The Ultimate Guide to Installing Composite Decking

Before You Install Your Composite Deck: The Golden Rules

Do not skip these critical preparation steps before breaking ground:

• Planning Permission: If you plan to raise your deck higher than 30cm off the original ground level, or if it will cover more than 50% of your total garden area, you may need planning permission. Always check with your local council.
• The 72-Hour Rule (Acclimatisation): Your composite boards must arrive at least 72 hours prior to your build. Place them flat on a ground sheet in your garden and keep them covered. Do not store them inside a warm garage. They must acclimatise to the exact outdoor temperature they will be installed in to prevent sudden expansion.

The Essential Tools For Install

Before you begin, please ensure you have the following equipment:

• Measuring: Tape measure, string line, spirit level (or laser level), and a chalk line.
• Cutting: A mitre saw or circular saw fitted with a 40-tooth fine blade (for uncapped boards) or an 80-tooth fine blade (for capped boards). If you aren’t sure which generation of board you are working with, check our review of composite decking types. You will also need a jigsaw for awkward cuts around posts.
• Fixing: A cordless impact driver/drill, Type 17 composite screws, starter clips, hidden fastener clips, and a rubber mallet (crucial for tapping boards tightly into clips without crushing the plastic edges).
• Safety & Groundwork: Safety glasses, gloves, a dust mask, a shovel, heavy-duty weed membrane, and MOT Type 1 Gravel (hardcore).

Part 1: Ground Preparation & Subframes

Setting a solid foundation is the most critical part of your build.

Scenario A: Soft Ground (Grass/Soil)

• Clear & Dig: Dig out the topsoil to a depth of 100-150mm.
• Weed Control: Install a heavy-duty weed membrane to prevent growth piercing up through your deck.
• The Base: Lay a 50-75mm bed of MOT Type 1 Gravel (hardcore) and compact it thoroughly. This ensures a solid, non-sinking foundation with excellent drainage.
• Airflow & Gradient: You must raise your timber subframe off the ground (ideally 100mm+) to allow air to circulate and prevent rot. Always build a 1.5% gradient (approx. 1:80 fall) into the frame, sloping away from your house, so rainwater naturally runs off the boards.

Scenario B: Existing Concrete

If your existing concrete patio is structurally sound and crack-free, you have two excellent options:

• Option 1: Decking Pedestals: These adjustable feet are fantastic for uneven concrete, allowing you to level the subframe perfectly without messing around with timber shims. Simply measure your area using pegs and string lines to determine the finished height, then adjust the pedestals to account for the gradient. (Important: Check with the pedestal manufacturer—some advise against using Composite or Plastic Joists due to expansion and advise using timber).
• Option 2: Composite Joists (The “Forever” Frame): For low-level decks on concrete, we recommend 50mm x 50mm or 25mm x 50mm Composite Joists. Unlike wood, composite joists absorb almost no water owing to their plastic fibres. They are heavily rot-resistant, ensuring the frame will last as long as the decking.
• Installation Tip: You must anchor these joists to the concrete because composite expands with heat; pre-drill the anchor holes 2mm larger than the screw head to allow for movement. If joining joists end-to-end, leave a 20mm expansion gap. In our experience, failing to leave this gap can cause the expansion force to breach brick walls!

Scenario C: Installation on a Slope

Sloped gardens require extra planning.

• Planning Permission: Check your local regulations. Raising a deck significantly (often over 30cm) may require planning permission.
• The Post Method: The traditional and strongest method is to dig foundations for posts. Dig holes for your posts (approx. 1/3 of the total post height, minimum 750mm deep). Ensure the posts are no more than 1.5m apart.
• Drainage & Setting: Place gravel or a brick at the bottom of the hole so the timber post doesn’t sit directly in wet soil. Use a connection batten to hold the post straight, then pour concrete to set it in place. Ensure the posts are level and plumb, leaving extra height for trimming later.

Part 2: Building the Framework (Joists)

Most customers opt for C24 Treated Timber for maximum strength.

