How to Install Composite Decking: Joist Spacing, Gaps and Fixings
Composite decking installation starts with four separate decisions: the board profile, maximum joist centres, expansion gaps and fixing system. Confirm them before building the frame. BOHAI's general guidance is 5–7 mm between boards, about 5 mm at walls, 3–5 mm at butt joints and double joists beneath every butt joint.
Composite Decking Spacing: The Quick Answer
Do not treat every measurement as the same kind of “spacing.” The values below perform different jobs.
| Detail | BOHAI general guidance | What must still be confirmed |
|---|---|---|
| Side gap between adjacent boards | 5–7 mm | The selected clip and exact board profile |
| Gap between a board and a wall or fixed edge | About 5 mm | Board length, profile and project conditions |
| End-to-end butt gap | 3–5 mm | Board length, profile and installation conditions |
| Support beneath a butt joint | Double joists | Joist material, fixing method and edge distances |
| Maximum joist centres | Profile-specific | Exact SKU, layout angle, use and written supplier data |
“Joist centres” means the distance from the centreline of one joist to the centreline of the next. It is not the clear space between them.
BOHAI's available reference drawings show that one illustrated dimension must not be treated as a range-wide rule. Confirm the current written maximum for the exact board on your quotation before setting out the substructure.
Start With the Exact Board Profile
Two boards sold as composite decking can have different thicknesses, core geometry, metre weight, edge grooves and fixing systems. Those differences can change the maximum support spacing and the number of clips and joists required.
Before installation, record:
- the full profile or SKU code;
- board width, thickness and stock length;
- hollow or solid construction;
- intended face and board direction;
- straight, diagonal, herringbone or picture-frame layout;
- residential, commercial or another stated use;
- the selected starter and intermediate clips;
- the current installation drawing or data sheet revision.
Use the BOHAI decking profiles and models page to shortlist a board, then ask for written confirmation of the installation values for the named profile. A category label such as “capped composite decking” is not enough to set joist centres.
Prepare a Drained, Stable Substructure
The deck boards are the visible layer, not the structural design. Foundations, beams, joists, pedestals and fixings must suit the site, local code, design loads and supporting surface. A local professional should confirm structural questions that sit outside the board manufacturer's scope.
Before laying boards:
- Confirm that the base and substructure are stable, level and in plane.
- Provide a drainage route beneath the deck so water cannot remain trapped.
- Keep the deck build-up clear of doors, vents, membranes and required inspection points.
- Check that every joist is securely restrained and offers enough fixing width at board ends.
- Set out additional joists for butt joints, borders, breaker boards and other changes in board direction.
Installation-stage reference photo. The visible joist layout illustrates the build-up only; it is not a universal spacing specification.
For a concrete slab, do not treat the slab as a substitute for the joist system. The slab condition, drainage direction, available build-up height and fixing method all need to be resolved first. See the dedicated guide to installing composite decking over concrete.
For roofs and podiums, coordinate the deck build-up with the waterproofing and load design. The separate pedestal decking for roof terraces guide covers that application.
Set Joist Centres for the Profile and Layout
Maximum joist spacing is a board-profile value, not an industry-wide number. Ask the supplier to identify the exact profile, the installation orientation and the basis for the stated maximum.
Five questions expose most weak spacing claims:
- Which exact model code does the value cover?
- Is the spacing measured on centre?
- Does it apply only when boards run perpendicular to the joists?
- Is it for residential use, commercial use, stairs or another loading condition?
- Which current drawing, test or engineering document states the value?
Straight layouts
When boards cross the joists at 90 degrees, each board takes the shortest route between supports. Use the maximum centres stated for the exact profile and do not exceed them to save joists.
Diagonal and patterned layouts
A board installed diagonally travels farther between supporting joists. Herringbone, diagonal fields and picture-frame borders can also need extra blocking or changes in joist direction. Do not reuse the straight-layout maximum without written confirmation.
Stairs, heavy use and concentrated loads
Stair treads, planters, equipment and other concentrated loads are not ordinary deck-field conditions. Provide the layout and load information for review. Use BOHAI's dedicated stair profile and its current written instructions rather than cutting a general deck board into an assumed stair detail.
