Stormwater – Kerb Discharge

If you are going to build on an existing house block you will probably have a planning permit condition that storm water must discharge to an approved point.

If there is no surface water system one option can be to discharge to the street.

This is normally by constructing a proper kerb outlet like the photo below:

But not like this dodgy installation on the right!

Or even discharging over people walking along the path like the one illustrated in this post: Overflow Fail

A kerb connection can be at a reasonable cost as long as the house is above the road.

If the house is below the road you will need either:


  • A pump and storage for the storm water, which can add significantly to the build cost.


For more information on issues like this see Guide to Buying a Block


Protecting Underground Pipes

One of the problems that happen all too often during a new house build is that Sewers and Drains get filled with concrete.

Its normally happens on:

  • A knock down and rebuild project where the sewer connection wasn’t sealed during demolition.
  • A Battle Axe block subdivision where the new home is built close to the pipeline.

During the pouring of the foundations the concrete gets into the pipeline through the unsealed connection, or a cracked pipe, and then flows along the pipe.

Our company then gets brought in to remove the concrete using a hyraulic impact cutter like this.

Don’t think its cheap though . . . . . . It normally takes a team of three plus a high pressure jetting truck so the costs is upwards of $200 an hour, and its not unusual for one job to take 2-3 days.

To understand more issues when purchasing land see: Buying a Block


Charged Drainage System

Thanks to SaveH2O, of Supadiverta

This diagram indicates a charged drainage system. These are sometimes called either a “wet” or a “pressure” system.

With this type of system a section of the pipe always remains full unlike a Conventional System.

As the pipes are under pressure it is essential all the joints in above ground and underground pipework are fully watertight


  • A neater appearance than having pipes above ground.
  • Allows you to discharge water at a height above the ground level at the down pipe.


  • It is more likely to block as the flow through the pipes can be fairly slow and the low points can collect silt. Silt will get deposited at flow rates below 0.6m/sec which will be the situation for most rainfall events.
  • It is also more difficult to unblock.
  • Can cost more with excavation and additional inspection fittings .
  • Potential for mosquito  breeding  in water unless appropriate screes are installed.


Also see Underground Pipes


Conventional Drainage

The vast majority of new houses will have a conventional (sometimes called a Gravity, or Open system) storm water drainage system discharging to either:

    • Public Surface Water Drain – Typically in Eastern States
    • Soakwells on Sandy Sites – Mainly in WA


With a conventional system like this the pipes are either vertical or at a slope towards the discharge point.

A feature of this system is that when there is no flow all the pipes are empty.


  • Simple and inexpensive to design and construct.
  • If well designed, and constructed,  the speed of flow in the pipes will prevents silting and subsequent blockage.


  • This type of system can look very untidy when taking water to a Rainwater Tank that is some way from many of the downspouts (It results in lengths of pipes suspended in mid air)
  • Difficult to transfer water to a discharge point that is above the ground level of the building, although below the gutter level. A problem often encountered on demolition and rebuild projects and battleaxe blocks.

If you are planning a rainwater tank or are having problems with getting storm water to a suitable discharge point you could consider a Closed System


Also see Underground Pipes


Underground Pipes

It’s important to make sure you are getting the right type of pipes and fittings that will be underground… don’t want to be digging up you garden, or paths, in case of blockages.


Some plumbers will want to use, and bury, 90 mm diameter rainwater pipes, basically plastic downpipes.

You need to make sure you are getting as a minimum 100 mm PVC pipes rated as DWV.(Stands for Drainage, Waste and Vent).

The reasons are:

      • The 90 mm pipe is thin walled and can be easily deformed once buried. This means you lose capacity without realising it. The DWV is a much thicker and thus stronger pipe.
      • Although the increase in pipe diameter is fairly small the flow capacity of the larger pipe is over 40% higher. That makes a difference in storm conditions.

Pipes are normally marked at 1m intervals with the type, manufacturer, nominal diameter, material, and the Standards reference (AS/NZS1260).

This is printed on the pipe every 1m.

Protect Underground Pipes

It’s important to protect your underground pipes.

One of the problems during a new house construction is that concrete tend to fill underground pipes, causing blockage.

A hydraulic impact cutter can remove concrete in drains and sewers.

Other problems include leaking or burst pipes caused by corrosion, tree roots, and collapsed pipes.

Roots tend to grow toward the direction of the water so a loose connecting or weak point in the underground pipes triggers tree roots to wrap around them until they burst.

That’s why the design of the pipe system is crucial to ensure a problem-free plumbing.

They should be away from trees and other structures to avoid these problems. 

You can use an experienced plumber to help protect your underground pipes.

A qualified and experienced plumber will detect common leak indicators in the underground pipes and repair them. They’ll test the repair and fill the trench.


Bend refers to a term for any change or offset of direction in the pipes, which includes elbows.

They’re fabricated as per piping specification requirement.

Elbows come in standard or pre-fabricated and are available off the shelf. 

Bends are available in 4 different angles for DFW pipes as follows: 15 degrees, 30 degrees, 45 degrees and 90 degrees.

Although 90 degree bends are available, I would NOT install them underground due to the blockage risk….. If you need a 90 degree change of direction underground:

      • For a drain or a sewer use a junction pit.
      • For a charged (pressure) rainwater system use two 45 degree bends with an inspection ‘T’ in the middle.

Why Aggi Drainage Pipes Have Slots In The Base

A lot of people aren’t sure how drainage pipes work.

“Why doesn’t the  water run out of the slots at the bottom?” is a question that many people ask.

The pipes work on the principle that the soil/gravel around the pipe is water logged. This water drains into the pipe as the bottom of the pipe is below the  soil water level.


When the water drops below the  pipe  the water then runs through the gravel in the trench getting into the pipe further along the trench. (Diagram below shows a section along a pipe)


See Agricultural Drains for more information


Retaining Wall Drainage

I’ve previously posted about the importance of Drainage Behind a Retaining Wall to minimise the forces acting on the wall…. here is some advice on how to make sure it works.

The first thing to consider is this is drainage of the water in the soil. You should aim to properly drain surface water away from the wall.

There are two sorts of retaining walls as far as drainage is concerned:

  • Permeable Walls – The water can flow through the face of the wall.
  • Impermeable Walls – The water is unable to flow through the face of the wall.

Permeable Walls

Examples of permeable walls are:

With permeable walls the water should seep out anywhere on the face.You still need to put a drainage layer behind the walls. This will ensure that there is a clear path to the seepage points and that any dirt is filtered out rather than staining the face of the wall. It can be worth using a drainage pipe behind the wall as extra security against water pressure building up.

Impermeable Walls

Examples of impermeable walls are:

  • Reinforced concrete walls.
  • Block Work or Brickwork Walls (see this link: Besser Brick Walls)
  • Sleeper walls…. With a plastic backing.

With these walls you need to make special provision for drainage.


  • Weep holes which are specially constructed holes through the structure at 1m, or less, spacing.


  • An  Agricultural Pipe(a perforated or slotted pipe see this Link Aggi Pipe) discharging to a surface water drain. A good idea is where possible have discharge points with silt pits (see this link: Silt Pit) at both ends of the wall so drainage will still occur if the pipe gets blocked.


Drainage Layer

  • For the drainage layer I would recommend  at least 100mm of 10mm or smaller gravel.
  • If you are going to have a pipe the width of the drainage layer should be 50-100mm greater than the pipe diameter.
  • Use this construction and I don’t think a geotextile is required.

For responsibility for Boundary Retaining Walls see Fairness

See Why Aggi Pipes Have Slots In The Base to learn more