Timber Recommendations
- Type of Timber: Use H4 or H5 treated pine or hardwood timber to withstand moisture and soil contact.
- Timber Size: For walls under 1.2 meters, 100mm x 50mm or 150mm x 50mm timber boards are ideal. For posts, use 150mm x 150mm or larger. If you wish to use round posts, and ensure if your posts have a minimum Small End Diameter (SED) of at least 150mm – 200mm.
Wall Height and Spacing Requirements
- Height Limit: Retaining walls over 1.5 meters typically require a permit in New Zealand, so keep DIY walls below this height unless approved by your council.
- Post Spacing: Space posts approximately 1 to 1.5 meters apart, depending on wall height and soil type. Closer spacing may be needed for higher walls or loose soil.
- Embedment Depth: Each post should be embedded to a depth of about one-third to one half of the wall’s total height for stability.
Step-by-Step Construction
- Mark the Area: Mark out the wall’s location using stakes and string lines for accuracy.
- Dig Post Holes: Excavate holes for each post based on your planned spacing. Ensure the holes are deep enough to support the wall height (e.g., 500mm – 700mm deep for a 1.5m wall).
- Set the Posts: Place each post in its hole, ensure it’s level, and fill with concrete to secure. Let the concrete set before continuing.
- Attach Timber Boards: Secure horizontal timber boards to the posts, starting from the bottom. Use galvanized nails or bolts for durability.
- Backfill and Add Drainage: Place gravel or drainage metal behind the wall to prevent water buildup, which could weaken the wall.
Legal Considerations
- Check with your local council on height limits and permits, especially for walls over 1.5 meters.
- Additional regulations may apply for walls retaining large amounts of soil or located near property boundaries.
Conclusion
- Regularly inspect and maintain the wall to ensure ongoing stability and safety.