• The Tradesman “Joist Tape” Secret: Timber frames almost always rot from the top down because rainwater sits trapped between the composite board and the timber joist. Before you lay your decking, run a layer of waterproof DPC (Joist Tape) over the top of every timber joist. This £20 addition will literally add a decade to the lifespan of your subframe.
• The “Box Frame” Method: Build your outer perimeter frame first to ensure it is perfectly square, then install the inner joists using Galvanised Joist Hangers.
• Joist Spacing (Critical): For standard residential composite decking, your joists must be spaced at a strict 300mm centre-to-centre (reduce this to 200mm for heavy-duty commercial projects). Do not push this to 400mm to save money; the boards will bounce and eventually snap.

Part 3: Installing the Composite Boards

Note: If you are using Bullnose edge boards or a Picture Frame border, these must be installed FIRST, not last!

Step-by-Step:

1. Starter Clips: Screw your starter clips into the joists along the starting edge of the deck.
2. The First Board (The 25mm Rule): If you are starting your deck against a house or solid wall, you must leave a 25mm gap between the brickwork and the first board. This allows for crucial airflow and gives the board room to expand in the summer sun without buckling against the wall. Push the grooved edge of your first decking board firmly into the starter clips.
3. Hidden Fasteners: Insert a hidden fastener clip into the groove on the opposite side of the board at every single joist (every 300mm). Screw the clip down into the timber.
4. Repeat & Gap Measurement: Slide your next board into those clips (use your rubber mallet to gently tap them flush), and repeat the process. The tradesman rule for gaps: Your hidden fastener clips will automatically set the perfect 3mm-6mm side-to-side gap for you. However, where two boards meet end-to-end (a butt joint), you must manually leave a strict 6mm expansion gap to stop them from pushing into each other during a summer heatwave.
5. The Last Board: On the final board, you may need to face-screw it. If doing so, pre-drill at a 90-degree angle, leaving a 2mm expansion gap for the screw head.

Part 4: Finishing Touches

Finish your deck like a pro with these accessories:

Bullnose Boards (The Professional Edge) Bullnose boards have a rounded edge, perfect for steps or the perimeter.

• Planning: These must be installed FIRST, not last. They require a starter clip underneath them.
• Structure: They almost always require a Double Joist underneath for full support.
• Overhang: Ensure the overhang is no more than 15mm.

Picture Framing This involves running a border board around the perimeter. It looks fantastic but requires a Double Joist underneath to support the border board and the main deck ends.

Fascia & Trims

• L-Corner Trims: These are typically 40mm x 50mm and are used to cap the ends of decking boards or cover corners for a clean, finished edge, protecting against water entering the internal structure.
• Fascia: Solid, wide boards that help cover the substructure of a raised deck. It hides your joists from view.
• The Tradesman “Glue Trick”: Standard L-trims can work loose over years of summer expansion and winter contraction. To ensure they stay permanently secure, lightly sand the back of the trim and the edge of the decking to create a “rough key.” Then, apply a high-strength marine adhesive like CT1 or Sumogrip, alongside your mechanical coloured screws. It will never move.

Common Misconceptions (Avoid These Mistakes!)

Don’t fall into these traps. These are the most common reasons for decking failure:

• ❌ “I Can Just Screw Straight Through The Top.” False. Never face-screw directly through the top of a composite decking board (especially hollow boards) like you would with traditional timber. Driving a screw through the face breaks the board’s moisture seal, causes winter cracking, and instantly voids your manufacturer warranty. You must rely exclusively on the hidden fastener clips.
• ❌ “Joist Spacing Doesn’t Matter.” False. Cheap composite often requires tighter joist spacing. Always check the supplier’s guide. If in doubt, stick to 300mm.
• ❌ “I Don’t Need Expansion Gaps.” False. Never butt composite boards tightly together. The plastic fibres expand in the heat. Without a gap, the boards will violently warp, buckle, and push each other off the frame.
• ❌ “No Need for Airflow.” False. Even at ground level, ventilation is required under the deck to prevent the subframe from rotting due to moisture and humidity buildup. These conditions encourage fungal/mould growth which thrives in damp environments.