Composite Fencing Installation Guide: Everything You Need to Know

Composite fencing has grown rapidly in popularity across the UK - and it's not hard to see why. It looks excellent, requires virtually no ongoing maintenance, lasts far longer than standard timber panels, and holds its colour and structural integrity through years of British weather without the rotting, warping, and splitting that plague timber fencing.

But composite fencing only delivers these benefits if it's installed correctly. A poorly installed composite fence - with inadequately set posts, wrongly spaced panels, or incorrect drainage provision - will move, crack, and fail long before its time.

This guide walks you through the complete installation process, from setting posts correctly through to attaching panels and finishing the fence line professionally. Whether you're a confident DIYer or a trade installer working with composite fencing for the first time, this is your reference guide.

At Paving and Slabs Ltd, we supply composite fencing alongside our broader range of composite decking, paving slabs, and garden products. We know what correct installation looks like - and what cutting corners costs.

Before You Start: Planning and Preparation

Check Boundaries and Permissions

Before installing any fence, confirm:

• Property boundary positions - install on your boundary, not over it
• Permitted development rights - in most UK gardens, fencing up to 2 metres high does not require planning permission (1 metre adjacent to a public highway). Check with your local authority if in doubt
• Party wall considerations - if a fence is on a shared boundary, your neighbour should be informed

Measuring and Calculating Materials

1. Measure the total fence run length accurately - measure from post centre to post centre for each bay
2. Standard bay widths for composite fencing are typically 1.8m - confirm your product specifications
3. Post quantity = (number of bays) + 1; add extra for corners and end posts
4. Panel quantity = number of bays
5. Post length = desired fence height + 600mm minimum below ground (750mm for fences over 1.5m)

Allow approximately 10% extra on all materials for cuts and wastage.

Mark Out Post Positions

Mark each post position along the fence line using string line and timber pegs. This ensures a straight fence line before a single hole is dug. Check that post positions avoid underground services - use a cable avoidance tool (CAT scanner) before digging.

Stage 1: Setting Posts

The posts are the most critical component of any fence installation. An inadequately set post will allow the entire fence section to move - and with composite panels, movement puts stress on the panel fixing channels and can cause cracking.

Post Material Options

Most composite fencing systems use either:

• Composite posts (matching the panel material)
• Galvanised steel posts (often sleeved or channel-profiled to receive the composite panels)
• Concrete posts with slot or groove profiles

All are valid; the right choice depends on the specific composite fencing system you're using. Always follow the manufacturer's specification - mixing post types not designed for each other will cause fitting problems.

Digging Post Holes

• Depth: Minimum 600mm for standard fence heights; 750mm+ for fences 1.5m+ high; 900mm for exposed positions or loose/sandy soil
• Diameter: The hole should be 3× the post diameter to allow adequate concrete surround - typically 200–250mm diameter
• Tool: Use a post hole borer (available from tool hire) rather than a spade for clean, correctly dimensioned holes
• Check for underground services before digging - gas, electric, water, and drainage pipes and cables can all be present in garden areas

Setting Posts in Concrete

Materials needed:

• Post mix concrete (pre-mixed bags - add water)
• Temporary struts or prop supports
• Spirit level (long - minimum 600mm)
• String line

Step-by-step:

1. Place the first post in its hole and use temporary struts to hold it upright
2. Set it to the correct height (all posts should be at the same height - use the string line for reference)
3. Check for plumb (vertical) on two adjacent faces using the spirit level
4. Pour post mix concrete into the hole around the post, filling to approximately 50mm below ground level
5. Add water as directed and tamp to remove air pockets
6. Recheck plumb immediately after tamping - concrete movement can shift posts
7. Leave to cure for a minimum of 24 hours before applying any load or fitting panels
8. Repeat for all posts - work from one end to the other, using the string line to maintain alignment

Important: Do not rush the curing process. Fitting panels to posts within the concrete cure period is the single most common cause of fence installation failure.

Post Caps

Composite post caps prevent water ingress into the top of hollow posts and improve the finished appearance. Fit post caps after panel installation is complete.

Stage 2: Fitting the Panels

Understanding How Composite Fencing Panels Fit

Most composite fencing systems use one of two panel fitting approaches:

Channel/slot system: The post has a channel or groove running vertically on each face. Horizontal composite boards (or composite infill boards) slot into these channels from above, stacking up to fill the panel bay. Additional boards are added one at a time until the desired height is reached.

Frame and panel system: A sub-frame fits between posts and the composite panel faces fix to this frame using hidden or visible fixings.

Always review the manufacturer's installation instructions for your specific system before proceeding - the details vary between products.

