Ground-Level Composite Decking for Backyards: Building a Stable Base on Soil
A ground-level composite deck can make a backyard easier to use without turning it into a raised structure. The board choice matters, but the long-term result is usually decided before the first board is fitted: by the ground, drainage route, support points and airflow below the frame.
This guide is for low-level backyard decks over grass or soil. It does not replace a local structural design, drainage plan or planning check. If the site is steep, recently filled, regularly waterlogged, close to a building, or intended for a heavy structure, have the base and support arrangement checked locally before ordering materials.
350 mm joist max
Design centre for 140 × 23 mm
140 × 23 mm board
Four-sided co-extrusion
3.25 kg/m
Board weight, 140 × 23 mm
20-day production
Standard catalogue sections
15-year warranty
Stepped cover, co-extrusion
Start with the Ground, Not the Boards
The useful question on a backyard site is not whether the ground looks level. It is where water will go once the deck covers the soil.
Remove turf, roots, loose topsoil and construction debris from the proposed deck area. Then inspect the site after rain if possible. A low deck can hide a drainage problem for a long time, so access to the outlet and to service points matters as much as the appearance of the finished surface.
The sub-base should lead water away from the building and avoid trapping it beneath the frame. Mark drainage runs, surface gullies, irrigation lines and any services before excavation.
Do not lay composite boards directly on grass or soil. The boards, clips and frame need a stable, drained structure beneath them. Direct contact makes it difficult to control movement, inspect the base or keep the underside clear of standing water and organic debris. Where the deck meets a house wall, the threshold and damp-proof detailing are covered on the separate garden terrace detailing page.
Choose the Base Route for the Site You Actually Have
Four starting conditions, four different answers. Identify which one you have before specifying anything.
| Starting condition | Practical route | What to check before proceeding |
|---|---|---|
| Firm, reasonably level soil | Clear the area, compact the formation, use a permeable separator and drained granular layer, then set the support system to the required level. | Does rain drain away without ponding? Is the soil undisturbed and consistent across the deck footprint? |
| Soft, made-up or uneven ground | Treat this as a foundation question, not a board-spacing question. A local professional may need to specify support locations, ground improvement or a different structure. | Has the ground been filled recently? Are there soft spots, roots, drains or visible settlement? |
| Existing concrete | Do not assume the soil-base method applies. Check concrete fall, drainage, finished height and the condition of the slab. | Can water leave the slab? Does the available height allow a ventilated subframe? |
| Sloping ground or a noticeable drop | Confirm the structural, guarding and local permission requirements before fixing a layout. | Where will loads transfer, and is fall protection required? |
The Build-Up in One Drawing
Everything below the boards, in the order it goes in. These layers decide whether a low deck stays level and drained.
Indicative section through a ground-level deck on prepared soil. Layer thicknesses are not to scale. The 350 mm figure is the maximum joist centre for the 140 × 23 mm board. Tap the drawing to open it full size.
Build a Base That Resists Uneven Settlement
The sequence is simple in principle. The details change with soil type, climate, deck size and finished height.
Separation and drainage are not a substitute for compaction
Use a permeable geotextile where it is appropriate to separate soil from the granular layer and reduce weed growth. A drained aggregate layer can help water move through the base rather than remaining under the frame. It is not, however, a substitute for compaction or a structural foundation. If the original ground is soft, the deck can still settle even when the surface looks neat on installation day.
Keep support points accessible
Support points should spread load onto prepared ground and allow the frame to be brought accurately to level. Keep the support system accessible enough for inspection and adjustment where the selected system permits it. Avoid creating low pockets around blocks, pads or pedestals that collect water after the surrounding soil is covered.
Plan the finished height before you order
The finished height must accommodate the board, clip, joist or rail, support system and the drainage and ventilation zone beneath. Trying to save a few millimetres by reducing the void below the frame can make cleaning, drainage and repair much harder later.
Do not close the perimeter tight
Low decks need a clear route for both drainage and ventilation. Do not close the perimeter with solid skirting if it prevents air movement or blocks a drainage outlet. Where a visual screen is required, use a ventilated detail and retain a removable access point for inspection.
Build Sequence on a Soil Base
The order matters. Several of these steps cannot be corrected once the boards are fixed.
Inspect the site, ideally after rain
Establish where water currently goes and mark drainage runs, gullies, irrigation lines and services before any excavation begins.
Strip the organic layer
Remove turf, roots, loose topsoil and construction debris across the whole deck footprint, not only where the supports will land.
Prepare and compact the formation
Compact the formation, then separate it from the granular layer with a permeable geotextile where appropriate, and form a free-draining base.
Set the support points
Spread load onto prepared ground and bring the frame accurately to level. Avoid low pockets around blocks, pads or pedestals that will collect water once the soil is covered.