Butt joints
Support every board end. BOHAI's general installation detail uses double joists at butt joints so each board end and its clips or fasteners have their own bearing and fixing zone. Do not force two board ends and both fixing sets onto an undersized single joist.
Allow the Correct Expansion Gaps
Composite boards need working space at their sides, ends and fixed boundaries. A neat joint at installation can become a problem if it leaves no room for movement or drainage.
Side-to-side gap: 5–7 mm
BOHAI's general side-gap guidance is 5–7 mm. The selected intermediate clip helps keep the gap consistent, but the installer should still check the result rather than assuming every clip and board combination creates the same opening.
The side gap also allows surface water and small debris to pass through. It does not replace the drainage design below the boards.
Wall and perimeter gap: about 5 mm
Leave approximately 5 mm where a co-extrusion board approaches a wall or other fixed edge under BOHAI's general installation detail. Do not wedge the final board tightly against masonry, posts, kerbs or another immovable element.
Long runs, unusual temperatures or project-specific constraints should be reviewed against the current instructions for the selected profile.
Butt gap: 3–5 mm
Where two board ends meet, BOHAI's general guidance is a 3–5 mm gap with double joists below. Keep the joint centred over the prepared support detail and fix each end independently.
If a project uses long continuous runs, repeated butt joints or a complex border, agree the joint layout before ordering. A breaker board can sometimes organise the visual joint line, but it also changes the framing and material quantities.
Install the Boards and Clips Step by Step
1. Confirm the layout and quantities
Mark the board direction, borders, steps, visible edges and every planned butt joint. Use the deck material calculator as a planning aid, then verify its inputs against the selected profile, current stock lengths and actual site dimensions.
2. Set out and check the joists
Install the joists at or below the confirmed maximum centres for the exact board. Add double joists at butt joints and extra support wherever the layout changes direction. Check the top surfaces with a straightedge or string line; composite boards will follow an uneven frame.
BOHAI system options include WPC joists at 40 × 30 × 3000 mm and aluminium joists at 40 × 25 × 2900 mm. The correct option depends on the project design and environment. View the aluminium joist as one available component, not as a universal structural prescription.
3. Fit the starter clips
Align the first row carefully because it controls the rest of the field. Secure the starter clips to the prepared edge support using the approved fastener for the joist material. Check the first board is straight before continuing.
4. Install intermediate clips
Seat the next board fully without hammering or deforming its edge groove. Insert the specified intermediate clips over each supporting joist, keep the board line straight and tighten consistently. Do not overtighten a clip until it crushes or distorts the groove.
5. Build butt joints over double joists
Place each board end over its own supported fixing zone. Maintain the 3–5 mm butt gap and do not bridge an unsupported board end between joists.
6. Maintain perimeter clearance
Check the approximate 5 mm clearance at fixed edges as the work progresses. It is easier to preserve a designed gap than to recut a completed field that has been installed tight to a wall.
7. Finish exposed edges
Where the design requires a trim, BOHAI's L-shaped trim is available at 50 × 50 × 3000 mm. The trim is a finish component; it must not block drainage or restrain the board ends that need movement space.
Adjust the Detail for Special Layouts
The general sequence does not make every application identical.
- Concrete slabs: check condition, drainage and build-up before selecting joists and anchors.
- Roof terraces: coordinate pedestals, joists, board mass, waterproofing access and local load design.
- Pool surrounds: define the correct slip-test method, drainage, pool-edge gap and fixing environment. See composite pool decking.
- Stairs: use the named stair profile and its current written support detail; do not transfer a flat-deck spacing value.
- Commercial areas: provide anticipated traffic, concentrated loads, cleaning regime and access requirements for review.
- Diagonal or herringbone fields: obtain a layout-specific support plan and allow for additional waste and blocking.
Final Quality-Control Checklist
Check the completed installation before handover.
| Check | Pass condition |
|---|---|
| Profile | Installed board matches the approved SKU and finish |
| Joist centres | Measured on centre and no greater than the written value for that profile and layout |
| Joist plane | Board surface is even, without unsupported high or low points |
| Side gaps | Consistent and within the specified 5–7 mm general range |
| Wall/perimeter gaps | Approximately 5 mm under the general BOHAI detail, unless the project instruction states otherwise |
| Butt joints | 3–5 mm gap, both ends supported, double-joist detail used |
| Clips and fasteners | Correct type for the board and joist; seated consistently without crushing the groove |
| Drainage | Water has a clear route through and away from the build-up |
| Borders and trims | Secure, aligned and not blocking required movement or drainage |
| Records | Profile code, installation drawing and any approved deviations retained with the project file |
Joist density, double-joist details, clips and trims all affect the bill of materials. Use the composite decking cost calculator to organise those cost lines rather than comparing board prices alone.
