Composite Decking Over Concrete: Slab, Drainage & Sleeper Guide
You can install composite decking over concrete, but the deck boards should not be treated as a finish that is bonded directly to the slab. The existing concrete first has to drain, remain stable and leave enough height for a supported, ventilated build-up.
This page is for installers, distributors, designers and project buyers assessing a patio, porch or slab. It separates the slab decision from the board decision, shows three build-up routes, links real BOHAI products and gives the measurements needed for a technical review and quotation.
The banner is an application illustration, not a completed BOHAI case study. Final anchoring, waterproofing, structural capacity, threshold and code decisions belong to the project designer and local authority.
Specification guides: Profiles, sections and metre weights · Roof terraces on adjustable pedestals · How to tell a real WPC factory from a trader
We also write for: Germany · the Netherlands · Belgium · Ireland · Poland · France · Spain · Italy · Greece · Austria · Switzerland · Sweden · Slovenia · Croatia · the Czech Republic · Slovakia · Hungary · Portugal · Denmark · Norway · Finland · Estonia · Latvia
Slab first
Condition and fall
Drainage route
Keep outlets open
Height check
Threshold before boards
350 mm max
Confirmed flat-board support
Matched system
Joists, clips and anchors
MOQ 70 m²
Factory decking orders
Can You Put Composite Decking Over Concrete?
Yes—when the slab is a suitable base and the boards are carried by a designed sleeper or joist system. “Over concrete” should not mean “directly onto concrete.”
A quick go / pause / stop test
Proceed to design when the slab is stable, drains away from the building, has no continuing movement and leaves enough vertical room. Pause for repair when water ponds, outlets are blocked, surface defects affect fixings or the threshold is tight. Stop and obtain local advice when the slab is sinking, structurally uncertain, part of a roof or waterproofed assembly, or expected to carry concentrated loads.
Why a substructure is still needed
Whether the brief says composite decking on concrete or a composite deck over concrete, the sleeper or joist layer creates a consistent fixing plane, supports the boards at the required centres, keeps water moving below them and leaves a route for inspection and cleaning. Composite deck boards are a walking surface, not structural joists and not a repair system for failed concrete.
Is the Existing Concrete Suitable?
Use this triage before choosing colours or ordering boards.
| Observed condition | Initial decision | What must be resolved |
|---|---|---|
| Sound slab, visible fall, outlet remains open | Suitable for a detailed sleeper-system check | Finished height, joist direction, anchors, edge detail and local requirements |
| Hairline cracks with no measured movement | Record and assess before covering | Cause, moisture route and whether the fixing pattern crosses or relies on the cracked area |
| Ponding after rain | Do not cover yet | Restore a positive drainage route; boards and gaps do not correct a flat or back-falling slab |
| Spalling, loose surface or damaged edges | Repair and reassess | Anchor substrate, edge distance and whether the remaining concrete is sound |
| Ongoing settlement or widening cracks | Stop | Structural or geotechnical cause; covering it only hides the evidence |
| Balcony, podium, roof or waterproof membrane | Use a project-specific system | Loads, membrane protection, penetrations, drainage and responsible designer approval |
| Door or sill leaves very little height | Compare low-profile route or deck tiles | The complete stack—not just board thickness—must clear the threshold detail |
The Build-Up in One Drawing
The water path and the tightest height normally decide the layout before the board colour does.
Indicative build-up — not to scale
Composite deck board · fixed to every support with the specified clip or screw system
Sleeper / joist layer · laid so it does not dam the slab's water route; maximum 350 mm centres for the confirmed BOHAI flat-board application
Leveling / isolation / fixing zone · selected for the slab, joist material, drainage and approved anchor design
Existing concrete slab · stable, checked for cracks and settlement, falling to an open outlet
high pointdrain / free edge
Do not trap waterSleepers must not create closed channels across the fall.
Do not hide failureActive settlement, major spalling or a blocked outlet should be resolved before covering the slab.
Do not guess the thresholdMeasure from the highest slab point to the lowest door, sill or fixed edge.
