How to: Build a retaining wall - Part 2

Now that you have a plan in place, know where you can dig and have a clear idea of what permits you will or won't need, it's time to get to work on preparing your sight and sinking your posts. This second part in our series will take you through these very important steps.

1. A clean working area

While tonnes of soil and a clean working area may not sound like they go hand-in-hand, this is a necessary step for getting the job done right. For small DIY jobs, a shovel should be adequate for cutting in, whereas bigger jobs may require a digger or bobcat.

You'll want to give yourself between 300 and 400 mm of space between the bank and the face of your wall. This will be backfilled later to ensure proper drainage, and also allows you to work your way around the entire wall.

2. Set your posts

Once you've used a mitre saw to cut all of your vertical posts to length, it's time to get them in the ground. Remember that every 100 mm  of wall needs 100 mm of support below ground, so your posts will be double your wall height. They also need to be adequately treated to avoid rotting in the ground.

Use a post hold borer with a 300 mm diameter to dig the holes for your posts and concrete. You will need to angle the holes so that the post is on a lean towards the bank. The lean should be 1:10 - in other words, for every 100 mm you go up, the post should angle towards the wall 10mm.

A purely vertical wall will start to sag over time, so this angle is important. When viewed from the front, the posts should appear completely vertical. Make sure to space your posts consistently and adequately for the rail and post sizes you have chosen.

Place some scoria in the post holes to act as drainage - about 100 mm. Position your two end posts at the correct angle and temporarily brace them with battens fastened with nails or screws.

Once your end posts are in place, you can use a string line with a 10 mm  packer to position the intermediate vertical posts. The packer will help to keep your string line straight if there are any posts slightly out of line.

With all of the vertical posts in place it's time to mix some concrete and pour it into each post hole. Allowing this to set for 48 hours not only ensure sufficient drying time, it also gives allows you to put your feet up and relax for a bit - it's been a hard slog so far!