How to lay composite decking

Video by Wickes. This guide draws on "How to Install a Composite Deck" from the official Wickes UK channel. It covers the sub-frame, clip fixing, and fascia boards from start to finish. The section on joist spacing for composite boards specifically is worth watching before you price up timber — composite decking needs more support than you might expect.

1. Plan the layout and prepare the ground

Mark out the deck area and decide on the direction of the boards. Boards running away from the house look natural; running parallel to the house can make a space feel wider. Either works — just decide before you build the frame.

Clear the ground underneath, lay weed membrane, and add a layer of gravel to help drainage. Composite decking needs airflow underneath to prevent moisture building up beneath the boards.

2. Build the sub-frame from treated timber

The sub-frame is usually built from 47 × 100 mm (or similar) pressure-treated joists. Fix an outer frame first using post anchors or concrete pads, then fit the internal joists at the spacing recommended for your specific decking boards.

Composite boards are heavier than timber and have less natural flex. Most manufacturers specify joist centres of 400 mm or less. Check your product's installation guide — using the wrong spacing can cause the boards to sag or crack over time.

3. Check the frame is level

A composite deck needs a slight fall away from the house (around 1:80) to shed rainwater. Build the fall into the frame rather than trying to add it later. Use a spirit level and packers to get every joist at the right height before screwing anything off.

Mind you, on larger decks this is where most problems start. Take your time getting the frame level — it is much easier to correct it now than after the boards are on.

4. Fit the starter board first

The starter board runs along the edge nearest the house and is face-screwed directly to the joist. Pre-drill to avoid splitting. This board sets the direction for the whole deck, so get it perfectly parallel to the house wall before fixing.

Leave a 10 mm gap between the starter board and any wall or structure. Composite material expands with heat and needs room to move.

5. Fit the boards using hidden clips

Slide a clip over the first board at each joist position and screw it down. The next board slots into the clip and is held firmly without any visible fixings on the face of the decking. Repeat along the run.

Leave a 5–6 mm gap between boards for drainage and expansion. Most clip systems space this automatically — do not be tempted to push boards tight together. Water needs somewhere to go and composite boards do expand in the sun.

6. Cut boards to finish the far edge

Composite boards cut cleanly with a standard circular saw or mitre saw, though they are noticeably denser than timber so the blade works harder. Wear a dust mask — the fibres are not pleasant to breathe.

Snap a chalk line across the boards before cutting to get a straight edge. Finish with a router or router plane if you want a clean chamfered edge rather than a sawn one.

7. Fit the fascia boards around the perimeter

Fascia boards clip or screw onto the exposed joist faces at the edges of the deck to hide the timber frame and give a clean, finished look. Mitre the outside corners at 45 degrees for the neatest result.

Some composite systems include matching fascia boards in the same colour as the decking. Others use a contrasting trim. Either way, the fascia goes on last and makes the difference between a deck that looks finished and one that looks like a work in progress.

When to call a handyman

Composite decking is a rewarding project but it involves a fair amount of frame building and some precise cutting. Call Richard if the ground is uneven and needs levelling, if you need the deck to be raised or stepped, or if you want it done without taking up your entire weekend.