How do I add trellis to the top of a fence?

Raising a fence with trellis is the cheapest privacy improvement in most gardens, and the most common way to pull a fence over in a storm. The difference is entirely in how the extra height is fixed and how much wind it catches.

Step by step

1. Check the height you are allowed first.In England and Wales, fences and walls next to a highway are generally limited to 1m, and elsewhere to 2m, without planning permission. Trellis counts towards the height. A deed or estate covenant may set a lower limit regardless.
2. Check the posts can take it.Rock each post. If any move now, adding half a metre of sail area at the top will finish them. Fix the posts before adding height.
3. Fix batten uprights to the post faces.A 50mm x 50mm treated batten screwed to the side of each post, projecting above by the trellis height. This puts the load into the post rather than the panel and lets you replace the trellis without touching the fence.
4. Check the battens are plumb and level across the run.Trellis makes any inconsistency along the top of a fence very obvious. Set a string line across the batten tops before fixing the trellis.
5. Choose open trellis, not close-woven screening.A 50mm-100mm square lattice passes wind. A close-woven willow or reed screen fixed to trellis is effectively a solid panel and doubles the load on the posts.
6. Screw the trellis to the battens, not the panel.Exterior screws with washers through the trellis frame into the batten, at least three per side. Never rely on nails into the top rail of a panel.
7. Cap the join if you want it to look deliberate.A thin timber capping across the joint between panel and trellis hides the batten fixings and sheds water off the panel top rail.
8. Treat everything before plants go on it.Once a climber is established you will never treat the trellis again. Do it now, including the batten backs.

Tips

• Fit the trellis before planting the climber, obviously — but also plan how you will get to it in five years when it needs treating. Hinged trellis panels exist for exactly this.
• A 300mm trellis strip adds useful screening without much wind load. A 600mm strip roughly doubles the extra load on the post.
• If neighbours are affected by the extra height, a conversation before the job is much cheaper than one after it.

Common mistakes

• Nailing trellis to the top rail of a fence panel — The top rail is a thin section carrying no structural load. The first gale rips the trellis off and usually takes the top of the panel with it.
• Adding a solid screen instead of open trellis — It behaves like extending the fence, catching all the wind at the point of maximum leverage on the post.

If it doesn't work

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