You can make a flat space in a sloping garden by digging away part of the sloping area and holding back the remaining earth with a retaining wall. Many garden landscapes use timber walls for retaining walls that create flat areas for patios, parking spaces or terraces. Timber retaining walls consist of posts, dug into the ground, and wooden slats fixed to these posts, which hold the earth back. Timber retaining walls provide a soft, organic visual effect to the landscape.
Remove unwanted earth from the sloping area using an earth mover. Unless you are an experienced earth mover operator, hire an operator -- incorrect use can be dangerous. Remove the unwanted earth, leaving a vertical wall of earth, which your wall will retain, and a flat surface at the base of this earth wall.
Run a string near to the ground, parallel to the earth wall, 10 inches away from the base of the wall to allow space for you to work on the wall. Use pegs to mark each end of the wall, and stretch the string tight between the pegs. Mark the ground at 3-foot intervals beneath the string. This is where you will place posts to support the retaining wall timbers.
Use a mechanical posthole borer to create holes in the ground at each of your post marks. The holes need to be as deep below the ground as the height of the wall, plus an extra 4 inches for base gravel to sit beneath the posts. Use a hole borer attachment that drills a 18-inch diameter hole in the ground.
Pour gravel into each posthole, and fill to a level of 4 inches. This provides a solid base for the posts to rest on, and will bind with the concrete used to hold the posts in place.
Place timber post in the holes, sitting on the base gravel. Mix concrete and pour into the holes surrounding the timber posts, filling the hole to the top. When the concrete sets around the base of a post, it fixes the post so that it won't be moved by the pressure of the earth it retains. Each post must be inclined toward the earth wall following the 10-to-1 rule. For example, if the wall is 36 inches high, the top of the post must be inclined toward the earth face by 3.6 inches. Use string to run from each of the end poles to align the intermediate poles. Give the concrete two days to set.
Nail 2-by-8-inch planks that are 6 feet long to the posts on the side nearest the earth wall. Fill the space between the planks and the earth with scoria gravel, which drains easily, to a height 1 foot below the top of the retaining wall. Use topsoil to fill to the top of the wall.
Use treated timber that is water-resistant. Use galvanised nails that will not rust.