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Before you build a retaining wall, first check if you need permission from your local council. If your wall is more than one metre high, call a professional. Otherwise, terrace a steep slope with two or more retaining walls. Water is the natural enemy of the retaining wall. Clay soils, or areas that collect water may require extensive draining. You can insert short lengths of pipes through the wall near the base. Another drainage method is to bury a perforated PVC pipe behind the wall. This pipe should extend beyond the end of the wall and slope 3mm every 300mm. Sandy soils may only require weep holes in the timber. Ask your local council for advice.

Which wood?
Treated radiata pine, oregon and Australian hardwoods are good choices for a timber retaining wall. (We used 200 x 200mm timbers for our wall.) Avoid old railway sleepers as these are treated with creosote, which harms some plants and can make the sleepers messy to work with. Reinforcing rods secure the structure.

Here's how:

1. Cut a bevelled trench into the slope, wet the trench and tamp it well. Set the first timber in place and level it. This course will be completely buried in the ground. We planned our wall to turn a corner, but the same technique applies to a straight wall.'''

2. Set a second timber on top of the first and bore a hole through the two timbers. Use a heavy-duty drill with an extension bit. A small bit burns out on long holes such as these. Drive a 19mm-reinforcing rod through the holes into the ground to hold the timbers firmly together.

3. Continue to lay the timbers, staggering the joints from one course to the next. Drill holes and use a reinforcing rod to pin each timber to the one below on each side of every joint. Cut timbers with a sharp chain saw, and wear goggles to protect your eyes from flying chips.'''

4. Backfill where necessary. For drainage, drill weep holes every 1.2m along the wall's length. Drill a row of holes 300mm above the ground. You can provide drainage by leaving a 25mm gap between the timbers, and plant trailing flowers in the crevices. However, the soil often compacts like cement, so weep holes are essential.

Source: Outdoor Structures (Murdoch Books)


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