Slatted Fence Topper
Add 300–600mm of slatted height to an existing fence — extra privacy without replacing the whole run. The easiest way to gain garden seclusion in a weekend.
What you'll need
Materials
- Battens, post extension brackets or new post sections, top rail, stainless screws, optional trellis or louvres instead of slats.
Tools
- Drill/driver, spirit level, tape measure, mitre or hand saw, clamps, stainless brackets or coach screws.
Material complexity: Low
Allow one spare batten per panel for any with a bad knot at a fixing point.
Main risk: Existing posts not strong enough for the extra height and wind — the whole fence fails sooner than it would have alone.
Cut list
One typical 1.8m panel × 600mm topper. Repeat per panel along the run.
- Post extensions ×275×75mm (or to match the existing post) × 900mm — 600mm above, 300mm overlap
- Rails ×247×50mm × 1800mm (top and bottom of the topper)
- Battens ×30–32: 38×19mm × 560mm at about 20mm gaps (38+20 = 58mm pitch across 1800mm). Buy 3.6m lengths and crosscut
- Cap ×170×20mm × 1800mm, sloped or chamfered
- Brackets: two post-extension brackets, or face-lap the new post to the old
- Through-bolts: M8 or M10, two per post — not a single bolt. Length = both posts plus washers (typically 180–200mm for two 75mm posts, 220–250mm for 100mm posts)
- Stainless 4.5×40–50mm screws, one per batten per rail
Step-by-step
Check the old posts before you buy timber
Push each post at the top. If it moves at ground level, or the timber is soft when you prod it with a screwdriver, replace that post first. A topper on a rotten post pulls the whole run over.
Watch out: If the finished height will go over 2m, check planning before you cut anything.
Bolt on the post extensions
Cut 900mm extensions (600mm showing, 300mm overlap). Clamp each one plumb to the existing post. Drill two holes and fit M8 or M10 through-bolts with washers both sides. Two bolts per post — a single bolt becomes a hinge in the wind.
Tip: Mount the extensions before you take off an old top rail, so you have something to clamp against.
Fit the two rails
Screw a 47×50mm rail at the top of the extensions and a second rail about 80mm above the old fence line. Check both are level across the 1800mm bay.
Fix the battens with a spacer
Cut 38×19mm battens to about 560mm. Cut a 20mm spacer block and work from one end. One stainless screw per batten per rail, through the centre line. Adjust the last few gaps by a millimetre so you finish on a full batten.
Tip: Pre-drill hardwood battens — they split if you drive screws straight in.
Cap the top and re-check the bolts
Fit a sloped or chamfered cap so the top rail does not soak. Recheck every through-bolt after the first windy week. Oil if you want to keep the colour.
Tip: Check the bracket-to-post fixings each spring — wind racks them most.
Tips & traps
- Topping a fence with rotten posts — the topper accelerates failure.
- Single-bolt brackets — wind racks them loose.
- Closing slat gaps tight for privacy — wind pressure increases a lot.
- Not checking planning when the finished height goes over 2m.
Planning & timber detail
Why build this?
A fence topper is the highest-impact garden privacy upgrade you can do in a weekend. It costs a fraction of a new fence, looks intentional, and immediately fixes overlooking from a neighbour's new extension or upper windows.
Where it works best
On structurally sound existing fences with posts in reasonable condition. Do not try to top a fence with rotten posts — replace those first.
Planning notes
Anything over 2m total height typically needs planning permission. Check neighbour boundary ownership before bolting to a fence — a lot of UK boundary disputes start with toppers.
Typical sizes
Default: one 1800mm panel, 600mm of extra height.
A 600mm topper on a 1.8m fence brings the total to 2.4m. Most rear-garden rules allow up to 2m without permission — check before you build over that.
Post extensions: 600mm showing, 300mm lapped onto the existing post.
Default dimensions
Default: 1800mm wide × 600mm high topper on an existing 1.8m fence.
Post extension: 900mm long (600mm showing, 300mm overlap).
Batten gaps: 15–25mm.
Design variants
Vertical slats (this page) — 38×19mm at 15–25mm gaps. The usual privacy add-on.
300mm topper — same method, shorter battens. Easier to stay under a 2m planning height on a 1.7m fence.
Trellis or louvres — swap the battens for a bought trellis panel if you want climbers rather than a tight screen.
Suitable timber options
Cedar is the no-maintenance choice if you want to leave it silver. Treated redwood matches most existing UK fences. Thermowood gives the cleanest, most dimensionally stable finish.
Fixing and finishing
Bolt post extensions to existing posts with M8 or M10 through-bolts, two per post, or heavy coach screws — not nails, and not a single-bolt bracket. Pre-drill hardwood battens. One stainless screw per batten per rail, through the centre line.
Mount post extensions BEFORE removing the existing top rail (if there is one) — gives you a stable point to clamp against. Cap the new top rail to shed water.
Maintenance
Check the bracket-to-post fixings each spring — wind racks them most. Re-coat any oil finish every 2–4 years.
If the existing fence reaches end-of-life in 5 years, the topper can usually be transferred to the replacement.
Timber behaviour
Durability
Use UC3 minimum — UC3 means timber kept off the soil (above ground). The topper itself is above ground. The weak point is the joint to the old post: wind loading at the top of a fence is significant. If an old post is rotten at ground level, replace it with UC4 timber (sits in soil or stays wet) or a shoe — do not hang a topper on it.
Movement
Slats shrink across width — single fixing per rail, leave gaps minimum 12mm so they don't close up.
Go deeper
Get the Timber Buying Companion
An 8-page practical guide to choosing better boards, avoiding waste and spotting common timber problems before you buy.