Sleeper direction follows drainage logic
Where practical, align the sleeper system so it does not form dams across the slab fall. If the design needs cross-members, packers or blocking, keep deliberate water passages through the layout. The exact direction and fixing pattern must be coordinated with board direction, edge restraint, structural requirements and outlet access.
Check Finished Height Before You Order
Installing composite decking over a concrete porch often fails at the door, not in the middle of the patio.
Arithmetic only. “Other allowance” can include packers, isolation, leveling, fastener zone and a project-defined drainage/ventilation allowance. It is not a universal minimum and does not approve a threshold detail.
What counts toward the stack
Include the deck board, joist or sleeper, leveling and isolation pieces, anchors or clips, the local drainage and ventilation detail, and any threshold or flashing allowance. Measure at the highest point of concrete and the lowest fixed obstruction. A 20 mm board plus a 25 mm aluminium joist is a 45 mm material stack before any other allowance; a 23 mm board plus a 30 mm WPC joist is 53 mm before those additions.
When the numbers do not fit
Do not shave away the drainage route or use a board as its own joist. Compare a purpose-made low-profile substructure, a direct-lay deck tile that is designed for the substrate, removal or rebuilding of the slab, or a different finish. The acceptable answer depends on the site and selected product.
How to Install Composite Decking Over Concrete
This sequence is a project-planning guide. It does not replace the selected profile instructions, anchor design or local code review.
Survey the slab after rain
Photograph ponding, mark the high and low points, record drains and measure from the highest concrete point to doors, sills, steps and fixed edges.
Decide whether the slab can stay
Investigate settlement, widening cracks, loose edges and spalling. Repairing a finish is different from proving the slab and anchor zones are suitable.
Set the water route
Keep the existing fall working or redesign it before covering. Do not place a continuous sleeper across an outlet or close the deck perimeter so tightly that water and debris cannot leave.
Choose the build-up route
Use an anchored sleeper system, a project-approved non-penetrating support system, or purpose-made direct-lay deck tiles. Do not mix details from unrelated products.
Lay out supports for the exact board
For the confirmed BOHAI flat-board application, use 350 mm centres or tighter. Diagonal patterns, special loads and different profiles may require closer supports.
Resolve joints and edges
Provide two supports at board-end butt joints so each end has its own fixing. Coordinate start clips, edge restraint, trims, access panels and any removable drain cover.
Install with documented gaps
BOHAI starting points are 5–7 mm between board sides, about 5 mm at a wall for capped boards and 3–5 mm at butt joints. Confirm the final values for the exact profile and site temperature.
Water-test and hand over
Before closing access, run water across the slab, verify that outlets remain clear, check board alignment and record the product, colour, batch, support layout and installation instructions.
Three Ways to Build Over Concrete
The correct route depends on the slab, height, drainage, access and whether penetrations are permitted.
| Route | Where it can fit | Main checks |
|---|---|---|
| Anchored sleeper / joist system | Stable patio or porch where the slab and anchor design are suitable | Concrete condition, anchor type and embedment, edge distances, drainage passages, corrosion environment and support spacing |
| Non-penetrating support or pedestal system | Waterproofed or protected surfaces where drilling is not permitted and the complete system is designed for the loads | Membrane protection, point loads, uplift, lateral restraint, minimum height, outlet access and responsible designer approval |
| Purpose-made composite deck tiles | Flat, sound concrete where the selected tile is designed for direct lay and its drainage grid remains open | Product-specific substrate limits, edge restraint, door clearance, drainage, cleaning access and local slip requirements |
Products to Compare for a Concrete Patio or Porch
These are real BOHAI product routes. The board profiles still require a joist or sleeper system; the deck tile is a separate direct-lay product and does not inherit the board specifications.
Co-extrusion Solid Board
20 × 140 mm · capped
The shallowest listed board profile on this page. Compare it where height is tight, while still allowing a complete support, drainage and threshold detail.
View product →
Co-extrusion Decking Board
23 × 138 mm · capped hollow
A main four-sided capped profile for a normal sleeper build-up. Use the quotation and profile instructions to confirm the exact surface, colour and fixing system.