Build the frame at 350 mm centres or tighter
For the confirmed 140 × 23 mm profile, 350 mm is the maximum joist spacing. Diagonal layouts, steps, high-use areas and local engineering requirements may require closer support.
Fit boards with the specified clip system and expansion gaps
Use the specified hidden-clip system so boards can move with temperature changes. Leave the required side, end and perimeter expansion allowances for the selected profile and local temperature conditions, and restrain the first and last board with the designated edge fixing detail.
Board Sections for a Ground-Level Backyard Deck
Four co-extruded sections suitable for a normal low-level backyard platform. Colour, variants, price and the full specification are on each product page.
Co-extrusion Decking Board
23 × 138 mm
Four-sided co-extrusion. The default section where a normal board-and-joist build-up fits the planned finished height.
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Co-extrusion Wide Board (Premium)
23 × 148 mm
Wider face in the premium M-profile range. Fewer joint lines across an open backyard platform.
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Co-extrusion Decking Board
23 × 148 mm
Standard co-extrusion in the same 148 mm width, as an alternative where the premium profile is not required.
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Co-extrusion Solid Board
20 × 140 mm
Solid co-extruded section, 3 mm shallower than the 23 mm board. Useful where the available finished height is marginal.
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Estimate Quantity and Compare Offcut
Enter the deck area and the longest single run. Quantity is calculated from the 145 mm effective coverage width rather than the nominal board width, and the four board lengths are compared for offcut.
When a Soil Base Is Not Enough
Pause and obtain local advice when any of these conditions applies. They affect the base and structure, not just the surface material.
• The ground is recently made up, visibly soft or shows existing settlement.
• The deck will bridge a drain, retaining wall, service trench or change in ground level.
• Water remains on the site after normal rainfall.
• The platform is elevated, close to a boundary, attached to a building, or needs guarding.
• The deck will carry a hot tub, masonry feature, outdoor kitchen, vehicle access or another concentrated load.
Already have a concrete backyard?
Composite decking over concrete is a separate installation route. The key checks are the slab's drainage fall, its condition, available build-up height and the way the subframe will be kept ventilated. Do not use a soil-base detail on a slab simply because both projects are low level. A dedicated concrete-base guide will cover that route.
Send your backyard measurements
Deck dimensions, a photo of the existing ground after rain, your country, the intended finished height and any proposed heavy features. We come back with a board section and accessory set that suits the build-up.
Ground-Level Backyard Decking: Common Questions
Base and drainage
Can composite decking go directly on grass?
No. Grass and loose soil do not provide a controlled support, drainage or inspection zone. Remove the organic layer and build a prepared base and subframe system instead.
What causes a low backyard deck to sink?
The usual cause is uneven movement in the ground or support points, often worsened by poor drainage. The boards cannot compensate for a base that was not prepared for the site's soil and water conditions.
Does a ground-level deck still need ventilation?
Yes. Drainage and airflow below the frame reduce prolonged damp conditions and make future inspection more practical.
Can I put composite decking over an existing concrete patio?
Possibly, but it needs a concrete-specific assessment of fall, drainage, height and subframe ventilation. It should not be treated as the same build-up as soil.
Boards and spacing
What joist spacing should be used on a backyard deck?
For the confirmed 140 × 23 mm profile, 350 mm is the maximum joist spacing. That number is not a universal rule for every WPC board, installation pattern or load condition. Diagonal layouts, special profiles, steps, high-use areas and local engineering requirements may require a closer support arrangement.
Does composite decking need regular treatment?
Do not apply a treatment unless it is compatible with the selected product and the manufacturer's instructions. Routine cleaning should follow the product guidance; do not use aggressive solvents or methods that could damage a capped surface.
Can we see the boards before committing?
Samples are free — you cover the courier, and we credit it against your first order.
Warranty
What warranty applies to these boards?
All four sections shown on this page are co-extruded boards, covered by the 15-year limited warranty for second-generation WPC decking. The cover is stepped, not flat: 100% of the value of the affected goods in years 1 to 3, 80% in year 4, 70% in year 5, 50% in year 6, 40% in year 7, 30% in years 8 and 9, 20% in years 10 to 12, and 10% in years 13 to 15, with no cover from year 16. The period runs from the date the goods arrive and are signed for at the destination port, not from the production date. Quote it that way in a contract rather than as a flat fifteen years.
Permissions
Do I need planning permission for a backyard deck?
Requirements vary by location, height, coverage, proximity to boundaries and the property type. Check with the local planning and building-control authority before construction.
Plan the right base before you order boards
Send the deck dimensions, a photo of the existing ground after rain, your location, the intended finished height and any proposed heavy features. We help identify a suitable board profile and accessory set, while the site-specific base, drainage and structural details are confirmed locally.