Constructing Timber Retaining Walls

Constructing timber retaining walls for anything up to a height of about 1 meter, or three feet, is a job that any DIY/home enthusiast can tackle. However, above the 1m mark you might be best advised to seek expert help from a builder or landscape gardener, as it involves calculating structural loads and other critical design work.

A well constructed timber retaining wall can easily last for twenty years enabling you to make better use of your garden or yard. If you want to build a retaining wall to support or extend a drive for your autos, then you’re recommended to check local planning/construction regulations before proceeding.

What you Need

Building a timber retaining wall isn’t a job to done with odd scraps of timber lying around your workshop, shed or garage. For a 1m high wall you need sturdy 150mm square posts that are 2m long, cross-members, or rails, that are either 150mm half-rounded or as a possible alternative 150mm by 75mm and interlocking 50mm retaining wall planks, all treated against decay.

If you ask at your DIY store they’ll probably have some purposely designed interlocking panels for timber retaining walls. You’ll also need some other bits and pieces, but we’ll come to those later.

To be sure that your retaining wall lasts more than a few years do take some time to plan and measure it properly. Your retaining wall will essentially be a series of posts and rails; however, the spacing of the posts is vital to the overall integrity of the wall. With 150mm square posts you want them spaced at 910mm centers, to allow for end-to-end jointing of the cross-members

Fitting the Posts

Dig out a suitable foundation for the base of retaining wall and then dig out at least a depth of 300mm behind where the back of the wall will be. Insert pegs at each end of where the wall will be and fix a line between them to indicate the line for the back of the wall, then measure and insert pegs where the center of each of the posts will be inserted.

Because you don’t want a lot of loose earth around the posts buy or hire a post borer and create 300mm diameter holes for the end posts to a depth of just over 1m. Angle the borer so that the posts will lean into the earth to be retained at about a 1:10 pitch.

Put some coarse gravel in the bottom of the holes for the posts to sit on, brace the posts in position and then fix the posts in place with a concrete mix. Using string lines between the end posts, you can now fit the intermediate posts to the correct angles. Leave the posts to set thoroughly for two or three days.

Rails and Planks

Fix the rails to the back of the posts working from the bottom up making sure the rails are level and very close to each other, butt joints can be used but make the joints staggered on alternate posts to add to the overall strength. The retaining planks can then be fixed to the front of the posts. Use 150mm galvanized nails for the rails and 100m ones for the planks.

You could just backfill with earth but you’re recommended to backfill with a drain coil sitting on a bed of gravel, overlaid with about 500mm of gravel that has a geotextile membrane above it. Use a piece of the membrane to create two compartments above this, fill in the side nearest the timber with gravel and the other side with earth, then withdraw the membrane. Finally top the whole thing off with top soil.