How to install composite decking
Every figure here comes from our own manufacturer guide, so the joist centres, expansion gaps and screw sizes are the ones your boards are made for. Read it through before you order your subframe.
Key facts. Joists at 350mm centres with 40mm of air under the frame. Boards fix with hidden T-clips and the steel screws supplied, never through the face. Expansion gaps of 9mm below 4°C, 6mm between 4°C and 24°C, 3mm at 25°C or above, and 20mm from any wall. Eleven steps; a 3.6m by 3m deck on a patio is one day for two people, on soil two weekends. Boards are £25 for 3.6m and £34 for 4.8m in every colour.
Step 1
Store the boards flat
If the boards are waiting to be laid, stack them flat on battens at no more than 400mm centres. That stops them bowing, keeps them out of standing water and avoids scratching. Cover them if they are stored outside, with something non-translucent.
Step 2
Plan the deck before you cut anything
The size, shape and orientation decide where your joists run, so settle them first. Pay attention to boundaries and to any door that opens onto the deck, as both limit your layout.
Step 3
Build the subframe
Never lay these boards without a subframe. Timber, metal or composite all work. Joist centres must be no more than 350mm, and you need to leave a 40mm space beneath the subframe so air can move under the deck.
Step 4
Set your expansion gaps
Composite expands and contracts with temperature, so the gap depends on the day you fit it. Below 4C leave 9mm end to end, between 4C and 24C leave 6mm, and at 25C or above leave 3mm. Keep at least 20mm from any fixed boundary such as a wall.
Step 5
Fix the starter clips
Work from the outside edge. Screw starter clips to the edge of the frame with the steel screws supplied, one every 350mm along the joist. Countersinking helps the screws sit flush. Clip your first board in.
Step 6
Clip and screw board by board
Lightly screw a T-clip in without tightening, place the next board against it, and lift the board edge slightly to slide it home. Check the board touches the clips along its whole length, then tighten fully. Repeat until you reach the last two boards.
Step 7
Finish the last two boards
Measure the last two boards with a 6mm gap between them and mark your joists, then fit steel finishing clips and clip the final board in. Slide the remaining T-clips down and screw them to the joists. If access will not allow it, screw the last board at an angle every 350mm instead and do not over-tighten, or the board will split.
Step 8
Picture frame border, if you want one
Where the border runs perpendicular to the main deck, it needs a double joist under every side. Fit starter clips on the inner joists to hold the inner board edge, slide the board in, then screw the outer edge. Keep the same 6mm to 10mm expansion gap between the mitred edges.
Step 9
Fit the edging and corner trim
Cut the trim to length, mitring the ends at 45 degrees around corners. Pre-drill 4mm countersunk pilot holes and fix with 50mm countersunk screws, finishing by hand so you do not overdrive them. Corner trims screw into the deck only, and need a 4mm gap where their ends meet.
Step 10
Building steps
Keep the rise no more than 190mm and the tread between 250mm and 450mm, consistent the whole way up, on a solid concrete or gravel landing pad. Stringer steps span from the subframe to the base. Box-framed steps stack boxes with joists at 250mm spacings, fixed to the subframe at every intersection.
Step 11
Allow for the colour to settle
Expect an initial fade back over the first three months as tannins in the wood react to the weather. After that the colour barely moves. Trims and boards can differ slightly during that period, particularly on the woodgrain face.
Frequently asked questions
How do you fix composite decking down?
With hidden clips, not screws through the face. Starter clips hold the first board at the edge of the frame, a T-clip sits in the groove every 350mm along each joist and is screwed to the joist, and finishing clips hold the last board. Face-screwing traps the board and is the cause of most warped composite decks; only angle-screw the final board if there is no access for the clip.
Can you lay composite decking on grass or soil?
Not directly. Strip the turf, lay weed membrane and gravel, and build a frame of recycled plastic joists on pads or posts with 40mm of air beneath it. A deck laid on the ground stays damp underneath, moves and fails first. Our guide to composite decking on grass or soil covers the frame in detail.
Can I lay composite decking on an existing patio or concrete?
Yes, and it is the easiest composite deck to build. Lay composite battens across the slabs at 350mm centres, pack them level, and clip the boards to them. No digging, no posts, and a 3.6m by 3m deck is a one-day job for two people.
How do you lay composite decking around a corner?
With a picture-frame border. Put a double joist under every side of the border, hold the inner edge with starter clips, mitre the corners at 45 degrees and keep a 6mm to 10mm gap between the mitred ends. Corner trims screw to the deck only, with a 4mm gap where their ends meet.
How do you join composite decking boards end to end?
Butt the ends over a double joist so each board end has its own support, leave the expansion gap for the day's temperature (9mm below 4°C, 6mm between 4°C and 24°C, 3mm above 25°C), stagger the joints across the deck, and finish exposed ends with end caps or a fascia board.
What joist spacing does composite decking need?
No more than 350mm between joist centres, 300mm if the boards run diagonally, with a double joist under every board end and under a picture-frame border. Wider spacing is why cheap composite decks bounce.
How long does it take to install composite decking?
A 3.6m by 3m deck on an existing patio takes one day for two people. The same deck on a lawn takes a DIYer two weekends because of the digging, levelling and frame. A raised deck with steps and a balustrade is about three weekends.
