Can You Lay Composite Decking on Grass, Soil or an Existing Patio?
You should never lay composite decking boards directly on grass or soil. Composite decking always needs a solid, level, well-draining subframe of joists — but you can build that subframe over grass, soil or a sound existing patio, as long as the ground is prepared properly. Get the base right and your deck will stay flat, firm and problem-free for decades; skip it, and you’ll get movement, poor drainage and a deck that fails early.
Here’s exactly what you can and can’t lay composite decking on, and how to prepare each type of ground.
Can You Lay Composite Decking Directly on Grass?
No — you can’t lay composite decking straight onto grass, but you can build a deck over a grassed area once it’s prepared. Laying boards or even a subframe directly on turf causes problems: the ground is soft and uneven, grass and weeds keep growing, water can’t drain freely, and there’s no airflow underneath.
To deck over grass properly:
- Remove the turf across the area.
- Lay a weed-control membrane to stop regrowth.
- Add a compacted sub-base (such as MOT type 1 or gravel) to create a firm, free-draining layer, or use concrete pads/footings to support the frame.
- Build a level subframe of composite joists on top, making sure it’s supported and level.
- Fit your composite decking boards to the frame with hidden clips.
Can You Lay Composite Decking on Soil?
Composite decking can go over soil, but only once you’ve created a stable, well-draining base and a supporting subframe — never boards straight onto bare earth. Soil moves, holds water and settles unevenly, which would leave your deck uneven and poorly ventilated.
The approach is the same as for grass: clear the area, lay a weed membrane, create a firm sub-base or set concrete footings/pads to carry the frame, then build your joist subframe on top. The key is that the finished frame is level, firmly supported and lifted clear of the ground so air and water can move underneath.
Can You Lay Composite Decking on an Existing Patio or Concrete?
Yes — you can lay composite decking over an existing patio or concrete base, provided it’s structurally sound, reasonably level and drains well. An existing hard surface is often a great starting point because the hard work of creating a stable base is already done.
A few things to check and do:
- Make sure the patio is solid, not cracked or sinking, and that water drains away rather than pooling.
- Build a subframe of joists on top of the patio rather than fixing boards flat to the concrete — the frame lets you level everything, creates airflow, and gives the clips something to fix into.
- Use packers or adjustable pads to correct any slight falls or unevenness in the old surface.
- Leave gaps for drainage and ventilation so water can escape beneath the deck.
Why Does Composite Decking Always Need a Subframe?
Whatever the ground, composite boards are designed to sit on a subframe of joists — never directly on the ground. There are four good reasons for this:
- Airflow: a gap underneath lets the deck breathe and water drain, preventing damp build-up.
- A level finish: the frame lets you create a flat, stable surface even over imperfect ground.
- Fixing: the hidden fixing clips that hold your boards need joists to fix into.
- Movement: composite decking expands and contracts with temperature, so it needs correct spacing and support to move freely without lifting or buckling.
What You’ll Need for the Base
For most projects over grass, soil or an old patio you’ll want a weed-control membrane, a sub-base material or concrete pads/footings, composite joists for the frame, and fixing clips and screws for the boards. Our free decking calculator helps you work out how many boards, joists and clips you’ll need once you know your area, and our full installation guide covers subframe layout, joist spacing and expansion gaps in detail. If you’re new to composite, it’s also worth understanding what composite decking is made of and why the subframe matters.
Frequently Asked Questions
Can you lay composite decking directly on grass?
No. You should never lay composite decking straight onto grass. Remove the turf, lay a weed membrane, create a firm sub-base or footings, then build a level joist subframe and fix your boards to that.
Can you lay composite decking on soil?
Only once the soil is prepared with a stable, well-draining base and a supporting subframe. Boards should never sit directly on bare earth, as soil moves, holds water and settles unevenly.
Can you lay composite decking on an existing patio?
Yes, if the patio is sound, level and drains well. Build a joist subframe on top rather than fixing boards flat to the concrete, and use pads or packers to level any uneven areas.
Do you need a subframe for composite decking?
Yes. Composite decking always needs a subframe of joists. It provides airflow, a level surface, something for the fixing clips to grip, and room for the boards to expand and contract.
Can you lay composite decking straight onto concrete?
It’s not recommended to fix boards flat to concrete. Build a subframe on top instead, so the deck has ventilation, drainage and room to move, and so the hidden clips have joists to fix into.
