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).

Brace and concrete posts

• Place a string line 50mm out from the front face of your post. Placing it 50mm away from the rough-sawn timber like this will ensure your line doesn’t accidentally snag or get pushed out of position.
• Start bracing your posts. Place a concrete block or layer of scoria in each hole to keep your post off the ground.
• Make sure posts are plumb, or are sitting at your desired 1:10 angle.
• Keep every post you brace 50mm off your string line. You can use an offcut of timber to measure this.
• Once posts are in position, concrete them in place and allow to dry as per instructions. Keep checking your posts are plumb while concrete is added, as they may move.
• Remove bracing when concrete has dried.

Fix boards to posts

• If your ground is flat and level, you can start at the bottom and move your way up. Run a level string line along the bottom of the posts to give your first row of boards a straight line to sit on.
• If your ground isn’t level, you can fix boards parallel to the ground or stagger them so they're level.
• If you’re staggering your boards run a level stringline from a low point to a high point and place a board along this line. Use this level line to stagger the rest of your boards. Aim to have full boards along the top row of your wall, and where necessary, contour the bottom boards along with the slope of your land.
• Place horizontal boards at back of posts, and fix with 100mm baton purlin screws. To make the job easier you can rest the first boards on a nail.
• Alternatively fix with galvanised nails at each post – 150mm nails for half round rails and 100mm nails for 50mm thick planks.
• Stagger the end joints in the rails so that butt joints occur on alternate posts.
• Make sure any joints are hidden behind posts.

Cut posts to height

• Once all boards are fixed to posts - cut the top of the posts to height at an angle to prevent water sitting on top.
• Paint timber preservative on the top of the post once cut.

Drainage

• Lay 100mm of drainage scoria along the back of the wall.
• Place a filter sock over the over the drain coil to prevent it silting up.
• Lay the drain coil on top of the scoria and lead the ends to where they can discharge away from the wall. Ensure the drain coil has some ‘fall’ - slopes down the length of the wall to allow water to run out.
• If possible, have the drain coil accessible at both ends so it can be flushed out.
• Cover the drain coil with 250mm of drainage scoria.
• Overlay it with the geotextile fabric – this will reduce the silting up of the drain coil.
• Continue filling with scoria, ensuring that there is a minimum of 300mm against the wall. The rest of the gap between the wall and the bank may be filled with well-compacted excavated material.
• Backfill to 100mm below the top of the wall.
• The top 100mm can be covered with topsoil where plants can be grown, lawn laid, or grass seed sown.