DIY Composite Deck Installation or Professional Installer?
A composite deck installation uses some framing logic familiar from a wood deck installation, but board profile, support spacing, movement gaps, clip geometry and butt-joint details must come from the selected composite system. This guide shows how to install composite decking without treating one profile's values as a universal rule.
Installing composite deck boards yourself
If you are learning how to install composite deck boards, confirm the substrate, drainage route, board direction and exact support centres before ordering. For a DIY deck install, mark every butt joint and edge detail on the layout, use the specified starter and intermediate clips, and keep the completed frame accessible for checks before boards hide it.
When to use composite decking installers
Experienced composite decking installers are appropriate when the project includes stairs, elevated framing, waterproofed roofs, complex drainage, diagonal patterns, heavy point loads or local structural requirements. Ask the installer to quote the named profile and fixing system rather than a generic composite allowance.
Compare installation cost separately
The cost to install composite decking and the price to install composite decking depend on site preparation, frame material, access, labour, pattern, edge work and local requirements. Compare composite deck installation costs with the composite decking cost calculator, then replace its planning inputs with local quotations. The installation guide owns the sequence and detailing; the calculator owns the budgeting intent.
What to Send BOHAI Before Ordering
For an installation pack and comparable quotation, send:
- project area and plan dimensions;
- intended application and expected traffic;
- substrate or support system;
- preferred board profile, colour and stock length;
- straight, diagonal, herringbone or picture-frame layout;
- stairs, borders and estimated butt-joint locations;
- destination port and required date;
- any project standards or test requirements.
BOHAI's decking MOQ is 70 m². Samples are free; the buyer pays the courier charge, which is credited against the first order. Review the physical sample and named profile before approving the installation data and final quantity.
FAQ
How do I install composite decking?
Start with the exact board profile and substrate, build a stable drained support system, set the verified joist centres, allow the specified gaps, fix each board with the compatible clips and check the completed deck before closing edges. The step-by-step sequence above provides the planning order; the current profile pack provides the final values.
What joist spacing should I use for composite decking?
Use the maximum on-centre spacing stated for the exact board profile, layout and application. Do not copy a dimension from a generic drawing or a different board. Obtain the current written data for the SKU on your quotation.
Can I use 16-inch joist centres for composite decking?
Not automatically. Sixteen inches is about 406 mm, which may exceed the maximum for the selected board. A value used for another brand, thickness or layout does not prove suitability. Check the exact product documentation before building the frame.
What gap should I leave between composite decking boards?
BOHAI's general side-to-side guidance is 5–7 mm. The specified clip helps maintain the gap, but the installer should verify the completed spacing and keep the drainage path below the deck clear.
What expansion gap should I leave at board ends and walls?
BOHAI's general installation detail shows 3–5 mm at end-to-end butt joints and about 5 mm at a wall or fixed edge. Board length, profile and project conditions can require written confirmation for the specific installation.
Do composite decking butt joints need double joists?
Yes under BOHAI's general installation detail. Double joists provide a separate bearing and fixing zone for each board end. Keep the 3–5 mm butt gap and fix both ends independently.
Can composite decking be installed directly on concrete?
Do not treat the concrete as the joist system. Check slab condition and drainage, then install the appropriate supported build-up. The available height, water route and anchoring method must be resolved before the boards are laid. If the base is soil or grass rather than a slab, use the ground-level composite deck guide for formation, drainage and support planning.
Which accessories are needed for a BOHAI deck?
The package may include joists, stainless starter clips, plastic intermediate clips, expansion screws and L-shaped trim. The exact quantities depend on the profile, joist centres, deck dimensions, board direction, butt joints and edge treatment.
Confirm the Profile Before the Frame Is Built
The safest installation value is the one tied to the exact board on the order. Request a profile-specific installation pack with your layout, substrate and project conditions before fixing joist centres or ordering the accessory package.
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