How do I fit a gravel board under a fence?

Almost every fence panel that has failed at the bottom failed because it was sitting in wet soil. A gravel board takes that contact instead, costs a fraction of a panel, and can be swapped in ten minutes when it eventually goes.

What you'll need

• Gravel boards — timber 150mm or 300mm, or concrete
• Gravel board clips or timber cleats
• Exterior screws
• A spirit level and a string line

Step by step

1. Decide between timber and concrete.Pressure-treated timber is cheap, easy to cut and lasts around ten years. Concrete lasts indefinitely and stops rabbits and dogs digging under, but it is heavy and only fits between concrete or well-set timber posts.
2. Set the height with a string line.Run a line between posts at the top of where the board will sit. The board should finish 25-50mm into the soil, with its top level so the panel above sits square.
3. Clear a shallow trench along the line.Take out the turf and enough soil for the board to sit in without being forced. Forcing a board bows it and lifts the panel above.
4. Fix timber boards with clips or cleats.Gravel board clips screw to the post and the board drops in. Alternatively screw a short timber cleat to each post and screw the board to that. Either way the board must be removable later.
5. Drop concrete boards into slotted posts.Concrete posts have a slot; the board slides down from the top before the panel goes in. They are two-person lifts — a 1.83m concrete board is around 40kg.
6. Check the board is level before the panel goes on.Everything above inherits this line. Ten minutes with a level now saves a fence that visibly steps along its length.
7. Sit the panel on the board, not on the soil.The panel base rests on the top edge of the gravel board with a small gap to the ground behind. Refix the panel clips to the posts as normal.
8. Treat any cut ends.Cutting a treated board exposes untreated timber. Brush end-grain preservative on every cut face before fitting.

Tips

• Fit gravel boards to the whole run at once even if only one panel has rotted — the rest are on the same clock.
• A 300mm timber board is worth the extra over a 150mm one where the ground slopes or the border soil is banked up.
• Leave a small air gap behind the board rather than backfilling tight against it, so it dries between wettings.

Common mistakes

• Burying the gravel board deep in the soil — It then rots in a couple of seasons instead of ten years. It only needs to be in the ground far enough to close the gap, not buried.
• Screwing the board directly through the panel — It becomes impossible to replace without dismantling the panel, which is the entire point of a sacrificial board.

If it doesn't work

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