Retaining walls define areas to create more usable space for seating, landscaping and gardens.
- Poles or piles
- Timber boards
- Timber and pegs
- Drainage pipe
What to do before you start:
If you’re planning to build a retaining wall that is over 1.5m high you need a building consent from your local council.
You’ll also need building consent if the wall carries extra load on top from traffic, nearby buildings or steeply sloping banks.
Soil types can have a significant effect on how a wall performs. For the purpose of this guide the ground has been taken to be a firm clay soil type. Other soils may require specific engineering design.
If in doubt, it’s best to check with your local council.
If your wall is over 1.5m and has any load above it you will most likely need to get it designed by an engineer. This will ensure that the wall you build meets all the necessary requirements.
This information is provided as a guide only and does not constitute a complete design aid. Crofts accept no responsibility with the design and construction of pole retaining walls or their suitability for any purpose.
Designing a retaining wall
Basics of retaining wall design
A retaining wall has to be strong enough to support the load of soil pressing against it. This load determines the size of poles and the thickness of retaining timber to be used so that the poles or timber do not break.
The most important part of the wall is the part below the ground that anchors it. If the footings, foundations or the soil is weak then there is nothing stopping it from falling over. Good drainage is critical to the success of a retaining wall. If a wall doesn’t drain well then the bank behind and footings can become waterlogged and the wall may fall over.
What materials do I need to build a retaining wall?
Poles or Piles
To meet the “ground contact load bearing” requirements, poles or piles should be H5 treated high density pine.
The timber for the wall should be H4 treated and can be rough sawn, machined or tongue and groove profile.
If the wall has been engineered the specifications may require specific timber treatment or stress graded material.
Poles should be set in 17Mpa concrete. Pre-bagged mix or 6:1 ratio of builder mix to cement.
Timber and Pegs
Timber and pegs allow you to temporarily brace the poles while the concrete sets around them.
Nails should be at least twice the thickness of the timber being fixed.
100mm, 125mm or 150mm flat head Hot dipped galvanised nails should be used to fix the boards to the back of the piles.
Any cut surfaces should be liberally painted with timber preservative (Copper Napthenate) before installation.
100mm diameter perforated polyethylene pipe, preferably fitted with a filter sock, to drain behind the entire length of the wall. You may need to add extra to reach appropriate discharge points for the drains.
At least 250mm thick free draining metal or scoria backfill is needed behind the wall. Geotextile cloth or polystyrene sheeting can also be added to assist with drainage.
A supply of ice cold lager at the end of each day will encourage assistance from “Mates”. Crofts recommends you drink in moderation as this can adversely affect the quality of the wall.
Building your wall
Steps to build a retaining wall.
1. Draw your plans.
2. Assess your design and check whether it will require specific engineering and or building consent.
2. Check that the proposed wall is clear of subsoil drains, electrical, water or gas services.
4. Calculate the quantity of materials required
5. Excavate the bank to give at least a 250mm gap between the back of the wall and the bank.
6. Make sure the ground behind the wall is level or slopes to one end to provide drainage.
7. Dig or auger the foundation holes. Depth of the poles will normally be at least equal to the height of the wall above the ground. Allow 100mm extra for the base concrete.
8. Put 100mm of concrete into the two end holes. Set posts on top of this concrete making sure that only uncut or preservative treated ends go into the holes. Check that the top of the posts sit high enough.
9. Set the posts at the recommended 1:10 inclination from vertical towards the bank. Fix the poles in place with temporary braces and ground pegs. (Small, low garden walls may be installed vertically as long as sufficient depth into the ground is provided)
10. Use two string lines (top and bottom) run from the end poles to align the back side of the intermediate poles. Set these poles in place in a similar fashion to the end poles.
11. Check the poles are all vertical and plumb when viewed from the front.
12. When all poles are set in their final position pour in the concrete foundation. Tamp the concrete down with a piece of timber and leave to set for at least two days.
13. Nail the horizontal rails to the back of the poles, starting at the bottom and working up to the top of the wall. Stagger and make joins only at the back of the poles.
14. Once the wall is complete lay 50mm of clean free draining metal along the bottom at the back of the wall and then lay over the drain coil (with sock). If possible protrude both ends of the drain-coil so it can be flushed. Cover with another 250mm of free draining metal.
15. The back of the wall can have a polystyrene sheet attached or backfilled with 250 to 300mm of scoria drainage material to within 300mm of the top of the wall. Finish to level with topsoil. (geotextile cloth can be used between the topsoil and the scoria if drainage is a concern)
16. Sit down, have a beer and admire your handy work!