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How to build a retaining wall

Adam_W
Valued Contributor

Difficulty: Intermediate

Retaining walls not only help ensure soil doesn’t wash away down a slope. They can also become a landscape asset, providing terracing for sloping gardens and lawns or creating level pathways. They are also a great sustainable addition as they’ll slow down water runoff.

 

This step-by-step guide will help you to build a simple retaining wall using treated pine sleepers. It also includes a set of steps using a flexible design that can be used in many different environments.

Video Tutorial

Steps

Step 1

Choose the right timber for the job. It’s critical that timber retaining walls be constructed from timber that is suitable for ground contact.

 

Timber has a number of classes and for most retaining jobs it’s the H or Hazard class that’s most important. For retaining you need to use timber with a minimum rating of H4. In treated pine this H-level is achieved through treating the timber. This is done under pressure with chemicals or other products such as micronized iron.

 

Commonly available CCA treated pine should not be used where you may have frequent contact with the timber, where any treatment may leach out, or in situations such as vegetable gardens. In these situations use ACQ or MicroPro treated timber.

 

1. Choose the right timber.jpg  

Step 2

Take some time to plan your retaining wall design, including a basic sketch. A simple, non-structural timber wall can easily be constructed by a anyone with just basic D.I.Y. skills and tools.

 

If you think you need a tall wall, consider whether you could use a series of smaller, tiered walls instead. You should seek professional advice if the wall height exceeds 400mm, if the wall is close to a boundary, or if it will be structural and carry a heavy or “live” load such as beside a driveway. Also check with your local council for any rules they have regarding retaining.

 

2. Take some time to plan.jpg

 

Step 3

Determine the area where the wall will be. Hammer a peg around 1m out from both ends, run stringline between, and mark the line with paint. Mark points for posts. Every 2.4m or 3m sleeper must have posts at either end and one in the centre.

 

3. Mark the line with paint.jpg

Step 4

Excavate as needed to reach desired levels and dig post-holes. In our case we installed the smaller downhill wall first as it did not require posts, just digging in. This gave us a level to work from. If you encounter rock you may need to use a small rock-breaker. Note that a post-hole should allow for around 100mm space at the front and back of the post and 50mm each side to allow for both positioning adjustments and adequate concrete.

 

The depth of the post below the ground can be variable depending on soil type and topography. For example, a 400mm high wall in heavy soil on a minor slope could have as little as 200mm in ground. But if soil was loose or slope greater, then you may need the same height in-ground as above. If in doubt, seek professional advice.

 

4. Excavate as needed.jpg

Step 5

Cut posts to desired length. We cut ours 800mm allowing for 400mm below ground. For extra support we used thicker 75mm sleepers for the posts. Use stringline, levels and tape measure to accurately set position and height. You can set posts a little high and trim off afterwards. Put around 30 to 40mm of gravel or concrete in the bottom of the hole, position post and add concrete following instructions on the bag. Check it is level on both the face and the sides. Repeat for all posts and allow to set.

 

5. Use concrete or gravel for post holes.jpg

Step 6

Our wall included a small set of steps. These were assembled and dropped into place affixing them to the first post and existing wall. See diagram for details of the design. The size can be adjusted to suit.

 

6.1 Our wall included steps.jpg  Steps.png

Step 7

Drop 75mm sleepers in behind posts. For a continuous, longer wall the sleeper end should be in the centre of the post. Check level is correct and use 100mm construction screws to secure. There should be a screw added at top and bottom of sleeper face at each post. At wall ends sleeper end should be flush with post and additional screws may be added for extra strength.

 

7. Use construction screws to secure the sleepers to the posts.jpg

Step 8

Position drainage pipe behind wall and run output end to suitable location where it can be accessed. Cover pipe with gravel and then cover this with filter fabric. (see diagram for cross section of wall and drainage)

 

8.1 Position drainage pipe behind wall.jpg  8.2 Cover pipe with gravel.jpg  Retaining Wall.png

Step 9

If used, connect un-slotted pipe to drain pipe output and run to the required point. Drainage water must never be drained to a boundary so direct it to a suitable stormwater drain on-site or into a garden area.

 

9. Connect to drain pipe.jpg

 

Step 10

The cut material can now be used to fill behind the wall. Pat it down to remove large air pockets but do not overly compress it. You can now add turf or plant out the area to finish it off.

 

10.1 Lay turf to finish.jpg  10.2 Completed wall.jpg  10.3 Completed wall and steps.jpg

Materials

To create a treated pine wall 2.4m long by 400mm high, we used the following materials:

 

  • Treated pine sleepers – Two sleepers for the rails measuring 200mm x 50mm x 2400mm, plus one for the posts measuring 200mm x 75mm x 2.4m
  • 100mm timber construction screws (small pack)
  • Pre-mixed Quickset concrete with a strength rating of at least 24MPa (minimum of 3 bags)
  • 65mm slotted agricultural drainage pipe
  • 65mm un-slotted agricultural drainage pipe (optional)
  • 20kg bag of blue metal drainage aggregate (minimum of 2)
  • Filter or drainage fabric
  • Set-out paint
  • Set-out pegs

Tools

  • Safety gear – eyes, ears, breathing, hands and feet
  • Digging spade or shovel and mattock
  • Wheelbarrow
  • Levels (long and short)
  • Tape measure
  • String line
  • Builder’s square
  • Power saw
  • Drill-driver

Images

1. Choose the right timber.jpg

2. Take some time to plan.jpg

3. Mark the line with paint.jpg

4. Excavate as needed.jpg

5. Use concrete or gravel for post holes.jpg

6.1 Our wall included steps.jpg

Steps.png

7. Use construction screws to secure the sleepers to the posts.jpg

8.1 Position drainage pipe behind wall.jpg

8.2 Cover pipe with gravel.jpg

Retaining Wall.png

9. Connect to drain pipe.jpg

10.1 Lay turf to finish.jpg

10.2 Completed wall.jpg

10.3 Completed wall and steps.jpg

10.3 Completed steps.jpg

10.4 Side view of steps and wall.jpg

AHW23151.jpg

 

12 Replies
fjp3012
Occasional Browser

Hi Adam,

Newbie question: does the ag pipe connect into stormwater / drainage pipes? I’ve always wondered and, as we’re just about to build our first retaining wall, it would be good to know. :smile: Thanks.

