Correct installation can make or break your decking project. A well-installed composite deck will look flawless for decades; a poorly installed one can buckle and fail within a single summer.
In this guide, we walk you through the entire installation process, sharing the exact “Tradesman Tips” we use to maximize the durability and performance of your new garden space.
Do not skip these critical preparation steps before breaking ground:
Before you begin, please ensure you have the following equipment:
Measuring: Tape measure, string line, spirit level (or laser level), chalk line.
Cutting: A mitre saw or circular saw fitted with an 40 tooth fine blade (uncapped) 80-tooth fine blade(capped), plus a jigsaw for awkward cuts. 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: Safety glasses, gloves and a dust mask.
Groundwork: Shovel, heavy-duty weed membrane, and MOT Type 1 Gravel (hardcore).
Setting a solid foundation is the most critical part of your build.
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.
If your existing concrete patio is structurally sound and crack-free, you have two excellent options:
These adjustable feet are fantastic for uneven concrete, allowing you to level the subframe perfectly without messing around with timber shims. (Note: Always use composite or plastic joists on pedestals, as timber expansion can knock them out of alignment).
Simply measure your area using pegs and string lines to determine the finished height.
Adjust the pedestals to account for the gradient, ensuring a flat walking surface.
Important: Check with the pedestal manufacturer—some advise against using Composite or Plastic Joists due to expansion and advise to use timber.
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 Tips
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 during expansion.
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 or walls!
Sloped gardens require extra planning.
Digging: 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: Place gravel or a brick at the bottom of the hole so the timber post doesn’t sit directly in wet soil.
Setting: Use a connection batten to hold the post straight, then pour concrete to set it in place. Ensure the posts are level and plumb and leave extra height for trimming later.
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.
Noggins (Blocking): Once your main joists are down, install solid timber “noggins” perpendicular between the joists every 1.2 metres. This locks the frame together and stops the joists from twisting or rolling as the timber naturally dries out over the years.
The “Double Joist” Rule: Wherever two decking boards meet end-to-end (a butt joint), or where you are installing a picture frame border, you must install Double Joists. You cannot share a single 50mm joist for two board ends—the hidden clips will not fit properly, and the ends of the boards will break under foot traffic.
Note: If you are using Bullnose edge boards or a Picture Frame border, these must be installed FIRST, not last!
Step-by-Step:
Starter Clips: Screw your starter clips into the joists along the starting edge of the deck.
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
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.
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.
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.
The Cutting Secret: When cutting composite, use an 80-tooth fine blade on your saw and cut slowly. A fast, coarse blade will create immense friction and literally melt the plastic polymers in the board, leaving a messy, burnt edge.
Finish your deck like a pro with these accessories:
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.
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.
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 and 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.
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 to moisture and humidity buildup. These conditions encourage fungal/mould growth which thrives in damp environments.
“I Can Share One Joist For A Join.” False. As mentioned above, where two boards meet end-to-end, they absolutely require a double joist. Each board end needs its own separate support and its own separate clip.
“I Don’t Need A Gradient.” False. Composite is highly water-resistant, but if water sits pooled on top of a perfectly flat deck, it will encourage algae growth and become a massive slip hazard. You must build in that 1.5% fall.
Installing a composite deck is an incredibly rewarding project that completely transforms how you use your garden. By following these professional tradesman rules—respecting expansion gaps, doubling your joists, and keeping your blades sharp—you guarantee that your deck will look just as flawless in ten years as it does on day one.
If you have your measurements ready or you are searching for the perfect colour to start your project, explore our premium range of decking or reach out to our team for expert advice on your specific build.