How to Build a Retaining Wall
Introduction
Building a retaining wall can help you create a useable space in your garden. It can help you level up an area in your yard, or help protect against erosion. Building one isn’t hard and done right, it’ll last for ages. Follow these steps, and you’ll be able to build one easy as.
Before you begin
Rules & Regulations
You can build a retaining wall up to 1.5m high without needing consent.
It must include ground water drainage behind the wall – otherwise your footings may become waterlogged, and the pressure can push your wall over.
If you’re retaining a wall higher than 1.5m or anything like a driveway, parking space or building above the wall, you will need an engineer to draw up your plans.
For retaining walls 1m or higher you may be required to install a safety barrier. This is likely to be required where the wall is beside an access path to the house.
If you are building your retaining wall on a boundary, the entire cut behind the wall, your fill, and drainage ALL need to be on your side of the boundary. So make sure you take this into account when planning.
As a general rule, you should limit your scope to walls with the cut set back from the boundary the same distance, or greater, than the height of the cut.
For these reasons it’s always best to check with your council before starting.
Check your council plans
Check your plans for the location of any pipes, you don’t want to hit those.
Call your electricity provider for information on any underground wires, as these may not be shown on your plans.
Draw a plan
Design and draw up a plan of the retaining wall you want to build, including:
- Wall height
- Round or square posts – this will determine post spacing and hole depth.
- Retaining timber
Once you’ve drawn up your plan, measure the quantity of materials you’ll need, and order them. Use these formulas to help you work out the volume of concrete you’ll need:
- For a square hole:
width x width x depth x number of holes
E.g. .3m x .3m x .45m x 10holes
= 0.405m3. - For a round hole:
Pi x radius squared x height = volume.
Measure the radius of the hole. To do this, measure the diameter, then divide by two.
E.g If the diameter is 6 cms, then the radius is 3 cms.
Measure the height of the hole from top to bottom which also represents how deep the hole is. Assume that the height or depth for your hole is 10 cms.
Calculate the Volume - This formula is pi x radius squared x height = volume.
Pi is 3.142. The radius is 3. The height is 10. So, plug in your values of 3.142 x 3^2 x 10 and calculate.
The volume is 282.78 cubic cms
Getting it done right
In this guide we are retaining a height between 500mm – 700mm, and will be using 125mmx125mm piles, with 200x50 retaining boards, as these are the commonly available sizes.
Mark out posts and dig holes
- Remember, if your retaining wall is on a boundary then the cut, the fill, and drainage all need to be on your side of the boundary, so allow for this when setting out your string line.
- When laying out your wall make sure you have enough space behind the wall to work in, and make sure there is room for scoria and drainage. Dig it out if need be.
- Set a string line to mark the position of the centre of your post holes.
- Mark out post holes along string line as per your plan.
- Dig holes to depth.
- If your retaining wall is over 1m you will need to angle your posts. Angle the hole to allow posts to lean back towards the area being retained at a ratio of 1:10 (so the posts lean 100mm for every 1m of height).