Where timber still makes sense
Timber was once the go-to for garden walls in Mandurah, and plenty of older back yards in the established suburbs still have one. Concrete sleepers and limestone have taken over for most structural walls, but timber still has a place:
- Low garden terraces, one to three sleepers high, to level beds and lawns.
- Vegetable garden and raised beds, where the look of timber is part of the appeal.
- Edging between lawn and garden, or along paths.
- Temporary walls on a block you plan to redevelop in a few years.
- Tight budgets, where a modest wall is needed now and a permanent one can come later.
It is not the right choice for tall walls, boundary walls with a fence on top, walls supporting a driveway, or anywhere a failure would cause real damage.
Pine or hardwood
Treated pine is the most common. It is treated with preservatives to resist rot and termites, and different treatment levels are rated for different exposure. For a retaining wall, the timber touching soil needs to be treated to an in-ground contact level, not just above-ground. Ask what treatment class the contractor is using.
Hardwood sleepers are heavier and more expensive, and some species are naturally durable. They have a rugged look that suits native gardens. Hardwood can split and twist as it dries, and quality varies between suppliers.
Whichever timber you choose, the posts are the part under most stress and most exposed to moisture in the ground, so they are worth spending on.
How a timber wall is built
- Set-out and excavation along the wall line, with enough room behind for drainage.
- Posts set in concrete footings at regular spacing, usually closer together for higher walls.
- Sleepers placed behind the posts (on the soil side) so the soil pushes them against the posts, not away from them.
- Lining with geotextile fabric or a plastic membrane behind the sleepers, to keep sand in and moisture off the timber.
- Drainage, with a slotted pipe at the base surrounded by gravel and an outlet.
- Backfill and compaction, in layers.
- Capping or a top rail, optional, to tidy the top edge.
A common shortcut is to put sleepers on the front of the posts, where they look neater but rely entirely on fixings to resist the soil pressure. That is not the way to build a retaining wall.
Why timber walls fail early
- Untreated or under-treated timber in contact with wet soil.
- Posts set without concrete or too shallow.
- No drainage, so soil and timber stay wet.
- Sleepers on the wrong side of the posts.
- Termites that have found their way in through the ground.
- Reticulation spraying the wall every day.
If you have an old timber wall that is rotting, leaning or bowing, it is usually cheaper and safer to replace than to patch. See repairs and rebuilds.
Timber in coastal conditions
Close to the ocean, salt air affects metal fixings more than the timber itself. Use galvanised or stainless steel bolts and brackets. On canal and estuary lots where the water table is high, the bottom of the wall may sit in damp ground for long periods, which shortens timber life. On those lots, concrete sleepers or limestone are often the better long-term call.
Approvals
Most low garden timber walls will sit below the height where a WA council would want a building permit, which is commonly about 500 mm of retained ground. Anything taller, on a boundary or carrying a load should be checked with the City of Mandurah or the Shire of Murray before work starts. Before You Dig Australia will show any underground services on the wall line.
Getting a quote
Call (08) 9515 3332 or use the form below with the length, height and a photo. To compare against other materials, see the cost guide.