General Information on drainage.


I first want to say that when it comes to drainage is that it’s always better to be safe than sorry. If there is any chance whatsoever of water or moisture building up to a point of putting any pressure against the wall then put a drainage system in.

Terracing systems although it might be tempting to allow the top one to flow onto the level of the lower ones, it is a better idea to keep them separate, connect them individually to a common storm water pipe running from the top level to the bottom,

If you are intending building a terrace set of walls and depending on how many terraces you have, how high each wall is, how long each wall is, you may end up with multiple systems. So much so that you may have to increase the size of your bottom pipe to transfer the volume of water collected away to its destiny. Just keep in mind that you cannot afford any pressure to build up behind those walls so do what you have to do!!!!

Closed Face Walls.

It makes no difference as to whether the wall is built from timer, brick or stone etc. they are all performing a task and that is holding back material that will swell when wet.

Collecting and Piping the water.

Basically your piping solution is using what is called an Agg pipe or slotted pipe. It’s like a corrugated pipe that is bendable with small holes all along the length of it.

This is usually sold in rolls of set lengths, 5, 10, 20m etc. It’s usually black in colour but can also be purchased socked, which is a white nylon covering along the length of the pipe which helps filtering what gets into it. The pipe usually comes in three diameters, 50mm (2”), 65mm (2.5”) and 100mm (4”)

The end of the pipe is either left open at the end of the wall or comes out along the length of the wall, somewhere at the lowest point or is plumbed into the storm water system.

Another idea that I have used a few times is to take the pipeline to a garden and just allow the water to be released there. Quite often this will seeping for a while after the rain has stopped, as it collects the seepage from behind the wall as well. Keep in mind this would only be used for the smaller walls where there volume of water is small.

With the higher walls you can also place an agg pipe half way up the wall or close to the top, these pipes will deal with the storm water and the bottom one will deal with seepage.

For the longer and or high walls you may have to have extra outlets to cater for the volume of water, these can be collected together with your own storm water piping system that is set in the ground in front of the wall. This will either be taken to a new seepage pit somewhere or out to the street, depending on your circumstances. You just have to make sure that the water has a good release or it will build up behind the wall.

Landscape Fabric or Weed Mat.

Another product we may use is the landscape fabric or as it’s commonly known as weed mat. When back filling, this would be placed between the fill and the gravel. It protects the gravel from being silted up from the fill.

If you are using boards to control the back filling process then this mat has to be placed under the very bottom of the gravel, and the boards have to be raised after about a foot of back fill. That way there is not too much pressure against them and they won’t pull the fabric out of position.

Here are drainage guidelines in relation to the wall heights.

  • 150 – 300mm (6”-12”) High

Although it is not absolutely necessary for a drain system to be placed behind this as it’s not very high, the important thing to remember there is NO absolute. If the material you are retaining is going to hold a lot of moisture, than it is always best to deal with it.

> An Agg pipe 50mm (2”) should deal with this.

> Socked pipe is recommended.

> Gravel over the pipe is optional.

> Fabric optional.

  • 300 – 600mm (1’ – 2’) High

> This height is good to start using the 65mm Agg pipe.

> It may still only need one pipe placed at the bottom.

> Gravel covering the pipe up to half way of the wall height.

> Fabric optional.


  • 600 – 750mm (2’ – 2.5’) High

> At this height use the 65mm Agg pipe.

> It may be worth considering two, one at the bottom, one half way up..

> Gravel covering the pipe to the top of the wall, at least 100mm (4”) thick behind the wall.

> Fabric optional.

  • 750 – 900mm (2.5’ – 3’) High

> At this height use the 65mm Agg pipe.

> It may be worth considering two, one at the bottom, one half way up..

> Gravel covering the pipe to the top of the wall, at least 100mm (4”) thick behind the wall.

> Fabric optional.

  • 900 – 1050mm (3’ – 3.5’) High

At this height  you need to check with Council regarding your areas  regulations. If your fine to build without a DA then use the following specs.

> Use the 100mm Socked Agg pipe for the bottom run.

> It may be worth considering two more, one half way up and one at the top, these you can use 65 or 100mm depending on the volume of water expected. The top pipe I would keep about 100mm (4”) buried beneath the top of the gravel.

> Gravel covering the pipe to the top of the wall, at least 100mm (4”) thick behind the wall if using the 65mm pipe BUT when using the 100mm pipe you want your gravel to be 200mm (8”) thick behind the wall.

> Fabric optional.

Stone Walls.

Stone walls can have an advantage over the timber walls in that even if it’s a cemented stone wall you are able to leave weep holes throughout the structure of the wall, as well as using the guidelines mentioned above, this is probably only advisable if at the base of the wall is either a garden or lawn. If you have paths or a driveway directly at the base then it’s not a good idea, in these cases it’s better to completely seal the wall up and use the same guidelines mentioned above for your drainage.

Brick and Block Walls.

Brick and Block can be treated exactly the same as stone when it comes to drainage, as well as using the guidelines mentioned above I have sometimes cemented tubes in allowing the water to escape, just use your discretion and the principles we have discussed here for your drainage system.

Open Face Walls.

Open face what ever material will have the same guidelines as above but Agg pipes and collecting the water is optional. the whole design of the wall works on the principle of the water being able to escape out of the face. However it’s important to remember that water will drain to the bottom and this may need an Agg pipe to remove it from behind the wall. Again work on the principles and guidelines mentioned here and your systems will work.


With all systems it is always better to air on the side of caution and over build, cater for that one or two times in the year we may have a deluge of rain, if the water can escape relatively quickly then you will not have problems later on down the track.

This probably doesn’t need to be said but I’ll say it anyway, water will NOT flow upwards, so you need to ensure that the pipes behind your wall are either level or flowing slightly downwards. The same with your storm water pipes, even just the slightest down slope the water will escape.