How to Lay Decking on Soil: A Step-by-Step Guide for Beginners

One of the most common questions from homeowners planning a new garden deck is: can I lay decking directly on soil? The short answer is yes—but how you prepare the ground makes the difference between a deck that lasts 25 years and one that rots, sinks, or becomes a pest harbor within five.

Laying decking over soil requires thorough ground preparation, robust bearer/post installation, and an elevated sub-frame. This guarantees that your structure remains stable, level, properly drained, and shielded from the moisture and frost naturally present in raw earth.

At Paving and Slabs Ltd, we supply low-maintenance composite decking that is ideal for soil-contact installations, alongside our core ranges of paving slabs and garden landscaping supplies. Follow this guide to build a safe, durable, and beautiful deck from the ground up.

Tools & Materials Checklist

Required Tools:

• Spade, garden fork, and wheelbarrow
• Tape measure, string line, and wooden layout pegs
• Spirit level (600mm+ length) or laser level
• Post hole digger or manual spade
• Drill, driver bits, and pilot bits
• Circular saw, mitre saw, or handsaw
• Cable avoidance tool (CAT scanner)
• Safety goggles, heavy-duty gloves, and dust mask

Required Materials:

• Heavy-duty woven weed membrane and fixing pins
• MOT Type 1 aggregate or clean 20mm gravel (for drainage layer)
• Post-mix concrete or adjustable deck pedestals
• Paving slabs (used as load-spreading pads under pedestals)
• Treated timber joists or composite sub-frame profiles
• High-quality boards from our composite decking range
• Stainless steel screws or hidden deck clip systems
• Matching composite fascia boards, trim, and end caps

Step 1: Plan and Mark Out Your Area

Begin with a accurate plan drawn to scale:

• Define the total deck footprint and target height.
• Plot post or deck foot positions (corner locations and supporting joist spans).
• Account for exterior doorways, garden steps, and drainage gullies.

Marking the Perimeter:

1. Drive timber pegs into the ground at each outer corner of your proposed layout.
2. Tie string lines between pegs to establish clear perimeter lines.
3. Verify squareness: Measure diagonally from corner to corner in an 'X' shape. If both diagonal measurements match exactly, your footprint is square.
4. Mark support positions along the string lines using additional pegs.

Underground Cable & Pipe Check: Before digging into soil, scan the area with a Cable Avoidance Tool (CAT scanner). Hitting buried electrical cables or gas lines is extremely dangerous and costly to repair.

Step 2: Ground Excavation & Drainage Sub-Base

Clear Vegetation and Turf
Strip away all top turf, grass, and surface plants. Dig out large tree or bush roots completely—leaving organic material underneath will cause rotting voids as roots decay over time.

Excavate the Base
Dig out the entire deck footprint to a depth of approximately 150mm. This removes soft, water-retaining topsoil and creates space for a solid drainage foundation.

Lay Heavy-Duty Weed Membrane
Cover the excavated ground with a heavy-duty geotextile weed membrane, overlapping joins by at least 200mm. Pin the fabric securely to the soil. This prevents weed growth through board gaps, keeps the sub-frame clean, and deters pests. Read our detailed guide on how to prevent rats under decking for more pest-proofing advice.

Add a Gravel Drainage Layer
Spread 75mm–100mm of clean 20mm aggregate or gravel over the weed membrane. Compact the stone firmly using a manual tamper or plate compactor. This creates a free-draining base that channels rainwater away from sub-frame timbers.

Step 3: Establish Posts or Support Pedestals

Never lay structural joists directly onto bare soil. Use one of the following two support methods:

Option A: Adjustable Deck Feet on Paving Pads (Recommended for Low Decks)
For low-profile ground decks (under 300mm high), this is the cleanest, most effective method:

1. Place dense concrete slabs from our patio slabs range (minimum 450x450mm) onto the compacted gravel base at each support grid point.
2. Position adjustable galvanised or plastic deck pedestals on top of each slab.
3. Adjust pedestals to level using a spirit level and string line.
4. Sit joists directly onto pedestals. This isolates all timber from earth moisture and eliminates ground rot risks.

Option B: Concrete Post Footings (For Elevated Decks or Sloped Ground)
For decks above 300mm or areas with soft soil:

1. Dig post holes to a minimum depth of 600mm (750mm+ on soft ground).
2. Insert treated timber or composite support posts, holding them plumb with temporary bracing.
3. Fill post holes with fast-setting post-mix concrete, troweling the surface so water sheds away from the post.
4. Allow concrete to cure for at least 24 hours before loading the frame.

Step 4: Construct the Sub-Frame

The sub-frame provides structural support for the deck boards:

Install Outer Bearers:
Connect perimeter bearers to your support posts or deck feet using heavy-duty joist hangers or structural timber screws. Ensure all outer framing members are plumb and level.

Set Internal Joists:
Place joists inside the perimeter frame at regular centers. For standard 25mm composite boards, joist centers should measure no more than 400mm center-to-center (or 300mm for commercial/heavy loads). Fit double joists where board ends meet to provide full support.

Incorporate a Drainage Fall:
Build a slight fall of 1:80 to 1:100 away from any adjacent buildings into the framing. This allows surface water to run off natural board grooves easily rather than pooling. Review our guide containing 10 tips on installing composite decking for joist specification guidelines.

Step 5: Lay and Fasten Decking Boards

Once the frame is secure, level, and properly pitched, begin board installation:

1. Position First Board: Lay the first board along the outer edge or adjacent wall, leaving a 10mm expansion gap against fixed masonry walls.
2. Use Recommended Fasteners:
   • Hidden Clips: Insert hidden starter clips into board grooves and screw into joists. Slide subsequent boards over clips for a screw-free surface appearance.
   • Face Screws: Pre-drill pilot holes and countersink screws at least 50mm from board ends to avoid splitting materials.
3. Maintain Consistent Gaps: Keep a uniform 5mm–6mm side gap between boards to allow air circulation and water drainage.
4. Thermal Expansion Gaps: Leave a 5mm–8mm end-to-end butt gap between board ends to allow composite materials to expand thermally. Learn how to finish board ends neatly in our guide on how to finish the ends of composite decking.

Step 6: Perimeter Trim & Edging

Complete your deck installation with proper edge finishing:

• Fix matching composite fascia boards around outer framing perimeter to conceal structural joists.
• Attach color-matched end caps to open profile board ends.
• Install anti-slip step edges or perimeter stair treads if working on elevated levels.

Common Mistakes to Avoid

• Skipping CAT Scans: Always check for hidden underground power lines or pipes before digging post holes.
• Omitting Weed Fabric: Skipping membrane installation allows aggressive weeds to grow through board gaps over time.
• Laying Flat Without Fall: Decks built completely flat trap standing water, increasing slip risks and timber decay. Read our post on stopping decking from rotting for maintenance strategies.
• Excessive Joist Spans: Spacing joists wider than 400mm centers causes composite boards to flex underfoot.

Summary

Laying decking over soil successfully requires systematic excavation, good drainage, sub-frame isolation, and precise spacing during board placement. Taking time to complete ground preparation properly ensures a stable deck that lasts for decades.

Complete your garden landscaping project by pairing your new deck with our luxury porcelain paving or traditional paving slabs collections. Match your outdoor space with stylish options from our composite fencing range or add interior character with sound-dampening acoustic wall panels.