View product →
Co-extrusion Wide Board
23 × 148 mm · capped hollow
A wider capped board that reduces the number of board lines across a patio. It does not reduce the need for correct supports, drainage or expansion allowance.
View product →
Co-extrusion Composite Deck Tile
Purpose-made direct-lay route
A separate click-lock tile route for a suitable flat substrate. Do not apply the 350 mm joist, board weight or board warranty statements to this product.
View product →
Joists, Clips and Edge Parts Are Part of the Specification
Two catalogue joist sizes
BOHAI lists a 40 × 30 × 3000 mm WPC joist and a 40 × 25 × 2900 mm aluminium joist. Their dimensions help with the height study; they do not by themselves approve a span, anchoring pattern, waterproofing detail or structural load.
Board-to-joist connection
Available parts include stainless-steel starter clips, plastic or stainless hidden clips and expansion screws. Match the clip and screw to the exact board groove and joist material. The person specifying anchors into concrete must account for the slab, loads, embedment, spacing, edge distance and exposure.
Finish without sealing the drainage path
The 50 × 50 × 3000 mm L-shaped trim can finish visible edges, but a solid perimeter closure must not block water, airflow or access. Provide a removable or ventilated detail where cleaning and inspection are needed.
Drainage Is the Non-Negotiable Part
Board gaps are not a substitute for slab fall
Water may pass between boards and still become trapped by a back-falling slab, a continuous sleeper or a sealed edge. The concrete must have a reliable route to a drain or free edge before the deck hides it.
Keep drains inspectable
A buried outlet is an eventual maintenance problem. Mark the drain position, provide a removable access panel where practical and record how the deck can be opened without destroying a full board run.
Design for debris as well as rain
Leaves, sediment and wind-blown dirt collect below low decks. The build-up needs enough access and open flow paths for the expected environment; a detail that drains on installation day can still clog if it cannot be cleaned.
BOHAI Board Installation Values Used on This Page
These are manufacturer starting points for the confirmed flat-board application. They are not universal values for every composite product.
| Item | Starting value | Scope |
|---|---|---|
| Maximum support spacing | 350 mm centres | Confirmed BOHAI flat decking application; use tighter spacing where the profile, pattern, load or local design requires it |
| Side gap | 5–7 mm | BOHAI installation starting point; confirm by profile and site conditions |
| Capped board to wall | About 5 mm | Perimeter movement starting point; the building threshold and waterproof detail still govern |
| Board-end butt joint | 3–5 mm plus double supports | Each board end requires its own support and fixing |
| Board lengths | 2.9 / 3.6 / 4.0 / 5.8 m | Use the selected length and board direction to reduce offcut |
| Factory MOQ | 70 m² | Decking order quantity, not 70 sets |
| Standard catalogue production | 20 days | Production only; delivery is quoted separately |
Estimate Board Quantity and Compare Offcut
Enter the deck area and longest board run. The tool uses the 145 mm effective coverage basis for the confirmed 140 mm board family and compares the four catalogue lengths. Final ordering must use the selected profile and layout.
What Changes the Cost of a Deck Over Concrete?
The slab may reduce excavation, but a difficult slab can add survey, repair, drainage and low-profile detailing costs.
Cost starts with the existing condition
Include crack or edge repair, drainage correction, outlet access, survey and any local engineering. Do not price only the visible boards when the main uncertainty is below them.
Height can change the product route
A low threshold may move the project from a standard 23 mm board and joist to a 20 mm board with a lower joist, a purpose-made tile, or a rebuilt substrate. Compare the complete system and its limitations, not only millimetres.
Price the accessory schedule
Boards, joists, clips, starter pieces, anchors, packers, trims, access details, waste, freight, unloading and local labour all belong in the estimate. Use the deck material calculator for the first take-off, then request a project quotation.
