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

One of the most common questions from homeowners planning a new deck is: can I lay decking directly on soil? The answer is yes - but how you do it makes all the difference between a deck that lasts 25 years and one that rots, sinks, or becomes a rat habitat within five.

Laying decking on soil requires proper ground preparation, correct post or bearer installation, and the right sub-frame construction to ensure the deck is stable, level, properly drained, and resistant to the moisture, pests, and frost that soil-contact installations inevitably face.

This step-by-step guide walks complete beginners through the full process - from clearing the ground through to laying the final board. Follow these steps and your deck will be safe, durable, and genuinely beautiful.

At Paving and Slabs Ltd, we supply composite decking that's ideal for soil-contact installations - alongside our core range of paving slabs and garden products. Here's how to get the ground preparation right.

What You Need Before You Start

Tools Required

• Spade and fork
• Tape measure
• String line and pegs
• Spirit level (600mm+ length)
• Long straight-edge or laser level
• Post hole borer or spade
• Drill and appropriate bits
• Circular saw or handsaw
• Hammer
• Safety equipment: eye protection, work gloves, dust mask

Materials Required

• Weed membrane (heavy-duty, not the thin garden centre variety)
• Compacted gravel or MOT Type 1 sub-base (for drainage layer)
• Concrete post mix (for post setting)
• Deck posts or adjustable deck feet
• Timber joists (treated) or composite/aluminium joists
• Composite decking boards (see our composite decking range)
• Appropriate fixings (stainless steel screws or hidden clip system)
• Composite fascia board and end caps

Step 1: Plan and Mark Out Your Deck Area

Begin with a clear plan on paper:

• Mark the deck position and dimensions
• Identify where posts will sit (at corners and at regular intervals to support joists)
• Consider the position of doors, steps, and any obstacles

Mark the area on the ground:

1. Drive timber pegs into the ground at each corner of the proposed deck
2. Run string lines between pegs to define the outer edge of the deck
3. Check the area is square by measuring diagonally corner to corner - if both diagonals are equal, the area is square
4. Mark post positions along the string lines using additional pegs

Underground services check: Before digging anything, use a cable avoidance tool (CAT scanner, available from tool hire) to check for underground pipes and cables. This is non-negotiable - hitting a buried cable or gas pipe is extremely dangerous.

Step 2: Clear and Prepare the Ground

Remove Existing Vegetation

• Cut existing grass or plants as short as possible
• Remove any large roots from within the deck area - roots that are left will continue growing and can disrupt the deck structure over time
• If you have significant weed problems, treat with a glyphosate-based weed killer and allow 2–3 weeks before proceeding

Excavate to the Right Depth

For most decking installations, you need to:

• Excavate the deck footprint to a depth of approximately 150mm - this removes the organic topsoil layer (which retains moisture and supports plant growth) and creates space for the drainage sub-base
• Dig post holes at all marked positions to the correct depth (see Step 3)

Remove all excavated material from site using a skip or wheelbarrow to a suitable disposal point.

Install Weed Membrane

This is an often-skipped step that proves its worth within two years:

1. Lay heavy-duty weed membrane across the entire excavated deck footprint
2. Overlap joins by at least 200mm
3. Pin the membrane to the ground using membrane pins at regular intervals
4. Cut crosses in the membrane at post hole positions

The membrane prevents weed growth that would otherwise push through gaps in the deck boards, holds the gravel layer in place, and keeps the underside of the deck cleaner. For more on why a tidy under-deck environment matters, see our guide on how to prevent rats under decking.

Lay Drainage Sub-Base

Pour compacted gravel (20mm clean stone or pea gravel) over the membrane to a depth of 75–100mm. This provides:

• Free drainage away from the deck structure
• A stable, non-compressible base for the deck frame
• Discourages burrowing pests

Compact the gravel firmly using a plate compactor (available from tool hire) or by treading thoroughly.

Step 3: Set Deck Posts or Bearers

The deck posts are the structural foundation of your deck. For a deck close to ground level (less than 300mm finished height), adjustable deck feet on paving pads are the most practical and effective approach.

