9 Step Composite Decking Install with Trex Transcend Over a 18-Square-Metre Frame

Trex Transcend boards expand and contract along their length. Across a 3.66 m board, the difference between a frosty morning and a 30 C afternoon can amount to several millimetres, so frame spacing and end gaps need to be settled before cutting starts.

The brief is a flat 18 m2 rectangle, roughly 6 m by 3 m, built over a treated softwood frame at ground-floor level. Worldwide product specification for Transcend gives 300 mm joist centres for a standard residential layout with boards running perpendicular to the joists. If the boards run at 45 degrees, the spacing tightens to 200 mm.

Step 1: Set the frame at 300 mm centres

A frame prepared for timber decking often has joists at 400 mm or 600 mm centres. Trex Transcend requires closer support: 300 mm on centre for boards laid at 90 degrees to the joists, and 200 mm for a diagonal pattern. On an 18 m2 rectangle with boards running across the 3 m width, that spacing gives about 20 joists across the 6 m length.

Use C16 or C24 treated softwood, 47 mm by 150 mm, set on edge. The outer frame forms the perimeter band. Internal joists can hang inside it on galvanised joist hangers or sit on noggins. Check the frame with a 2 m spirit level in both directions. A fall of 1:80 toward the garden, about 12 mm per metre over the 3 m run, sends surface water away. Composite does not absorb water the way timber does, so standing puddles remain until evaporation unless the fall moves them off the boards.

Double any joist that carries a board joint. Two Transcend board ends meeting mid-span need a 90 mm bearing surface, which a single 47 mm joist cannot provide. A pair of joists bolted together, or a single 90 mm timber, provides the required landing.

Step 2: Lay a perimeter starter clip

The first board along the outer edge fixes with a starter clip or a stainless screw through the face into the joist. Snap a chalk line first, then set the board parallel to the house wall within 3 mm over the 6 m length, because that first edge controls every row after it.

Step 3: Drive the hidden fasteners and hold the gap

Trex sells the Hideaway hidden fastener system for grooved Transcend boards. Each clip seats in the groove on the board edge, and its screw drives down into the joist. The clip body sets the side-to-side gap at around 5 mm, giving the boards space for thermal movement and letting water drop through.

Use the stainless or coated screws supplied with the clips. A standard zinc screw corrodes against the aluminium-bearing clip and the wet timber within a couple of seasons. Once the head has rusted, it strips before it backs out. A cordless impact driver on a low torque clutch seats the screw without crushing the clip. Over-driving splits the clip and loses the set gap.

The Transcend installation specification gives a butt-joint gap that changes with installation temperature. A board fitted at 5 C will lengthen by August, so the cold-weather gap has to be wider than a warm-weather gap. As a working figure, fit about 5 mm to 6 mm end-to-end at moderate temperatures, go narrower above 25 C, and go wider below 5 C. The boards come with the gapping guidance printed on the wrap.

Mark each cut end so the factory-sealed end faces the visible perimeter where possible. That small sorting step keeps the most uniform ends in view after the field boards are installed.

The hidden-fastener gap also serves the drainage path. Water runs between boards, down the joist faces and out under the perimeter. If debris blocks that route, the timber frame stays wet. A frame that never dries can rot even while the composite surface above it remains flat and clean.

Step 4: Cut with a fine-tooth blade

A 40-tooth or higher carbide blade on a mitre saw gives a clean edge on Transcend. A coarse rip blade tears the capped surface and leaves a fuzzy edge. Cut outdoors. Composite dust is fine and clings, and a dust mask keeps it out of the lungs during a full day of cutting.

Step 5: Ventilate the underside

Ground-level decks often fail at the airflow detail under the frame. Transcend boards cap the top and sides, but the joists are timber, and timber needs air movement to dry after rain and condensation.

Allow at least 25 mm of clear air below the lowest joist, with cross-flow from at least two open sides. On a fully enclosed 18 m2 deck with no airflow, humidity remains high underneath. The visible surface can look perfect while the frame degrades from below.

Where the deck sits on soil instead of a slab, lay a permeable membrane and a 50 mm bed of 20 mm angular gravel under the frame. The gravel keeps the lowest joist out of direct ground contact and gives water somewhere to drain. Leave the perimeter fascia with a vented gap or use a proprietary vented trim instead of sealing the edge solid to the ground.

Step 6: Stagger the joints

Butt joints should not line up across adjacent boards. Stagger them so no two joints sit on the same joist line in neighbouring rows.

On a 6 m run with 3.66 m boards, each row needs one full board and one cut piece. Alternate which end receives the cut piece. The joints then spread across the deck surface instead of forming a single visible seam line down the middle.

Step 7: Fit the fascia and picture-frame the edge

A fascia board closes the exposed joist ends and the board ends along the perimeter. Many installers picture-frame the deck first, using a perimeter board mitred at the corners and fixed through the face with colour-matched Trex screws or plugs. The picture frame hides every cut field-board end, leaving only factory edges visible from above.

Fix the fascia to the rim joist and leave a small gap at the top so water is not trapped against the deck board edge. Pre-drill fascia fixings. Transcend fascia is thinner than the deck board and can split if a screw is driven in cold weather without a pilot hole.

Step 8: Check the deflection underfoot

Walk the finished field before fitting the last fascia run. A board that flexes underfoot usually points to a joist centre that opened past 300 mm or a hanger sitting low. Lift the board, correct the joist, then refit it. If the fault is found after the fascia is plugged, the edge has to come apart before the low support can be corrected.

Step 9: Clear the gaps before winter

Leaves and grit pack into the 5 mm board gaps and block the drainage path the structure depends on. A stiff brush down the line of the gaps once in autumn keeps the channels open. A pressure washer on a wide fan at low pressure can clear compacted debris if the jet stays moving so it does not score the cap.

A worked board count for 18 m2

Trex Transcend boards measure roughly 140 mm wide. With a 5 mm gap, each board covers about 145 mm of width. Across a 3 m board run, 3000 divided by 145 gives around 21 board widths.

Each width needs a 6 m length. With 3.66 m boards, that means two pieces per width, so the board order lands near 42 boards of 3.66 m before fascia is added. Add 10 percent for cuts and waste.

Order the clips by the box. One Hideaway box covers a stated square-metre area, and an 18 m2 deck with 300 mm joists needs the clip count that matches roughly 60 linear metres of joist contact.

The number that decides whether an old timber deck can stay is the joist centre. An inherited frame reading 400 mm has to be re-joisted between the old members or replaced, and the cost comparison sits outside the board order. How much of the old frame is still worth keeping once the spacing has been measured?