So you're thinking of building a timber retaining wall in your garden. Good choice: timber is natural, attractive and easier to work with than concrete or stone. But there are a few things to get right first if you want a wall that lasts. Start with these questions:
How high do you want the wall to be?
The height will determine how much soil it needs to hold back and the size of timbers required.
What sort of soil do you have?
Clay, sand or rock - each needs a different approach.
Do you want it to curve or have corners?
Angled sections need extra support, so think about how you'll fix the timbers in place.
And most importantly, what timber should you use?
There are many options from pine to hardwoods, each with pros and cons.
This article walks you through each one so you can design and build a wall that lasts.
What Is a Timber Retaining Wall?
A timber retaining wall is a landscape structure that holds back soil and prevents erosion on sloping ground. It’s made of wooden posts, boards, logs or sleepers that are anchored into the ground.
Timber retaining walls are popular for residential gardens because wood is attractive, durable, and blends well. The natural appeal of timber complements plants and landscaping. Timber walls are also more affordable and easier to construct than concrete or stone.
There are a few types of timber retaining walls:
- Post and board walls use vertical posts with horizontal boards attached. This style has gaps between each board, allowing for drainage and plant growth.
- Log walls stack whole logs on top of each other, with each tier stepped back from the one below it. Log walls create a rustic look but require more timber.
- Sleeper walls use sleepers laid horizontally and stacked, usually between posts or in a stepped pattern. Avoid second-hand railway sleepers unless you know how they were treated: many are creosote-treated and not suitable near vegetable gardens or where kids play. New H4 treated pine or hardwood landscaping sleepers are the safer choice.
The key considerations for a timber retaining wall are:
- Choose durable, weather-resistant timber: treated pine rated H4 (in-ground) or H5 (in-ground, structural and wet), or a durability class 1 or 2 hardwood. Sleepers 75mm thick are better than 50mm.
- Bury about a third of the posts or sleepers in the ground for stability. Secure them in concrete for extra support. Always ensure that there is at least 100mm of drainage gravel placed at the base of each post hole. This will help compact and reduce the instance of settlement and post rot by keeping the end of the post above water that may accumulate in the base after rain.
- Step the wall back as it gets higher, with each tier angled slightly towards the slope. This helps the wall resist the pressure from the soil.
- Include drainage behind the wall. Add a gravel or crushed rock layer and install drain pipes to prevent water building up. Drainage gravel is your friend when building a retaining wall, so don't skimp on it!
- Backfill and compact the soil behind the wall as you build up. This anchors the structure in place.
- Consider the slope and any extra loads like buildings or vehicles near the wall. Get professional advice for more complex sites.
Timber Qld has developed two fantastic technical data sheets all about retaining wall design and construction considerations. View them by clicking on the button below:
Key Design Considerations for Timber Retaining Walls
Slope and drainage
The slope of the ground in front of and behind the wall is critical. Steeper slopes mean more pressure on the wall, so you’ll need thicker timber and deeper footings. Make sure water drains away from the wall in both directions. Add drains, swales or pipes if needed. Standing water is the enemy here.
Choice of timber
Use durable hardwoods that can handle ground contact and better resist moisture and pests. Pressure-treated pine is also a good, affordable option. The bigger the timber, the more stable the wall. For most small residential walls, 200mm x 75mm is a good size when using treated pine and 200x50mm for hardwood.
Anchoring and footings
Bury about a third of the timber in the ground for stability. Concrete footings under the first row of timber help prevent settling. You'll also want to anchor the wall to the ground, especially for higher walls. Deadman anchors, concrete pads and rebar rods are all options. If you use threaded rod attached to deadmen you can add a small amount of adjustment to your wall to account for future settlement.
Backfill
The material you use to fill behind the wall is important. Choose a free-draining granular fill like drainage gravel and compact it as you go. Leave at least 150mm between the fill and the top of the wall for drainage and settling, and top it up 6 months after the wall is installed.
Get help when needed
In Queensland, a retaining wall needs building approval if it's over 1m high, within 1.5m of a building or another retaining wall, or carries a surcharge load like a driveway, pool or building. In those cases you need a structural engineer. They'll assess the site, design the wall to resist the loads, and handle complex jobs like walls on steep slopes or footings that bridge over services (stormwater, sewer or electrical trenches). The engineer provides a Form 15 for the design and, after inspecting the footings and other stages, a Form 16 certifying the work.
With the right materials and techniques, a timber retaining wall can create usable yard space and add value to your property. But do it right the first time—it’s not a job you’ll want to do over again!
Construction and Installation of Timber Retaining Walls
Now that you've designed your wall and have the materials, it's time to build. For a DIY homeowner it usually takes a few weekends. Go slowly and make sure each layer is level and secure before moving on. One of the main decisions is the spacing of your uprights. 1.8m centres is the maximum I would ever use; if you want your wall to stay straighter for longer, reduce it to 1500mm or 1200mm. Closer spacing obviously means more material and more labour.
Excavation
Before you dig any post holes, lodge a free enquiry with Before You Dig Australia (BYDA) to locate underground services. Then mark out the wall with stakes and string lines and dig out the soil to the required depth, about 150mm to 200mm deeper than the first course of timber to allow for drainage material. Wheelbarrow the soil away or stockpile it nearby for backfilling later.