Eight Common Failures to Avoid
1. Fixing deck boards directly to concrete with no supported drainage zone.
2. Covering a slab that still moves or ponds after rain.
3. Letting sleepers act as dams across the fall.
4. Measuring height at an easy point instead of the highest slab and lowest threshold.
5. Using the board itself as a joist or assuming all composite profiles are structural.
6. Leaving a butt joint on one narrow support instead of giving each board end its own fixing.
7. Closing the perimeter with solid trim so water, air and debris cannot move.
8. Copying another manufacturer’s clearance, spacing or fastener detail onto a BOHAI profile without confirmation.
Related Planning Guides
Different substrate, different page
Building on soil or grass is covered by the ground-level backyard decking guide. A concrete slab is a separate route because settlement, drainage, anchors and height are assessed differently.
Where the deck meets the building
Use the garden terrace detailing page for thresholds, walls and drainage direction. Use the types of composite decking guide to compare capped, uncapped, hollow, solid and tile products.
Board installation and sun exposure
The composite decking installation guide covers clips, board order and general gaps. For an exposed patio, also compare colour and shade on the full-sun decking page.
Technical Sources and Scope
Manufacturer guidance agrees on the principles—stable substrate, sleepers, drainage, airflow and product-specific spacing—but published dimensions belong to each named product.
Cross-checks used for this page
TimberTech Composite Installation Guide · Fiberon concrete-patio guidance · MoistureShield sleeper detail · ICC residential deck provisions.
How those sources are used
They support the decision sequence and the need for drainage, airflow, support and local review. Their brand-specific clearances, spans and fasteners are not copied into the BOHAI specification. BOHAI numbers on this page come from the verified product record and current installation materials.
Send the slab measurements before choosing the board
Include the patio dimensions, tightest finished height, photos after rain, drain or free-edge position, country, expected traffic and whether drilling is permitted. We will match a board, joist and accessory route for quotation; site structure, drainage and anchors remain subject to local approval.
Composite Decking Over Concrete FAQ
Slab and build-up
Can you put composite decking over concrete?
Yes, if the concrete is stable, drains, leaves enough height and can support the selected sleeper or joist system. Do not treat the boards as a direct bonded finish or use them to hide a failing slab.
Can composite decking be installed directly on concrete?
Standard deck boards should be fixed to a designed support system rather than laid directly on the slab. Purpose-made direct-lay deck tiles are a separate product route with their own substrate and drainage requirements.
Can I install composite decking over a concrete porch?
Potentially. A porch adds threshold, step, edge, railing and sometimes structural questions. Measure the complete build-up and have the slab, anchors and local requirements checked before covering it.
What if the concrete patio has cracks?
Record the cracks and determine whether they are stable. Widening cracks, continuing settlement, spalling or uncertain anchor zones need repair or professional assessment; decking should not hide unresolved movement.
Does the slab need to be level?
The walking surface needs a consistent support plane, but the slab also needs a positive drainage route. Do not make the slab “level” in a way that creates ponding; use a designed leveling and support system while preserving fall to the outlet.
Supports, gaps and products
Which way should sleepers run over concrete?
Set them out so they do not dam water across the slab fall. The final direction also depends on board direction, blocking, edge restraint and the approved structural and fixing layout.
What is the joist spacing for BOHAI composite decking over concrete?
Use 350 mm centres or tighter for the confirmed flat-board application. Different profiles, diagonal layouts, concentrated loads or local engineering may require closer supports.
How much gap should composite decking have over concrete?
BOHAI starting points are 5–7 mm between board sides, about 5 mm at a wall for capped boards and 3–5 mm at butt joints. Each butt joint needs double supports, and final values must match the profile and site.
What is the lowest-profile BOHAI board and joist combination?
The listed 20 mm solid capped board with the 25 mm aluminium joist creates a 45 mm material stack before packers, isolation, anchors and any project-defined drainage or ventilation allowance. It is not an automatic approval for a 45 mm threshold.
Can composite deck tiles go directly on concrete?
A purpose-made click-lock deck tile can be considered on a flat, sound substrate when its own installation limits are met and drainage remains open. Board-specific spacing, weight and warranty statements on this page do not transfer to deck tiles.
Measure the slab before selecting the surface
Send photos after rain · Record the tightest height · Mark drains and edges · Choose the support route · Then compare board samples and price