Jason
Community Manager
Community Manager

Hi @fjp3012,

 

Many thanks for joining in the discussion on Workshop. Let me tag @Adam_W for you so he is alerted to your question about whether to connect the ag pipe. 

 

Let me extend a very warm welcome to the community. We trust you'll get loads of helpful information, advice and inspiration for all your projects around the house and garden from our wonderful members like Adam. Please don't hesitate to post whenever you need a hand or have something to share. 

 

Jason

 

Caron
Experienced Contributor

Great information - thank you.

Tash_B
Budding Browser

We are seeking retrospective approval for a Ridgi retaining wall. Just wondered where we can get a hold of this company's generic engineering safety certificate? Thanks in advance.

MitchellMc
Bunnings Team Member
Bunnings Team Member

Welcome to the Workshop community @Tash_B. Many thanks for your question, I can assist with that.

 

I am in the process of contacting the manufacturer in regards to your request and will let you know when I have an answer from them.

 

Mitchell

 

MitchellMc
Bunnings Team Member
Bunnings Team Member

Welcome to the Bunnings Workshop community @Rasilon. It's great to have you join us.

 

Let me mention @Adam_W so they are alerted to your kind feedback. Are you planning on building a retaining wall? We would encourage you to let us know if you need any assistance or would like to share your project with the community. I trust you'll find loads of inspirational projects from our knowledgable members, like this one, for around your house and garden.

 

Mitchell

 

pete64
Newbie

I am looking to build a low retaining sleeper wall using H section joiners, bit confused with the online sizing, planning on using 75mm sleepers, which size H section should I use?

MitchellMc
Bunnings Team Member
Bunnings Team Member

Welcome to the Bunnings Workshop community @pete64. It's great to have you join us and many thanks for your question.

 

For your 75mm thick sleepers, there are the galvanised joiners in 450mm750mm and 1100mm lengths. There is also the option to have black powder-coated joiners in 450mm750mm and 1100mm lengths. As a general rule of thumb, you'll want as much post in the ground as you have above the ground. For a single height sleeper wall using 200mm wide sleepers, you would get the 450mm joiner and concrete 250mm into the ground.

 

Here a step-by-step D.I.Y. video on how to install Retain-It which I trust you'll find useful.

 

@Adam_W might also like to join the discussion and share his knowledge on the subject.

 

Please let me know if you need further information or had any questions.

 

Mitchell

 

Adam_W
Valued Contributor

Hi @pete64 you should find that the sizes they use to describe the joiners are in-fact the sleeper size that they accept.
So to use this Retain-it as an example... the size is expressed (in this case) as 75 x 450mm.

It's a bit confusing because the 450 is the external length whereas the 75mm is the internal sizing of the 'U'.

Normally a size is expressed as external length x width x depth but here it's length and internal dimensions. I guess it would be even more confusing if they gave the external width which would be about 85 to 90mm & then tried to somehow add the extra dimension...
You'll find this is the same with most of these products - internal dimension & overall length.
And just a tip too... you'll notice the sizing increments? This is to help make sure you get the right amount of post in the ground. So a 450mm is for 1 x 200mm sleeper. A 750mm is for 2 x 200mm sleepers high. An 1100mm is for 3 x 200mm sleepers high.
In most circumstances anything above 600mm high will require council approval and engineering designs etc. so always worth checking with your local council as it can depend on circumstances. For example a 400mm retaining wall supporting a driveway would likely need approval. It gets into a rather complex area of static and 'live' loads.

melvah6
Newbie

What are the best type of sleepers to place as a retainer wall just slightly in the ground. Pine or hardwood - I understand whichever must be at least h4 treated, but is pine going to last longer than hardwood, or the other way round???

Jason
Community Manager
Community Manager

Hi @melvah6

 

Welcome to the Bunnings Workshop community. We're so pleased to have you join us and look forward to reading about all your projects and plans for around the house and garden. We're sure you will get plenty of helpful information, advice and inspiration from our amazing community members.

 

Let me tag the wonderful @Adam_W for his thoughts on what timber is best suited to your retaining wall project. 

 

Jason

 

Adam_W
Valued Contributor

Hi @melvah6, It can be a bit 6 of one 1/2 dozen of the other to be honest. Treated pine would typically last longer in a wider range of situations but in say a dry situation hardwood may outlast treated pine. Remember that any timber that isn't treated is prone to attack from termites so that can be the downfall (pardon the pun...) of hardwood.  You'll find however that many of the hardwood sleepers are chemically treated now too to extend their life still further and protect against bugs.
The thing with hardwood is it is a lot harder to work with - heavier, harder to cut, all screw holes will need to be pre-drilled etc.
Personally I'd just go microshades pine. A lot cheaper too 200 x 50 x 2.4 microshades is around $16.50. Hardwood same size is $23.43.
If you don't like the colour of pine it's easy enough to stain or even paint. Just do so before you install.

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