Base preparation
Once excavated, level and compact the base. Lay a 150mm layer of free-draining material like gravel or road base and rake it smooth. This gives stability and lets water move away from the wall. Install an agricultural pipe at the base of the wall to carry water to a suitable outlet, and cover it with drainage material.
Post-installation
Seal any cut ends of the posts with a brush-on copper-based end-cut preservative (H4 and H5 treated pine comes treated, but cutting exposes untreated timber). Place at least 100mm of drainage gravel in the base of each post hole to prevent post rot. Stand the first posts in place, spaced to your design, and brace them plumb. Concrete them into the holes and check each post with a level before the concrete sets.
Installing wales
With the posts installed, lay out the wales (horizontal members) and mark where they cross the posts. Cut notches in the posts so the wales sit flush, then bolt the wales to the posts. Stagger the joints between wales for maximum stability, and install corner bracing if your design needs it.
Drainage Installation
Before backfilling, install drainage behind the wall. At the bare minimum, that's an agg line (a flexible slotted PVC pipe) at the base of the wall. There's debate about sock-wrapped agg lines: can the sock clog with sediment and make the pipe useless? To err on the side of caution, skip the sock but make sure at least 200mm of drainage gravel fully surrounds the pipe, so water filters through into the agg line and discharges to the stormwater line. The main job of the agg line is to relieve water pressure behind the wall, which otherwise tries to push it over (the "overturning moment").
If you'd like to go "full defcon-3" on retaining wall drainage, I highly recommend the Geofabrics or Polyfabrics drainage cell products and the Terradrain sheet drain. To install drainage cell, first lay your agg line and drainage gravel, then assemble the drainage cell and pin it to the back of the wall with 65mm roofing nails. It captures water moving sideways through the soil and directs it into the agg line.
Links
Terradrain Sheet Drain - LINK
Drainmasta DrainCel - LINK
Geofabrics - Geosheet - LINK
Backfilling
Once the frame is complete, start backfilling behind the wall with the soil excavated earlier, compacting it in layers as you go. With each layer, check the wall is still plumb and adjust if needed. Leave about 150mm between the final backfill level and the top of the wall to allow for settlement.
Capping (optional)
You can install timber capping or fix metal gang nail plates to the top of each post for added protection. The caps help prevent water from getting into the wall and improve the appearance, and plates will help reduce the tops of the posts splitting. Secure them in place with construction adhesive and galvanised batten screws for the best result. Treat the tops of all posts with a timber preservative to help reduce moisture absorption.
Maintenance Tips for Timber Retaining Walls
Regular maintenance and inspections are key to keeping your timber retaining wall in good shape for years to come.
Check your wall frequently, especially after heavy rains or other extreme weather events. Look for signs of excess movement, cracks, drainage issues or damage. If you notice the wall leaning or bulging, contact a professional immediately. Small cracks can often be repaired, but an expert must address major structural issues immediately.
Inspect the drainage system and clear any blockages so water flows freely. Blocked drains or gutters build up water pressure behind the wall, which makes it unstable.
Every few years, re-treat any cut or exposed timber with a suitable timber preservative, especially where it meets the soil, and re-stain or paint the visible face as needed. Don't use creosote or sump oil: creosote isn't approved for home garden use, and used oil contaminates your soil.
Trim back any plants or trees growing next to the wall. Their roots can damage the structure and drainage pipes. Likewise, don't pile heavy materials or equipment against the wall.
For log walls, check that the logs are still tightly joined with no major gaps. You may need to re-seal or chink the logs to prevent soil erosion.
Get an expert to assess the wall if it moves more than normal seasonal movement. How much movement to expect depends on your soil. Check the engineer's site and soil report for the Ys value (the expected ground surface movement): stable sites have low Ys values, reactive sites have higher ones. Small adjustments can often stabilise the wall, but severe cases may need rebuilding, so deal with problems early.
When to Engage an Engineer for Timber Retaining Walls
Sometimes a timber retaining wall project becomes complicated enough that you’ll want to bring in a structural engineer. It’s best to do this before starting construction to ensure your design is sound and safe.
If your wall will be over 1m high, within 1.5m of a building or another retaining wall, or carries a surcharge load, engage a structural engineer (and in Queensland you'll need building approval, as above). They'll specify the post sizes, depths, spacing and bracing to handle the load.
Sloping backfill or surcharge loads also call for engineering input. The angled soil increases the horizontal pressures, and extra loads like vehicles, equipment or buildings add more weight the wall must sustain. An engineer can design the wall to suit the site’s unique conditions. They may specify larger posts, closer spacing, extra bracing or anchoring to provide adequate support.
Drainage and moisture conditions impact a wall’s stability. Reactive soils that expand and contract with moisture changes require careful design consideration. An engineer will evaluate the site and design a wall able to handle potential soil movement to avoid issues like cracking or bowing. They can also design an effective drainage system to properly control moisture levels.
In areas with aggressive insects like termites, an engineer may specify particular wood treatments or alternative materials not susceptible to damage. They have the expertise to choose options suited to your local conditions and the wall’s expected lifespan.
The short version
Get the drainage, timber grade and post depth right and a timber wall will last for decades. Over 1m, near a building, or holding up a driveway? Call an engineer first.