Option A: Adjustable Deck Feet on Paving Pads (Recommended for Low Decks)

This is the best approach for most domestic low-level decks:

1. Place a concrete paving slab (minimum 450mm × 450mm) at each post position - these distribute the deck's load over a wider area and prevent the deck feet from sinking
2. Sit an adjustable galvanised or stainless steel deck foot on each slab
3. Set the deck feet to the correct height using the spirit level and string line
4. The joist frame sits directly on the deck feet - no ground contact for any timber component

Advantages: No concrete mixing; deck feet are adjustable after installation; post positions can be adjusted during installation; excellent drainage; no direct timber-to-ground contact eliminates the primary rot risk.

Browse our patio slabs range for suitable paving pads to use as load-spreading bases under deck feet.

Option B: Concrete Post Bases (For Higher Decks or Soft Ground)

For decks over 300mm high, or on soft or unstable ground:

1. Dig post holes to the correct depth - minimum 600mm; 750mm+ for high decks
2. Place the post in the hole, supported temporarily by timber props
3. Check plumb on two adjacent faces
4. Fill with post-mix concrete and allow to cure for minimum 24 hours before attaching any structural components

Step 4: Build the Sub-Frame

The sub-frame consists of perimeter bearers and internal joists. This is the structural skeleton that the deck boards lay across.

Bearer Installation

1. Fix perimeter bearers between posts at the correct height - these define the outer edge of the deck frame
2. Use the string line and spirit level to ensure all bearers are level with each other
3. Use appropriate joist hanger brackets or direct screw fixing at all connections

Joist Installation

1. Fix internal joists between the perimeter bearers at regular intervals - typically 400mm centre-to-centre for 25mm composite boards
2. All joists must sit at exactly the same height as each other - use a laser level or long spirit level to check
3. At board join positions, double joists provide support for the board ends of two adjacent boards meeting in the span

Critical: Build a fall of 1:80 to 1:100 away from the house into the frame. This means the far edge of the deck should be 12–13mm lower per metre of deck depth than the house-side edge. Use packing shims under deck feet or bearers to achieve the correct fall.

For detailed installation advice, see our 10 tips on installing composite decking which covers sub-frame specification in detail.

Step 5: Lay the Deck Boards

With a solid, level, correctly falling frame in place, board laying is the most satisfying part of the project.

Starting the Layout

1. Plan your board layout across the frame width - see our installation tips for avoiding awkward narrow strips at one edge
2. Leave a gap of minimum 10mm between the first board and any adjacent wall or structure
3. Mark centre-board positions on each joist before laying

Fixing the Boards

1. For hidden clip systems: Slide the first clip into the board groove, position the board on the frame, and screw the clip to the joist. Position the next board against the clip and repeat.
2. For face-fixed systems: Drill pilot holes and fix through the board face into the joist - always pre-drill within 50mm of board ends to prevent splitting.
3. Maintain consistent gap between boards (typically 5–6mm) throughout

Expansion Gaps

Leave a 5–8mm gap at each board end between the board and any fixed structure (wall, step, post). Composite boards expand thermally and need this space. See our detailed guide on how to finish the ends of composite decking for how to handle board ends neatly.

Step 6: Edge Finishing

The perimeter finish makes the deck look professional:

1. Fix composite fascia boards to the frame perimeter, covering board ends
2. Apply end caps to any exposed cut board ends not covered by fascia
3. Add composite post caps to any exposed post tops
4. Install steps if required - composite step systems are available as accessories

Common Beginner Mistakes to Avoid

• Not checking for underground services before digging - always use a CAT scanner
• Skipping the weed membrane - you'll regret it when weeds appear through the boards within 18 months
• Not allowing concrete to cure before loading the frame - 24 hours minimum
• Building without a drainage fall - a flat deck holds water and accelerates timber decay; our guide on stopping decking from rotting explains why drainage is critical
• Underestimating joist spacing requirements - too wide and boards flex uncomfortably underfoot

Summary

Laying decking on soil successfully requires careful ground preparation, correct post or bearer installation, a level sub-frame with adequate drainage fall, and careful board laying with correct expansion gaps. None of these steps is technically difficult - but each one matters.

For the best long-term result, choose composite decking over timber - its resistance to moisture and rot makes it particularly well-suited to soil-contact installations where moisture management is critical.

Browse our full composite decking range and contact our team for free expert advice on materials, quantities, and installation specifics for your project.