How to Build Decking

Well-built garden decking can transform an unusable slope or an awkward ground-level area into a usable outdoor space. The key to decking that lasts, rather than turning soft, lifting at the edges, or becoming hazardously slippery, is getting the subframe right, using appropriately treated timber, and paying attention to drainage gaps and ventilation underneath.

A typical softwood deck costs £60–£140/m² in materials and takes a reasonably confident DIYer a weekend per 15–20 m² of finished area.

Planning and Permissions

Before you start, check whether your project needs consent:

Permitted Development allows householders to build decking without Planning Permission provided:

• The deck is no more than 300 mm above ground level at its highest point. This is the threshold in the order’s own definition: a platform is raised once it is higher than 0.3 m, and Class E does not permit the provision of a verandah, balcony or raised platform at all¹
• The total area of ground covered by buildings, enclosures and containers within the curtilage, other than the original house, does not exceed 50% of the curtilage, and the curtilage figure for that sum excludes the ground area of the original house¹
• The structure is not within the curtilage of a listed building
• On article 2(3) land, which includes conservation areas, nothing may sit between a side elevation wall of the house and the boundary of the curtilage
• In an AONB, National Park, the Broads or a World Heritage Site, structures more than 20 m from any wall of the house must not cover more than 10 m² in total

If your deck exceeds 300 mm, sits close to a side or rear boundary in a restricted area, or the property is Listed or in an Article 4 area, check with the Local Planning Authority before starting. In these cases, a planning application (Householder Application, £548 in England as of 2026) is required.²

Freestanding garden decking is not normally building work under the Building Regulations at all. The 600 mm figure people quote is the Part K guarding threshold: Approved Document K asks for pedestrian guarding capable of preventing injury from a fall of more than 600 mm in a dwelling, so a raised deck attached to the house, or one reached from a door, is where guarding becomes the question.³ Structural posts set in concrete and any electrical connections, outdoor lighting and sockets included, have to comply with the relevant regulations either way.

Tools and Materials

Tools

• Circular saw or mitre saw
• Cordless drill and screwdriver bits
• String lines and pegs
• Spirit level and long straight-edge
• Post hole digger or auger
• Mallet and chisel
• Tape measure
• Jigsaw (for curves and cutouts)
• Safety goggles and hearing protection

Materials (for a 20 m² ground-level deck)

Timber treatment: always use UC3 (Use Class 3) pressure-treated softwood for joists and bearers in an outdoor application, and UC4 for any posts set into or close to the ground. Green-treated timber (pre-impregnated with preservative) is available from most builders’ merchants.

Step-by-Step Build Process

Step 1: Mark Out and Prepare the Ground

Mark the deck perimeter with string lines and pegs. Dig or clear all vegetation within the footprint. Lay 75–100 mm of compacted hardcore, topped with weed-suppressing membrane, to prevent weed growth and improve drainage underneath the deck. Good airflow under the deck is essential to prevent timber decay.

Step 2: Set Your Posts or Bearer Supports

For low-level decks (under 150 mm), treated timber pads or adjustable steel saddles set on concrete pads are used to support the bearers. For raised decks, concrete-set posts are necessary.

Post holes should be at least 450 mm deep for a deck up to 600 mm high, or 600 mm+ for anything higher. Mix post-mix concrete dry into the hole, pour in water, and allow 24 hours to cure before loading.

Step 3: Fix the Bearer Frame

Bearers (the lowest horizontal timbers) span between post heads and carry the joists. Set them level using a spirit level and string line. Bearer spacing depends on joist size:

• 100 × 50 mm joists → bearers at 1,800 mm maximum centres
• 150 × 50 mm joists → bearers at up to 2,400 mm centres

Where the deck adjoins the house, fix a ledger board directly to the wall using frame anchors, with a step down of at least 150 mm below the damp-proof course. Seal around fixings with exterior sealant.

Step 4: Install the Joists

Joists run perpendicular to the direction of the decking boards. Joist spacing determines the rigidity of the deck surface:

Fix joists using galvanised joist hangers at each end, plus intermediate support on bearers. Ensure all joist tops are in the same horizontal plane, check with a long straight-edge and adjust packer shims under bearers as needed.

Step 5: Lay the Decking Boards

Run boards across the joists, leaving a 5–6 mm gap between each for drainage and thermal expansion. Timber decking dries and moves after installation, if boards are laid green (freshly treated), allow for more movement initially.

Fix each board with two screws per joist crossing, pre-drill to prevent splitting. Alternatively, use hidden clip fixings (available for grooved-edge boards) for a cleaner surface finish. Stainless steel fixings are strongly recommended; galvanised fixings will rust and stain within a few seasons in wet climates.

At the perimeter, cut boards flush using a circular saw guided by a chalk line or straight-edge clamped along the deck edge. Install a fascia board to finish the ends of the joists and give a clean vertical edge.

Step 6: Finish and Treat

Sand any rough surfaces using 80-grit orbital sanding. Apply an exterior-grade penetrating oil or decking stain within a week of completion, before the wood begins to grey. First application typically soaks in fully, a second coat after 2–4 hours improves protection. Reapply annually or when water stops beading on the surface.

Common Mistakes to Avoid

• Insufficient ground clearance: less than 100 mm between soil and joists leads to rot within a few years. Aim for a minimum 150 mm clear airspace.
• Boards running parallel to the house: water can pond against the house wall. Run boards at 90° or diagonally to ensure run-off away from the structure.
• Skipping the membrane: without weed membrane, grass and weeds push up through gaps and trap moisture against the timbers.
• Overtightening board gaps: boards installed in winter shrink together in the first warm spring. Fit in summer at 5 mm gap; fit in winter at 8 mm gap.
• Ignoring the anti-slip requirement: textured or ridged decking boards are important on north-facing or shaded decks, which grow algae in UK conditions. Sand regularly, treat with an anti-algae wash, or specify composite boards with anti-slip texture.

Timber vs Composite Decking