Welcome to the third and final part of our series on constructing your own retaining wall. One of the bigger jobs in the DIY gardening repertoire, this project requires a certain amount of skill and hard work. Placing boards, supplying drainage and backfilling are the final three steps in the process.
1. Fixing your horizontal boards
After your concrete has dried you can remove your battens with your claw hammer or electric screw driver. Run a string line along the bottom of your posts to ensure your rails are properly horizontal.
Place your boards on the back of the posts (between the posts and the bank) ensuring that all joints are hidden behind the vertical posts. Stagger your joints to ensure maximum rigidity.
Half rounds are a common choice for horizontal boards, although thick planks or other materials such as rail way sleepers or unique woods can be used to great affect. Remember to use galvanised nails for fixing your rails, and allow at least an extra 50 mm beyond the thickness of your materials.
Adequate drainage is essential to ensuring your wall has a long service life. Place between 100 and 150 mm of scoria behind the lower part of the wall, and lay a drain coil on top of this. The ends should discharge outside of the wall, or into a storm water drain if suitable. Having both ends accessible is ideal, as it allows you to flush it out when necessary.
Cover your drain coil with a further 200-250 mm of drainage material, and top this with a geotextile fabric layer to avoid the drain coil silting up in wet weather. Scoria should be used up to the topsoil layer up to around 300 mm back from the face of the wall. The rest of space between the wall and the bank can be filled with compacted excavated material or supplied soil.
You can use a polystyrene, metal sheet, plywood or hardboard sheet to separate your drainage material and backfill soil, pulling it out as you fill the gap. The last 100 mm should be topsoil on top of a geotextile fabric layer (at least over the drainage material), on which you can spread lawn or ground cover plants. A popular DIY gardening trick is to use plants that droop over the tops of the wall, or a vertical garden fixed to the front, to give it a very classy, landscaped look.