For Channel/Slot Systems:

1. Begin at the bottom of the panel bay - if your system uses a gravel board at the base, fit this first
2. Slot the first horizontal board into the post channels on each side
3. Continue adding boards, stacking from bottom to top
4. Check level periodically as you build up - even small misalignments accumulate over the height of the panel
5. The top board may need to be cut to height to achieve the correct finished fence height - use a fine-toothed saw or composite-rated circular saw blade

For Frame and Panel Systems:

1. Fit the sub-frame between posts according to the manufacturer's specification
2. Ensure the frame is level and square before attaching panels
3. Attach composite panels using the specified fixings - typically stainless steel screws or hidden clip systems
4. Check alignment after each panel

Cutting Composite Fencing Boards

Composite fencing can be cut with:

• Fine-toothed handsaw - works well for occasional cuts
• Circular saw with a composite/fine-tooth blade - faster and cleaner for multiple cuts
• Mitre saw - ideal for accurate angled cuts at corners or line ends

Always wear eye protection and a dust mask when cutting. Composite cutting dust is a respiratory irritant. Cut boards slightly long, then trim to final dimension for a clean fit.

For advice on cutting composite materials generally, see our companion article on 10 tips on installing composite decking - many of the same cutting and fitting principles apply.

Stage 3: Corners, Ends, and Gate Posts

Corner Posts

Corners require a post that can accept panels from two directions simultaneously. Most composite fencing systems include a specific corner post profile for this purpose. If you're adapting a system without a dedicated corner post, consult the manufacturer before attempting a workaround.

End Posts and Wall Fixings

Where a fence run terminates against a wall or building:

1. Fix a starter post to the wall using appropriate wall fixings (masonry anchors for brick/block walls; coach bolts for timber)
2. Ensure the post is plumb and secure before fitting panels

Gate Posts

Gate posts require significantly more secure fixing than standard fence posts because they carry the dynamic load of the gate - both its weight and the swinging motion.

• Increase post hole depth to minimum 900mm for gate posts
• Use concrete, not post mix - slow-curing concrete develops higher strength
• Allow concrete to cure for minimum 48 hours before hanging the gate
• Check that your composite fencing system is compatible with standard gate hinges or uses proprietary gate hardware

Stage 4: Gravel Boards and Base Details

A gravel board at the base of the fence protects the bottom of the composite panels from ground contact, standing water, and soil splash. Even though composite panels resist moisture better than timber, direct soil contact can still cause surface staining and may compromise some fixing systems.

Standard composite gravel boards sit in the same post channels as the fence boards and sit on compacted gravel or a concrete footing. Ensure a minimum 25mm gap between the bottom of any composite panel and the soil surface.

This principle of keeping composite materials clear of the ground is consistent across our product range - see our how to lay decking on soil guide for the parallel approach applied to composite decking installations.

Stage 5: Finishing and Maintenance

Final Checks

• Check all posts are plumb and secure
• Check all panels are level and consistently spaced
• Ensure all panel ends are neatly trimmed and no raw cuts are exposed
• Fit post caps to all posts
• Apply any manufacturer-specified end caps to exposed board ends

Maintenance

One of composite fencing's greatest advantages is minimal ongoing maintenance. Standard care involves:

• Annual wash-down with warm soapy water or a composite cleaner to remove algae and grime
• Inspect post fixings annually - in particular, check that ground-level fixing points have not degraded
• No painting, staining, or oiling required

For composite cleaning guidance including the use of pressure washers, our jet washing composite decking guide covers the principles that apply equally to fencing boards.

Common Installation Mistakes to Avoid

1. Insufficient post depth - the most common cause of fence failure, particularly in the first winter
2. Fitting panels before concrete has cured - creates stress on posts and panel channels
3. Not maintaining consistent post spacing - variable bay widths look unprofessional and may leave boards too loose or too tight
4. Cutting boards too short - always check the fit before final cutting
5. Ignoring drainage at post bases - waterlogged post holes accelerate post degradation even with concrete surrounds

Summary

Composite fencing installation is well within the capability of a confident DIYer if the process is followed correctly. The critical stages - correct post setting depth, adequate concrete cure time, and accurate board fitting - determine whether the fence stands for decades or fails within years.

Browse our full composite fencing range at Paving and Slabs Ltd for a complete selection of panels, posts, and accessories. We supply composite fencing to homeowners and trade customers across the UK with nationwide delivery.

For personalised advice on your fencing project - including post quantities, panel calculations, or product compatibility questions - contact our expert team. We're here to help you get it right first time.