A rough DIY materials and labour estimate for a straightforward timber post-and-board retaining wall. Enter your wall length, retained height, and ground conditions to start — then tap any post in the diagram to set its own ground level and top level by hand; posts in between rake (slope evenly) between whichever posts you've set.
Rule-of-thumb bands this calculator looks up by soil type and retained height — not a citation of a specific code table. Embedment is shown here at each band's tallest wall (its conservative design case); your actual figure scales with the exact height you entered. Your current selection is highlighted.
| Soil | Up to | Post size | Max spacing | Embedment* |
|---|---|---|---|---|
| Firm clay | 0.6 m | 100 x 100mm | 1.50 m | 300 mm* |
| Firm clay | 1.0 m | 100 x 100mm | 1.35 m | 500 mm* |
| Firm clay | 1.5 m | 150 x 150mm | 1.20 m | 825 mm* |
| Loose or sandy soil | 0.6 m | 100 x 100mm | 1.30 m | 330 mm* |
| Loose or sandy soil | 1.0 m | 100 x 100mm | 1.15 m | 600 mm* |
| Loose or sandy soil | 1.5 m | 150 x 150mm | 1.00 m | 975 mm* |
| Made-up or fill ground | 0.6 m | 100 x 100mm | 1.20 m | 360 mm* |
| Made-up or fill ground | 1.0 m | 150 x 150mm | 1.05 m | 680 mm* |
| Made-up or fill ground | 1.5 m | 150 x 150mm | 1.00 m | 1125 mm* |
* plus a ~100mm gravel base pad under the post for drainage and bearing — not counted as structural embedment.
Tap a post to set its ground level and top level by hand.
Embeds deeper than a simple fence post — it has to resist the retained soil's push over its whole buried length, not just wind load.
Assumes simple butt joints at whole-board lengths, and each bay's own local height — not an optimised cutting plan.
Compacted free-draining gravel behind the facing boards, roughly 150mm thick.
Rough guide for one experienced DIYer working alone — add time for a helper-dependent job, hard ground, or bad weather.
Rule-of-thumb estimate for straightforward ground and access — not an engineered design. Ground conditions vary a lot in practice; if in doubt, especially near a boundary, driveway, or structure, get a professional opinion before you dig.