbing work has to comply with the Plumbing Code of Australia and the relevant Australian Standards, including AS/NZS 3500. The 135 rule is a handy guideline, but what you actually need to meet depends on your drainage system and the code provisions that apply to it.

If you’re in an area like Plumber Brunswick West, a licensed local plumber will know the standards and local requirements. They can also explain the paperwork and give you a Certificate of Compliance when one is needed.

The takeaway

The 135 rule is a simple way of saying that the number and type of bends in a drain matter. Too many sharp turns, or the wrong fittings, can slow the flow, cause blockages and make cleaning a nightmare. Plumbers avoid this by using suitable fittings, keeping the right slope and adding access points where needed.

If you’re planning a renovation or drainage repair, ask your plumber how they intend to lay out the pipes and what they’ll do to prevent blockages down the track.

Essendon Plumbing Services handles drainage installations and repairs. Their licensed plumbers can design and install drainage that meets Australian standards and Victorian requirements, and good planning up front goes a long way toward avoiding recurring problems.